domain: info math

text:
Thomas Fel
email: aithomas_fel@brown.edu

Rémi Cadène
email: remi_cadene@brown.edu

Mathieu Chalvidal

Matthieu Cord

David Vigouroux

Thomas Serre

Look at the Variance! Efficient Black-box Explanations with Sobol-based Sensitivity Analysis

We describe a novel attribution method which is grounded in Sensitivity Analysis and uses Sobol indices. Beyond modeling the individual contributions of image regions, Sobol indices provide an efficient way to capture higher-order interactions between image regions and their contributions to a neural network's prediction through the lens of variance. We describe an approach that makes the computation of these indices efficient for high-dimensional problems by using perturbation masks coupled with efficient estimators to handle the high dimensionality of images. Importantly, we show that the proposed method leads to favorable scores on standard benchmarks for vision (and language models) while drastically reducing the computing time compared to other black-box methods -even surpassing the accuracy of state-of-the-art white-box methods which require access to internal representations.

Introduction

Deep neural networks are now being deployed in numerous domains including medicine, transportation, security or finances with broad societal implications. Yet, these networks have become nearly inscrutable, and for most real-world applications, these systems are used to make critical decisions -often without any explanation. In recent years, numerous explainability methods have been proposed [START_REF] Simonyan | Deep inside convolutional networks: Visualising image classification models and saliency maps[END_REF][START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF][START_REF] Tobias Springenberg | Striving for simplicity: The all convolutional net[END_REF][START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF][START_REF] Ramprasaath R Selvaraju | Grad-cam: Visual explanations from deep networks via gradient-based localization[END_REF][START_REF] Ruth | Interpretable explanations of black boxes by meaningful perturbation[END_REF][START_REF] Bachoc | Entropic variable boosting for explainability & interpretability in machine learning[END_REF][START_REF] Sundararajan | Axiomatic attribution for deep networks[END_REF][START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]. In addition to helping improve people's trust in these systems, these methods have helped identify and correct biases in datasets [START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF][START_REF] Ramprasaath R Selvaraju | Grad-cam: Visual explanations from deep networks via gradient-based localization[END_REF][START_REF] Dancette | Beyond question-based biases: Assessing multimodal shortcut learning in visual question answering[END_REF]. This has, in turn, helped improve these systems' robustness and accelerate their broad deployment. An important limitation of standard explainability methods is that they require access to the system's internal states including hidden layer activations or input gradients [START_REF] Simonyan | Deep inside convolutional networks: Visualising image classification models and saliency maps[END_REF][START_REF] Ramprasaath R Selvaraju | Grad-cam: Visual explanations from deep networks via gradient-based localization[END_REF][START_REF] Ruth | Interpretable explanations of black boxes by meaningful perturbation[END_REF][START_REF] Kapishnikov | Xrai: Better attributions through regions[END_REF]. As a result, these so-called white-box methods cannot be applied in the most general situations for which the internal states of network are not publicly accessible. For instance, it is common for companies to use neural networks provided by third parties (e.g., through web APIs or specialized hardware). However, only a handful of so-called black-box methods have been proposed to address this challenge with limited successes [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF][START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF][START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]. It is thus critical to develop more general methods that can reliably interpret and characterize the underlying decision processes of a wider array of models.

Common approaches to explaining a model's prediction consists of attributing a score for each input dimension such as image pixels for computer vision systems or individual words for natural language processing. Shown in Fig. 1 is an example image and associated importance map for an image categorization system, whereby scores for individual pixels are displayed as heatmaps where Figure 1: (Left) Sobol Attribution Method overview. Our method aims to explain the prediction of a black-box model for a given image. We first sample a set of real-valued masks M drawn from a Quasi-Monte Carlo (QMC) sequence. We apply these masks to the input image through a perturbation function Φ (here the Inpainting function) to form perturbed inputs X that we forward to the black box f to obtain prediction scores. Using the masks M and the associated prediction scores, we finally produce an explanation S Ti which characterizes the importance of each region by estimating the total order Sobol indices. While S Ti encompasses the effects of first and all higher-order non-linear interactions between pixel regions, we can also produce the first-order Sobol indices S i that reflect the importance of a region in isolation (e.g., the eyes of the cats). (Right) Sample explanations for ResNet50V2. Comparing explanations produced with S i and S Ti helps highlight the importance of individual image regions in isolation vs. jointly (e.g., the lynx tips are important but conditioned on the presence of the presence of an eye).

hotter locations correspond to pixels that contribute the most to the system's final prediction. In the context of black-box models, a core challenge is to derive these heatmaps using only the output predictions available through the network's forward pass. A simple approach consists in applying a given perturbation at a specific location on the input image to then measure how the corresponding prediction is affected. In the case of image models, pixel intensities are simply set to a default value corresponding to a pure black or gray value; in the case of language models, individual words are removed entirely from the text [START_REF] Arras | what is relevant in a text document?": An interpretable machine learning approach[END_REF][START_REF] Arras | Explaining recurrent neural network predictions in sentiment analysis[END_REF]. However, evaluating the impact of these perturbations one dimension at a time fails to identify all of the non-linear interactions between input variables that are known to prevail in a complex system such as a deep neural network. However, estimating the combined effect of perturbations across multiple locations quickly becomes combinatorially intractable. Methods have been recently proposed to try to address some of these issues by grouping dimensions together, such as by grouping pixels within a neighborhood of the image (superpixel) [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF] or sampling perturbation masks that affect multiple regions of the input [START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF][START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]. A first limitation of these approaches includes the use of Monte Carlo sampling methods which require a high number of forward passes -making these approaches computationally expensive. A second limitation is that they rely on relatively simple perturbations such as flipping pixels on or off [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF][START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF][START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]. This severely constrains the space of perturbations considered and limits the efficiency with which the space of perturbations can be explored.

We address these limitations by introducing an attribution method that leverages variance-based sensitivity analysis techniques, and more specifically Sobol indices [START_REF] Sobol | Global sensitivity indices for nonlinear mathematical models and their monte carlo estimates[END_REF]. These methods were initially introduced to help identify the input variables that have the most influence on the variance of the output of a non-linear mathematical system [START_REF] Saltelli | Making best use of model evaluations to compute sensitivity indices[END_REF]. These were traditionally used in physics and economics to estimate the part of the variance induced by a group of variables on a system's output [START_REF] Saltelli | Variance based sensitivity analysis of model output. design and estimator for the total sensitivity index[END_REF][START_REF] Iooss | Uncertainty management in Simulation-Optimization of Complex Systems: Algorithms and Applications[END_REF][START_REF] Wagener | What has global sensitivity analysis ever done for us? a systematic review to support scientific advancement and to inform policy-making in earth system modelling[END_REF]. Our main contribution is a general framework to explain predictions from black-box models by adapting Sobol indices to be used in conjunction with perturbation masks. One of the originalities of our attribution method is that it is designed to support standard perturbations and real-valued intensity perturbations that can generate a continuous range of perturbations. Our second contribution is a tractable method for the calculation of Sobol indices. This is done by first sampling the perturbation masks following Quasi-Monte Carlo sequences, which efficiently covers the space of perturbations. This can be done best by leveraging the most efficient estimator borrowed from the sensitivity analysis literature [START_REF] Michiel | Monte carlo estimation of uncertainty contributions from several independent multivariate sources[END_REF][START_REF] Michiel | Analysis of variance designs for model output[END_REF][START_REF] Janon | Asymptotic normality and efficiency of two sobol index estimators[END_REF][START_REF] Puy | A comprehensive comparison of total-order estimators for global sensitivity analysis[END_REF]. As a result, the proposed method can be efficiently applied to high-dimensional inputs such as images. It produces on par with or better explanations than the state-of-the-art with at least half the number of forward passes. Another benefit of our method is that it allows to characterize not only the main effect of image regions but also higher-order interactions by decomposing the variance of the system's prediction using the Sobol indices. We run extensive experiments to demonstrate the benefits of the proposed method on several recent complementary benchmarks including the "Pointing Game" and "Deletion". Our primary focus is image classification, but our method is general and we include results on text classification benchmarks as well.

Related work

Attribution methods for white-box models A widely used family of attribution methods uses gradient computations via the backpropagation algorithm. The first such method was introduced in [START_REF] Baehrens | How to explain individual classification decisions[END_REF] and later refined in [START_REF] Simonyan | Deep inside convolutional networks: Visualising image classification models and saliency maps[END_REF] in the context of deep convolutional networks. The idea is to backpropagate the gradient from a classification unit all the way down to the input pixels. The resulting gradient image indicates which pixels affect the decision score the most. Other gradientbased methods including DeConvNet [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF] and Guided Backprop [START_REF] Tobias Springenberg | Striving for simplicity: The all convolutional net[END_REF] were specifically developed to deal with certain activation functions such as ReLU. However, this family of methods are limited by the fact that they focus on the influence of individual pixels in an infinitesimal neighborhood around the input image in the image space. For instance, it has been shown that gradients often vanish when the prediction score to be explained is near the maximum value [START_REF] Sundararajan | Axiomatic attribution for deep networks[END_REF]. Integrated Gradient [START_REF] Sundararajan | Axiomatic attribution for deep networks[END_REF] partially addresses this issue by accumulating gradients along a straight interpolation path from a baseline state to the original image.

Yet another class of gradient-based methods [START_REF] Ruth | Interpretable explanations of black boxes by meaningful perturbation[END_REF]24] iteratively optimize a perturbation mask over regions of an image. It leads to a different type of explanation given by the final perturbation mask. Such an approach might leverage more meaningful perturbations but it turns out to be computationally very slow with 20 seconds per explanation -limiting its broad applicability. In comparison, our proposed approach also works on image regions but it does not require iterative gradient computation -and can thus be parallelized.

Another family of attribution methods relies on the neural network's activations. Popular examples include CAM [START_REF] Zhou | Learning deep features for discriminative localization[END_REF] which computes an attribution score based on a weighted sum of feature channels activities -right before the classification layer. GradCAM [START_REF] Ramprasaath R Selvaraju | Grad-cam: Visual explanations from deep networks via gradient-based localization[END_REF] extends CAM via the use of gradients to reweigh each feature channel to take into account their importance for the predicted class. These methods also work on image regions because they are computed on the output of feature maps which are themselves at a coarser resolution than the original image. In comparison, our proposed approach is model-agnostic and hence does not require access to internal computations.

Attribution methods for black-box models Most similar to our approach are attribution methods that can be used to explain the predictions of truly black-box models. These methods probe a neural network's responses to perturbations over image regions and combine the resulting predictions into an influence score for each individual pixel or group of pixels. The simplest method, "Occlusion" [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF], masks individual image regions -one at a time -with an occluding mask set to a baseline value and assigns the corresponding prediction scores to all pixels within the occluded region. Then the explanation is given by these prediction scores and can be easily interpreted. However, occlusion fails to account for the joint (higher-order) interactions between multiple image regions. For instance, occluding two image regions -one at a time -may only decrease the model's prediction minimally (say a single eye or mouth component on a face) while occluding these two regions together may yield a substantial change in the model's prediction if these two regions interact non-linearly as is expected for a deep neural network.

Our work, together with related methods such as LIME [START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF] and RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF], addresses this problem by randomly perturbating the input image in multiple regions at a time. Obviously, perturbating multiple image locations simultaneously leads to a combinatorial explosion in the number of combinations and methods have been proposed to make these approaches more tractable. For instance, a popular method, LIME [START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF], takes superpixels as regions to perturbate instead of individual pixels. An influence score is then computed for a set of connected pixel patches indicating how strongly a patch is correlated to the model predictions.

RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF] relies on Monte Carlo sampling to generate a set of binary masks, each value in the masks representing a pixel region. By probing the model with randomly masked versions of the input, RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF] produces a importance map by considering the average of the masks weighted by their associated prediction scores. Instead of using binary masks, our method considers a continuous range of perturbations which allows for a finer exploration of the model's response. Our method can still use the same perturbations as used in Occlusion [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF], LIME [START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF] and RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF], but it also enables the use of more advanced perturbation functions that take continuous inputs.

More importantly, the aforementioned methods lack a rigorous framework. Here, we introduce a theoretical framework that decomposes the influence score of each individual region between multiple orders of influence. The first-order approximates Occlusion [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF] by considering the influence of one region at a time, while the second-order considers two regions at a time, etc. The decomposition also includes higher-orders.

Variance-based sensitivity analysis Our attribution method builds on the variance-based sensitivity analysis framework. The approach was introduced in the 70s [START_REF] Ri Cukier | Study of the sensitivity of coupled reaction systems to uncertainties in rate coefficients. i theory[END_REF] and reached a cornerstone with the Sobol indices [27]. Sobol indices are currently used in many fields (including those that are said to be safety-critical), especially for the analysis of physical phenomena [START_REF] Iooss | Uncertainty management in Simulation-Optimization of Complex Systems: Algorithms and Applications[END_REF]. More recently, connections have been successfully made between these indices and existing metrics of fairness [START_REF] Bénesse | Fairness seen as global sensitivity analysis[END_REF].

They are used to identify the input dimensions that have the highest influence on the output of a model or a mathematical system. Several statistical estimators to compute these indices are available [START_REF] Saltelli | Variance based sensitivity analysis of model output. design and estimator for the total sensitivity index[END_REF][START_REF] Marrel | Calculations of sobol indices for the gaussian process metamodel[END_REF][START_REF] Janon | Asymptotic normality and efficiency of two sobol index estimators[END_REF][START_REF] Owen | Better estimation of small sobol'sensitivity indices[END_REF]31] and have asymptotic guarantees [START_REF] Janon | Asymptotic normality and efficiency of two sobol index estimators[END_REF][START_REF] Da | Efficient estimation of sensitivity indices[END_REF][START_REF] Tissot | Bias correction for the estimation of sensitivity indices based on random balance designs[END_REF]. We build on this literature by adapting these Sobol indices in the context of black-box models to compute the influence of regions of an image on the output predictions using perturbation masks.

Sobol attribution method

In this work, we formulate the feature attribution problem as quantifying the contribution of a collection of d real-valued features x = (x 1 , ..., x d ) with respect to a model decision. Specifically, we consider a black-box decision function f : X → R k whose internal states and analytical form are unknown (for instance, f can score the probability for the input to belong to a specific class). Our goal is to quantify the importance of each feature to the decision score f (x), not just individually but also collectively. To capture these higher-order interactions, our method consists in estimating the Sobol indices of the features x by randomly perturbating them and evaluating the impact of these perturbations on the prediction of the black-box model (Fig. 1).

Considering variations of f (x) in response to meaningful perturbations of the input x is a natural way to interpret the local behavior of a the decision function f around x. Several methods build on this idea, e.g., by removing one or a group of input features [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF][START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF][START_REF] Ruth | Interpretable explanations of black boxes by meaningful perturbation[END_REF][START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]24] or by back-propagating the gradient to the input space through the model [START_REF] Simonyan | Deep inside convolutional networks: Visualising image classification models and saliency maps[END_REF][START_REF] Sundararajan | Axiomatic attribution for deep networks[END_REF][START_REF] Smilkov | Smoothgrad: removing noise by adding noise[END_REF][START_REF] Ramprasaath R Selvaraju | Grad-cam: Visual explanations from deep networks via gradient-based localization[END_REF]. Most of these methods use the model's internal representations and/or require computing the gradient w.r.t. the input, which makes them unusable in a black-box setting. Moreover, these methods focus on estimating the intrinsic contribution of each feature, neglecting the combinatorial components. Our method applies perturbations directly on the input in order to deal with a black-box scenario, and allows us to estimate higher-order interactions between the features x = (x 1 , ..., x d ) that contribute to f (x).

Random Perturbation

Formally, let us define a probability space (Ω, X , P ) of possible input perturbations and a random vector X = (X 1 , ..., X d ) as a stochastic perturbation of the original input x (see Fig. 1). There are several ways to define random perturbations corresponding to different coverage of the data manifold around x. For instance, we can consider the perturbation mask operator Φ : X × M → X which combines a stochastic mask M = (M 1 , ..., M d ) ∈ M (i.e., an i.i.d sequence of real-valued random variables on [0, 1] d ) with the original input x. This formulation encompasses Inpainting perturbations:

Φ(x, M ) = x ⊙ M + (1 -M )µ with µ ∈ R
a baseline value, and ⊙ the Hadamard product. This consists in linearly varying the pixel intensities towards a baseline intensity such as a pure black with a value of zero [START_REF] Ruth | Interpretable explanations of black boxes by meaningful perturbation[END_REF][START_REF] Tulio Ribeiro | why should i trust you?" explaining the predictions of any classifier[END_REF][START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF][START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]. Similarly, Blurring consists of applying a blur operator with various intensities to certain regions of the image [START_REF] Ruth | Interpretable explanations of black boxes by meaningful perturbation[END_REF]. Different perturbation domains can be considered for other types of data such as textual or tabular data that we discuss further in the experimental section. In the next section, we explain how we adapt the Sobol-based sensitivity analysis using a class of perturbations to explain the predictions of a black-box model.

Sensitivity analysis using Sobol indices

We first briefly review the classical Sobol-Hoeffding decomposition from [START_REF] Hoeffding | A class of statistics with asymptotically normal distribution[END_REF] and introduce the Sobol indices. Let (X 1 , ..., X d ) be independent variables and assume that f belongs to L 2 (X , P ). Moreover we denote the set U = {1, ..., d}, u a subset of U, its complementary ∼u and E(•) the expectation over the perturbation space. The Hoeffding decomposition allows us to express the function f into summands of increasing dimension, denoting f u the partial contribution of variables X u = (X i ) i∈u to the score f (X):

f (X) = f ∅ + d i f i (X i ) + 1⩽i<j⩽d f i,j (X i , X j ) + • • • + f 1,...,d (X 1 , ..., X d ) = u⊆U f u (X u ) (1)
Eq. 1 consists of 2 d terms and is unique under the following orthogonality constraint:

∀(u, v) ⊆ U 2 s.t. u ̸ = v, E(f u (X u )f v (X v )) = 0 (2)
Furthermore, orthogonality yields the characterization

f u (X) = E(f (X)|X u ) -v⊂u f v (X)
and allows us to decompose the model variance as:

Var(f (X)) = d i Var(f i (X i )) + 1⩽i<j⩽d Var(f i,j (X i , X j )) + ... + Var(f 1,...,d (X 1 , ..., X d )) = u⊆U Var(f u (X u )) (3)
Building from Eq. 3, it is natural to characterize the influence of any input subset u as its own variance w.r.t. the total variance. This yields, after normalization by Var(f (X)), the general definition of Sobol indices.

Definition 1 (Sobol indices [27])

The sensitivity index S u which measures the contribution of the variable set X u to the model response f (X) in terms of fluctuation is given by:

S u = Var(f u (X u )) Var(f (X)) = Var(E(f (X)|X u )) -v⊂u Var(E(f (X)|X v )) Var(f (X)) (4)
Sobol indices give a quantification of the importance of any subset of features with respect to the model decision, in the form of a normalized measure of the model output deviation from f (X). Thus, Sobol indices sum to one : u⊆U S u = 1.

For each subset of variables X u , the associated Sobol index S u describes the proportion of the model's output variance explained by this subset. In particular, the first-order Sobol indices S i capture the intrinsic share of total variance explained by a particular variable, without taking into account its interactions. Many attribution methods construct such intrinsic importance estimator. However, the framework of Sobol indices enables us to capture higher-order interactions between features. In this view, we define the Total Sobol indices.

Definition 2 (Total Sobol indices [START_REF] Homma | Importance measures in global sensitivity analysis of nonlinear models[END_REF]) The total Sobol index S Ti which measures the contribution of the variable X i as well as its interactions of any order with any other input variables to the model output variance is given by:

S Ti = u⊆U i∈u S u = 1 - Var X∼i (E Xi (f (X)|X ∼i )) Var(f (X)) = E X∼i (Var Xi (f (X)|X ∼i )) Var(f (X)) (5)
Where E X∼i (Var Xi (f (X)|X ∼i )) is the expected variance that would be left if all variables but X i were to be fixed. S Ti is the sum of the Sobol indices for the all the possible groups of variables where i appears, i.e. first and higher order interactions of variable X i .

Since the total interaction index contains the first order index, it is natural that it is greater than or equal to the first order index. We thus note the property which can easily be deduced: ∀i, 0 ≤ S i ≤ S Ti ≤ 1.

We will now see why these two indices and the difference between them make them relevant for the explainability of a black-box model.

These statistics quantify the intrinsic (first-order indices) and relational (total indices) impact of each variable to the model output. A variable with a low total Sobol index is therefore not important to explain the model decision. Also, a variable has a weak interaction with other variables when S Ti ≈ S i , while it has a strong interaction when the difference between its two indices is high. A strong interaction means that the effect of one variable on the variation of the model output depends on other variables. Thus, using Sobol indices allows to describe fine grained interactions between inputs which leads to the model decision. We next present an efficient method to estimate these indices.

Efficient estimator

As models are becoming more and more complex, the proposed estimator must take into account the computational cost of model evaluation. Many efficient estimators have been proposed in the literature [START_REF] Iooss | Uncertainty management in Simulation-Optimization of Complex Systems: Algorithms and Applications[END_REF]. In this work, we use the Jansen [START_REF] Michiel | Analysis of variance designs for model output[END_REF] estimator which is often considered as one of the most efficient [START_REF] Puy | A comprehensive comparison of total-order estimators for global sensitivity analysis[END_REF]. Jansen is typically used with a Monte Carlo sampling strategy. We improve over Monte Carlo by using a Quasi-Monte Carlo (QMC) sampling strategy which generates low-discrepancy sample sequences allowing a faster and more stable convergence rate [37]. For more information, see appendix D. We will now describe the procedure to implement these estimators.

We start by drawing two independent matrices of size N × d of perturbation masks from a Sobol low discrepancy LP τ sequences. N will be our number of designs and we recall that d is our dimensions (e.g, 121 for 11 by 11 masks). Once the perturbation operator is applied to x with these masks, we obtain two matrices A and B of the same size as the perturbed inputs (i.e., partially masked images). We note A ji and B ji the elements of the matrices such that i = 1, ..., d the number of variables studied and j = 1, ..., N the number of samples in each matrix. We form the new matrix C (i) in the same way as A except for the fact that the column corresponding to the variable i is now replaced by the column of B. We denote

f ∅ = 1 N N j=0 f (A j ) and the empirical variance V = 1 N -1 N j=0 (f (A j ) -f ∅ ) 2 .
The empirical estimators for first ( Ŝi ) and total order ( ŜTi ) can be formulated as:

Ŝi = V -1 2N N j=1 (f (B j ) -f (C (i) j )) 2 V ŜTi = 1 2N N j=1 (f (A j ) -f (C (i) j )) 2 V (6)
Hence, to compute the set of first order and total indices, it is necessary to perform N (d + 2) forwards of the model. We study in section 3.3 how to choose a sufficient number of forwards (N ). To ease understanding and demonstrate that these estimators can be easily implemented, we show in Algorithm 1 a minimal pythonic implementation of the total order estimator that outputs ŜTi indices. The input Y contains the prediction scores of the N * (d + 2) forwards. The scores are ordered following the same QMC sampling ordering of their associated mask. The output STis contains d importance scores, one for each dimension of the mask. In the case of images, we obtain our final explanation map by applying a bilinear upsampling to match the dimensions of the input image.

Signed estimator

Although the proposed Sobol-based attribution method allows us to determine the impact of any variables for a given prediction and thus to identify diagnostic ones, it lacks the ability to highlight the type of contributions made, whether positive or negative. Simple methods such as "Occlusion" typically include this information. Hence, we propose a variant that combines the importance scores of [START_REF] Zhang | Top-down neural attention by excitation backprop[END_REF]). The first and second best results are bolded and underlined. Results are based on PyTorch re-implementations using the TorchRay package. The reported execution time is an average over 100 runs on ResNet50 using an Nvidia Tesla P100 on Google Colab and a batch size of 64. Lower execution time can be reached with higher batch size.

the total Sobol indices with the sign of the occlusion. We compute the difference in score between the prediction on the original input x and a partial version x \i with the variable x i occluded. Intuitively, this provides an estimate of the direction of the variations generated by the variables studied with respect to a reference state.

Ŝ∆ Ti = ŜTi × sign(f (x) -f (x \i )) (7)
4 Experiments

To evaluate the benefits and the reliability of the Sobol attribution method, we performed multiple systematic experiments on vision and natural language models using common explainability metrics.

For our vision experiments, we compared the plausibility of the explanations produced on the Pointing Game [START_REF] Zhang | Top-down neural attention by excitation backprop[END_REF] benchmark. We evaluate the fidelity of our explanations using the Deletion metric for 4 representative models commonly used in explainability studies: ResNet50V2 [START_REF] He | Identity mappings in deep residual networks[END_REF] , VGG16 [START_REF] Simonyan | Very deep convolutional networks for large-scale image recognition[END_REF], EfficientNet [START_REF] Tan | Efficientnet: Rethinking model scaling for convolutional neural networks[END_REF] and MobileNetV2 [START_REF] Sandler | Mobilenetv2: Inverted residuals and linear bottlenecks[END_REF] trained on ILSVRC-2012 [START_REF] Deng | ImageNet: A Large-Scale Hierarchical Image Database[END_REF]. In addition, we also compared the speed of convergence of the proposed estimator with that of the leading approach, RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF], on the same models. For our NLP experiments, we fine-tuned a Bert model and trained a bi-LSTM on the IMDB sentiment analysis dataset [START_REF] Maas | Learning word vectors for sentiment analysis[END_REF] before comparing fidelity scores using word-deletion for representative methods.

Throughout this work, explanations were generated using the Sobol total estimator ŜTi on the target class output. In the supplementary material, we demonstrate the effectiveness of modeling higher-order interactions between image regions by comparing ŜTi against Ŝi which only models the main effects. For the experiments involving images, the masks were generated at a resolution of d ′ = 11 × 11 pixels, then upsampled with a nearest-neighbor interpolation method before being applied with the Inpainting perturbation function. Finally, N was set to 32 which is equivalent to 3, 936 forward passes (see Section 3.3 for details). For Ŝ∆ Ti , an occlusion using the same resolution as the masks was used to sign ŜTi , with zero as baseline. For RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF], we have followed the recommendations of the original paper with 8, 000 forward passes for all models.

Pointing game

Different evaluation methods have been proposed to compare attribution methods and their explanations [START_REF] Samek | Evaluating the visualization of what a deep neural network has learned[END_REF][START_REF] Hooker | A benchmark for interpretability methods in deep neural networks[END_REF][START_REF] Bhatt | Evaluating and aggregating feature-based model explanations[END_REF][START_REF] Fel | Representativity and consistency measures for deep neural network explanations[END_REF]. The first common approach consists in measuring the plausibility of an explanation as the correlation between attribution maps and human-provided semantic annotations. Here, we focused on the Pointing Game used in [START_REF] Zhang | Top-down neural attention by excitation backprop[END_REF][START_REF] Ruth | Interpretable explanations of black boxes by meaningful perturbation[END_REF]24,[START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]. For each attribution method, we compute a contribution score for each pixel of a given class of objects, e.g., bike or car. We then calculated the percentage of times the pixel with the highest score is included in the bounding box surrounding the object of interest. In this benchmark, a good attribution method should point to the most important evidence of the object appearance in accordance with a human user.

In Table 1, a report results for the Pascal VOC [START_REF] Everingham | The pascal visual object classes (voc) challenge[END_REF] and MS COCO [START_REF] Lin | Microsoft coco: Common objects in context[END_REF] datasets using VGG16 [START_REF] Simonyan | Very deep convolutional networks for large-scale image recognition[END_REF] and ResNet50 [START_REF] He | Identity mappings in deep residual networks[END_REF]. In the last column we report the computation times for each method averaged over 100 MS COCO samples for the ResNet50 model. We subdivided explanation methods into two categories: white-box methods which require the use of backpropagation, such as Gradient [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF] and Extremal Perturbation [24], and/or access to the internal states of the model, such as GradCAM [START_REF] Ramprasaath R Selvaraju | Grad-cam: Visual explanations from deep networks via gradient-based localization[END_REF] versus black-box methods such as Occlusion [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF], RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF], or the proposed Sobol method S Ti which only require the final model predictions. The proposed method outperforms RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF] on all of the tested cases, while reducing the number of forward passes by half. Surprisingly, white-box methods do not always lead to higher scores, and indeed Ŝ∆ Ti is the leading method for Pascal VOC / VGG16 and our two estimators Ŝ∆ Ti , ŜTi prevail on COCO / VGG16. Also note that our signed version of the estimator obtains higher scores overall. This might be due to the fact that images from VOC and COCO often feature several types of objects. Thus, the maximum variance in the output is not always induced by the object of interest but can be due to the masking of another object in the image. This result suggests that our signed version Ŝ∆ Ti should be used on multi-label datasets, while ŜTi should be used on multi-class datasets. We indeed confirm this in the next set of experiments on a multi-class dataset.

Fidelity

There is a broad consensus that measuring the plausibility of an explanation alone is insufficient [START_REF] Adebayo | Sanity checks for saliency maps[END_REF][START_REF] Ghorbani | Interpretation of neural networks is fragile[END_REF]. Indeed, if an explanation is used to make a critical decision, users expect an explanation to reflect the true underlying decision process of the model and not just a consensus with humans. Failures to do so could have disastrous consequences. A first major limitation of current evaluation methods based on human-provided groundtruth such as the pointing game is that they do not work when a model prediction is wrong. In this case, an explanation method can be penalized for not pointing to the correct evidence even though explaining prediction errors is a critical use case for explanation methods. Another limitation of these evaluation methods is that they make the implicit assumption that the models should be relying on the same image regions than humans for recognition [START_REF] Ullman | Atoms of recognition in human and computer vision[END_REF][START_REF] Linsley | What are the visual features underlying human versus machine vision[END_REF][START_REF] Linsley | Learning what and where to attend[END_REF], which is likely to be an incorrect assumption. We thus use the fidelity metric as a complementary type of evaluation. This metric assumes that the more faithful an explanation is, the quicker the prediction score should drop when pixels that are considered important are reset to a baseline value (e.g., gray values).

In Table 2, we report results for the Deletion Metric [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF] (or 1 -AOP C [START_REF] Samek | Evaluating the visualization of what a deep neural network has learned[END_REF]) for 4 different pretrained models: ResNet50 [START_REF] He | Identity mappings in deep residual networks[END_REF] , VGG16 [START_REF] Simonyan | Very deep convolutional networks for large-scale image recognition[END_REF], EfficientNet [START_REF] Tan | Efficientnet: Rethinking model scaling for convolutional neural networks[END_REF] and MobileNet [START_REF] Sandler | Mobilenetv2: Inverted residuals and linear bottlenecks[END_REF] on 2,000 images sampled from the ImageNet validation set. TensorFlow [START_REF] Abadi | TensorFlow: Large-scale machine learning on heterogeneous systems[END_REF] and the Keras [58] API were used to run the models. Several baseline values can be used [59], but we chose the standard approach with gray values. We observe that the proposed Sobol ŜT i is the most faithful black-box methods with the lowest deletion scores across all models. Overall ŜTi is able to match the scores of the most faithful white-box method, namely Integrated Gradients [START_REF] Sundararajan | Axiomatic attribution for deep networks[END_REF], and gets the lowest score on ResNet50V2 with 0.121 against 0.123 (lower is better). We also report that our signed version Ŝ∆ Ti is less faithful that the standard Sobol ŜT i . This can be explained by the fact that ImageNet images contains only one object and therefore the main variance area generally coincides with the class to be explained. This confirms our observation on the previous pointing game benchmark that ŜT i should be preferred in a multi-class setup and Ŝ∆ Ti in a multi-label setup. Another metric called Insertion has been proposed by the authors of RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]. Instead of deleting pixels in the original image like with Deletion, Insertion consists in adding pixels on a baseline image, e.g. one gray image, starting with pixels that are associated with the highest importance scores for a given explanation method. An issue with Insertion is that the score computed along the insertion path is highly influenced by the first inserted pixels which contributes disproportionately. A good score on this metric therefore requires exploring a region very far from the original image and closer to the baseline. For this reason, we rather preferred to focus our study on Deletion than Insertion. However, we also report results on Insertion in the supplementary material using the same hyperparameters as used in Deletion.

Efficiency

The black-box methods presented so far compete with white-box methods that do not require access to the internal representation of the model at the cost of a large number of forward passes, e.g., around 8, 000 for RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]. This weakness leads us to take a more serious look at the performance of the proposed method. It seems critical for the deployment of black-box methods to lower the amount of compute required to produce correct explanations. We describe an experiment to show that beyond producing higher quality explanations, our estimator converges quickly. We first generate an explanation with a high number of forward passes that is large enough to reach convergence, e.g. 10, 000 forward passes. Then we compare this explanation that "converged" to other explanations obtained with lower numbers of forward passes. It allows us to measure the stability and rate of convergence towards this explanation that "converged", but more practically to find the proper trade-off between the amount of compute and the quality of explanations. This procedure requires defining a measure of similarity between two explanations. Since the proper interpretation method is to rank the features most sensitive to the model's decision, it seems natural to consider the Spearman rank correlation [START_REF] Spearman | The proof and measurement of association between two things[END_REF] to compare the similarity between explanations. Prior work has provided theoretical and experimental arguments in line with this choice [START_REF] Ghorbani | Interpretation of neural networks is fragile[END_REF][START_REF] Adebayo | Sanity checks for saliency maps[END_REF][START_REF] Tomsett | Sanity checks for saliency metrics[END_REF][START_REF] Fel | Representativity and consistency measures for deep neural network explanations[END_REF].

Figure 2: Comparison between the rate and stability of convergence of Sobol ŜTi and RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]. A high Spearman correlation rank corresponds to producing an explanation that is similar to the explanation that "converged", i.e. with 10, 000 forward passes. We report mean and variance computed over 500 images from the ImageNet dataset using EfficientNet.

In Fig. 2, we compare the proposed Sobol attribution method ŜTi against RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF], which is the current state-of-the-art for black-box methods. We use their respective gold explanation generated after 10, 000 forwards. To allow a fair comparison, both methods use masks generated in 7 × 7 dimensions (as recommended by RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]). We report the average results and variance over 500 images from the ImageNet validation set using Efficient-Net, a convolutional neural network optimized for fast forward computing times. We observe that our method exhibit higher convergence rate by getting higher Spearman's rank correlation of 0.8 after only 1,000 forwards against 0.65 for RISE, and consistently obtain higher scores until reaching 0.97 with 7,000 forwards against 0.73 for RISE. Additionally, we observe that Sobol has a more stable convergence by getting an overall lower variance than RISE. This implies that the number of forward passes used in Sobol can be greatly reduced to accommodate computational resources constraints compared to RISE. Indeed, RISE pro- 3: Word deletion scores, obtained on 1,000 sentences. Delete up to 20 words per sentence accordingly to their relevance and track the impact on the classification performance. Lower is better. The first and second best results are bolded and underlined. poses to use 8, 000 forwards, but our method is faster and reaches better results with half the number of passes. We report similar results for other neural network architectures in the supplementary material D. Finally, we perform an ablation study of Sobol to show the impact of lowering the number of forwards on the Deletion benchmark. We report competitive scores with 16 times fewer number of forwards than RISE by reaching 0.151 in Deletion score with 492 forwards. For reference, Sobol was reaching state-of-the-art results of 0.121 with 3,936 forwards. We report additional scores in the supplementary materials S2.

Word deletion

For NLP, black-box methods require the use of perturbations that can be applied to the space of characters, words or sentences. For instance, a common perturbation consists in simply removing one word of the sentence to be explained. Therefore, the Inpainting perturbation that we used with Sobol to reduce the intensity of pixels in a continuous manner cannot be directly applied in this context. Instead, we adapt it by binarizing the masks such that Φ(x, M) = x ⊙ ⌈M -0.5⌉, i.e., if the value is greater than 0.5 the word is kept, otherwise it is removed. We then verify that our Sobol method can be used to identify words that support a specific decision of a text classifier. Inspired by previous work [START_REF] Arras | what is relevant in a text document?": An interpretable machine learning approach[END_REF][START_REF] Arras | Explaining recurrent neural network predictions in sentiment analysis[END_REF][START_REF] Bach | On pixel-wise explanations for non-linear classifier decisions by layer-wise relevance propagation[END_REF], we introduce an experimental benchmark on the IMDB Review dataset [START_REF] Maas | Learning word vectors for sentiment analysis[END_REF]. It is similar to the previous Deletion benchmark for images in that it focuses on assessing the faithfulness of the explanation and does not require specific human annotations. More precisely, we first trained two models: a bi-LSTM from scratch and a BERT model fine-tuned for the task (see appendix H for details). We generate explanations on 1, 000 sentences from the validation dataset. An explanation associates an importance score to each word. Similar to Deletion, we use these scores to successively remove the most relevant word of the sentence and measure the corresponding drop in the prediction score of the model.

In Table 3, we report results for explanation methods that are commonly used in NLP. Both our two Sobol methods have a better faithfulness than all the tested gradient-based white-box methods including Saliency, Grad-Input, SmoothGrad, and Integr-Grad. For instance, Sobol Ŝ∆ Ti even reaches a low Word deletion scores of 0.461 (lower is better) for the bi-LSTM compared to 0.529 for the best white-box approach. However, the proposed methods are only the second and third most faithful methods. Occlusion (often called Omit-1 in NLP) reaches the lowest score of 0.440 for LSTM and 0.531 for BERT, against 0.461 and 0.598 for Sobol Ŝ∆ Ti . This is due to the fact that the default distribution of masks used in Sobol is centered around 0.5, which corresponds to removing on average half of the words as opposed to a single word for Occlusion. In IMDB, this causes the frequent removal of critical words that support the model decision. Indeed, we report comparable results with Occlusion (e.g., 0.527 for BERT) for a lower threshold of 0.05 to remove far fewer words. Since Sobol can model higher-order interactions between words, we believe that it could successfully be used for NLP tasks that are more complex than sentiment classification on IMDB.

Conclusion

We have presented a novel explainability method to study and understand the predictions of a blackbox model. This approach is grounded within the theoretical framework of sensitivity analysis using Sobol indices. A non-trivial contribution of this work was to make the approach tractable and efficient for high-dimensional data such as images. For this purpose, we have introduced a method using perturbation masks coupled with efficient estimators from the sensitivity analysis literature. One additional benefit of the approach is that it provides a way to study the importance of not just the main effects of input variables but also higher-order interactions between them. We showed that our method can be efficiently used to explain the decisions of image classifiers. It reaches performance on par with or better than the current best black-box methods while being twice as fast. It even reaches comparable results to the best white-box methods without requiring access to internal states. We also showed that our method could be applied to language models and reported initial competitive results. It is our hope that this work will guide further efforts in the search for more general and efficient explainability methods for black-box models and that further links will be made with the field of sensitivity analysis.

A Broader Impact

This work is part of the burgeoning field of explainable AI which has a potential positive impact on society including transportation, science and medicine. There is now broad consensus that many (if not all) the AI systems deployed in real-world settings exhibit significant biases including gender and racial biases. Understanding how these systems arrive at their decisions is a necessary first step before these biases can be corrected. We described a method that provides explanations for predictions made by black-box models that are as faithful as possible (in the sense that it reflects the true inner-working of the model). We thus feel it is important for any explainability method to be built on a rigorous theoretical framework. Here, we borrowed methods from Sensitivity Analysis which has been extensively used to evaluate critical systems and that we showed compare favorably to other approaches on fidelity benchmarks. Nevertheless, progress in explicability should not result in a blind trust in the explanations of the models, which should always be used in the knowledge of their associated flaws. Finally, the attribution methods presented in this work are sensitive to adversarial attacks that can be used to hide the behavior of a model [START_REF] Slack | Fooling lime and shap: Adversarial attacks on post hoc explanation methods[END_REF][START_REF] Ghorbani | Interpretation of neural networks is fragile[END_REF] which is still an open problem in the frame of attribution methods.

B Qualitative comparison

Regarding the visual consistency of our method, Fig. S1 shows a side-by-side comparison between our method and the other methods tested in the Fidelity benchmark. The images are not hand-picked but are the first images from the ImageNet validation set. To allow better visualization, the gradient-based methods were 2 percentile clipped. The only black box methods are Occlusion, Rise and S Ti . We found that S Ti consistently provides a sparser map than RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF] while being equally consistent. On the other hand, we found that in general, the gradient-based method provides the sharpest map, but some are prone to failure (fourth row in the Fig. S1), which is a known problem [START_REF] Adebayo | Sanity checks for saliency maps[END_REF].

C Effectiveness of modeling higher-order interactions

We introduced two approaches, Sobol ( ŜTi ) and Sobol signed ( Ŝ∆ Ti ), that combine effects of first-and all higher-orders interactions between image regions. For comparison, Occlusion [START_REF] Matthew | Visualizing and understanding convolutional networks[END_REF] only accounts for the first order as it removes one region at a time, while RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF] accounts for higher-order by removing around 50% of regions at a time. As seen in Table S1, RISE already surpasses Occlusion on ImageNet in term of Deletion scores, which may indicate that using higher-order information is effective.

To further demonstrates that it is critical to model the higher orders, we evaluate Sobol first-order (S i ) on our Deletion benchmark. We report that Sobol (S Ti ) reaches lower deletions scores (lower is better) than Sobol first-order (S i ) with 0.121 against 0.170 respectively on ResNet50v2, and similar differences on VGG16, EfficientNet and MobileNetV2.

Method

ResNet50V2 VGG16 EfficientNet MobileNetV2

D Efficiency of Sobol estimator

Regarding the estimation of the Sobol indices, we notice that we can derive a 'brute-force' (or often called double-loop method [START_REF] Sobol | Global sensitivity indices for nonlinear mathematical models and their monte carlo estimates[END_REF]) estimator from the definition 1: However, one the main problems with this estimator is the cost of computation, which can be too heavy, especially with complex models such as large neural networks. This difficulty is particularly true for the calculation of total Sobol indices.

S i = [ f (X)dX ∼i ] 2 dX i -( f (X)dX) 2 f (X) 2 dX -( f (X)dX) 2 (8)
Since the perturbation masks are used to approximate these integrals, an efficient way to proceed is to generate those masks from a low discrepancy sequences, also called Quasi-random sequences. These sequences allow to efficiently integrate functions on the hypercube [0, 1] d . In fact, they have a faster convergence rate compared to ordinary Monte Carlo methods [37] (with f sufficiently regular). This difference being due to the use of a deterministic sequence that covers [0, 1] d more uniformly. In our experiments we used Sobol sequences [START_REF] Il | On the distribution of points in a cube and the approximate evaluation of integrals[END_REF], we refer the readers to [START_REF] Leobacher | Introduction to quasi-Monte Carlo integration and applications[END_REF] for more informations. The efficiency of the estimator and the sampling is shown on Figures S2,S3 and S4 where our estimator consistently converges faster than RISE [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF].

We also perform an ablation study of the number of forwards on the Deletion benchmark. In Table S2, we show that competitive scores can be obtained with lower number of forwards such as 0.151 in Deletion score with 492 forwards instead of 0.121 with 3936 forwards which is our default number of forwards.

E Explanation methods

In the following section, the formulation of the different methods used in the experiment is given. We define f (x) the logit score (before softmax) for the class of interest. An explanation method provides an attribution score for each input variables. Each value then corresponds to the importance of this feature for the model results.

Saliency is a visualization techniques based on the gradient of a class score relative to the input, indicating in an infinitesimal neighborhood, which pixels must be modified to most affect the score of the class of interest. [START_REF] Shrikumar | Not just a black box: Learning important features through propagating activation differences[END_REF] is based on the gradient of a class score relative to the input, element-wise with the input, it was introduced to improve the sharpness of the attribution maps. A theoretical analysis conducted by [START_REF] Ancona | Towards better understanding of gradientbased attribution methods for deep neural networks[END_REF] showed that Gradient ⊙ Input is equivalent to ϵ-LRP and DeepLIFT [START_REF] Shrikumar | Learning important features through propagating activation differences[END_REF] methods under certain conditions: using a baseline of zero, and with all biases to zero.

g SA (x) = ||∇ x f (x)|| Gradient ⊙ Input [
g GI (x) = x ⊙ ||∇ x f (x)||
Integrated Gradients consists of summing the gradient values along the path from a baseline state to the current value. The baseline is defined by the user and often chosen to be zero. This integral can be approximated with a set of m points at regular intervals between the baseline and the point of interest. In order to approximate from a finite number of steps, we use a Trapezoidal rule and not a left-Riemann summation, which allows for more accurate results and improved performance (see [START_REF] Sotoudeh | Computing linear restrictions of neural networks[END_REF] for a comparison). The final result depends on both the choice of the baseline x 0 and the number of points to estimate the integral. In the context of these experiments, we use zero as the baseline and m = 80.

g IG (x) = (x -x 0 ) 1 0 ∇ x f (x 0 + α(x -x 0 )))dα
SmoothGrad is also a gradient-based explanation method, which, as the name suggests, averages the gradient at several points corresponding to small perturbations (drawn i.i.d from a normal distribution of standard deviation σ) around the point of interest. The smoothing effect induced by the average help reducing the visual noise, and hence improve the explanations. In practice, Smoothgrad is obtained by averaging after sampling m points. In the context of these experiments, we took m = 80 and σ = 0.2 as suggested in the original paper.

g SG (x) = E ε∼N (0,Iσ) (∇ x f (x + ε))
Grad-CAM can be used on Convolutional Neural Network (CNN), it uses the gradient and the feature maps A k of the last convolution layer. More precisely, to obtain the localization map for a class, we need to compute the weights α k c associated to each of the feature map activation A k , with k the number of filters and Z the number of features in each feature map, with

α c k = 1 Z i j ∂f (x) ∂A k ij and g GC = max(0, k α c k A k )
Notice that the size of the explanation depends on the size (width, height) of the last feature map, a bilinear interpolation is performed in order to find the same dimensions as the input.

Occlusion is a sensitivity method that sweep a patch that occludes pixels over the images, and use the variations of the model prediction to deduce critical areas. In the context of these experiments, we took a patch size and a patch stride of 20.

g OC i = f (x) -f (x [xi=0] )
RISE is a black-box method that consist of probing the model with randomly masked versions of the input image to deduce the importance of each pixel using the corresponding outputs. The masks m ∼ M are generated randomly in a subspace of the input space, then upsampled with a bilinear interpolation (once upsampled the masks are no longer binary).

As recommended in the original paper, we used N = 8, 000 and E(M) = 0.5 for all the experiments.

g RI (x) = 1

E(M)N N i=0 f (x ⊙ m i )m i

F Fidelity with Insertion

Insertion is an evaluation procedure introduced in [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF] at the same time as Deletion. Deletion assumes that the more faithful an explanation is, the faster the prediction score should drop when pixels that are considered important are reset to a baseline value (e.g., gray values). Insertion is the opposite of Deletion in that it assumes that the prediction score should go up faster for the most faithful explanations when pixels from the original image that are considered important are added to a baseline image (e.g., gray image). Metrics similar to Insertion are less common than Deletion in the literature that is why we focus on Deletion in the main paper. Moreover, just like Deletion, the Insertion score is largely influenced by the first steps [START_REF] Vitali Petsiuk | Rise: Randomized input sampling for explanation of black-box models[END_REF]: the first pixels removed from the original image for deletion, and the first pixels inserted in the insertion case. Maximizing Insertion means exploring in a space close to the baseline, while maximizing Deletion means exploring a space around the original image. We suggest that for this reason, the Insertion score is not as relevant as Deletion.

Method

G Sanity check

We followed the procedure used by [START_REF] Adebayo | Sanity checks for saliency maps[END_REF], namely the progressive reset of the network weights. We used an Inception V3 [START_REF] Szegedy | Rethinking the inception architecture for computer vision[END_REF] model, each images shows the S Ti explanation for the network in which the upper layers (from logits) were reset. Fig. S5 shows that our method passes the sanity check: it turns out to be sensitive to the modification of the model weights.

H Word Deletion

For the bidirectional LSTM [START_REF] Hochreiter | Long short-term memory[END_REF], the word embedding is in R 300 and is initialized with the pre-trained GloVe embedding [START_REF] Pennington | Glove: Global vectors for word representation[END_REF]. The layer has a hidden size of 64 (bidirectional architectures: 32 dimensions per direction). The resulting document representation is projected to 64 dimensions then 2 dimensions using fully connected layers, followed by a softmax and reached an accuracy of 89% on the test dataset.

For the BERT-based models, we use the Transformers library from HuggingFace [START_REF] Wolf | Transformers: State-of-the-art natural language processing[END_REF] and more specifically the bert-base-uncased model. The final layer is tuned to minimize cross-entropy, with Adam optimizer [START_REF] Diederik | Adam: A method for stochastic optimization[END_REF] and initial learning rate of 1e -3 to reach an accuracy of 92% on the test dataset.

The observation that local perturbation: with the majority of words present, gets a better score is verified by playing on the threshold of the perturbation function. By decreasing the percentage of words removed on average we observe that a better deletion score is obtained.

ŜTi ∆ 50% Ŝ∆ Ti 90% ŜTi ∆ 95% Occlusion Deletion 0.598 0.553 0.527 0.531 Table S4: Word deletion scores on the Bert based model when the perturbation threshold is modified to control the average presence of words in each generated perturbated input. Lower is better.

Algorithm 1 :

 1 def total_order_estimator(Y, N=32, d=11 * 11): fA, fB = Y[:N], Y[N:N * 2] fC = [Y[N * 2+N * i:N * 2+N * (i+1)] for i in range(d)] f0 = mean(fA) V = sum([(val -f0) ** 2 for val in fA]) / (len(fA) -1) STis = [sum((fA -fC[i]) ** 2) / (2 * N) / V for i in range(d)] return STis Pythonic implementation of the Total Order indices ( ŜTi ) calculation.

Figure S1 :

 S1 Figure S1: Qualitative comparison with other explainability methods. The heatmaps are normalized and clipped at 2 percentile for Saliency, Guided-Backprop, DeconvNet, Smoothgrad and Integrated-Gradients. Explanations are generated from a ResNet50V2.

Figure S2 :

 S2 Figure S2: Spearman rank correlation of explanations as a function of the number of forwards, compared to an explanation generated with 10, 000 forwards. The model used is a ResNet50V2.

Figure S3 :

 S3 Figure S3: Spearman rank correlation of explanations as a function of the number of forwards, compared to an explanation generated with 1, 0000 forwards. The model used is a VGG16.

Figure S4 :

 S4 Figure S4: Spearman rank correlation of explanations as a function of the number of forwards, compared to an explanation generated with 1, 0000 forwards. The model used is a MobileNetV2.

Figure S5 :

 S5 Figure S5: Sanity Check model weights are progressively reinitialized from Mixed 6 to Mixed 1 in InceptionV3 [70], demonstrating our method's sensitivity to model weights.

Table 1 :

 1 Pointing game. Accuracy over the full test set and a subset of difficult images (defined in

			Pointing Game Deletion Time (s)
		Baseline Center	27.8	0.235	-
		Saliency [1]	37.7	0.174	0.031
	White box	Guided-Backprop. [3] MWP [38] cMWP [38] Integ.-Grad. [45] GradCAM [5]	39.1 39.8 49.7 49.7 54.2	0.142 --0.123 0.141	0.051 0.039 0.040 0.040 0.015
		ExtremalPerturbation [24]	51.5	-	26.48
	Black box	Occlusion RISE [9] Sobol ( ŜTi ) (ours)	35.6 50.8 54.6	0.350 0.127 0.121	1.134 13.19 6.381

Table 2 :

 2 Deletion scores obtained on 2,000 ImageNet validation set images. Lower is better. Random consists in removing pixels at each step at random. The first and second best results are bolded and underlined.

		Method	ResNet50V2 VGG16 EfficientNet MobileNetV2
		Baseline Random (ours)	0.235	0.168	0.124	0.137
		Saliency [1]	0.174	0.134	0.105	0.125
	White box	Guided-Backprop. [3] DeconvNet [2] Grad.-Input [45] Integ.-Grad. [8] SmoothGrad [34]	0.142 0.159 0.140 0.123 0.130	0.138 0.146 0.096 0.095 0.106	0.105 0.105 0.093 0.091 0.094	0.102 0.111 0.103 0.093 0.098
		GradCAM [5]	0.141	0.118	0.130	0.122
	Black box	Occlusion [2] RISE [9] Sobol ( ŜTi ) (ours) Sobol signed ( Ŝ∆ Ti ) (ours)	0.350 0.127 0.121 0.145	0.357 0.121 0.109 0.114	0.252 0.119 0.104 0.147	0.357 0.114 0.107 0.141

Table S1 :

 S1 Deletion scores obtained on 2,000 ImageNet validation set images. Lower is better.

	Sobol first-order ( Ŝi )	0.170	0.147	0.129	0.143
	Sobol ( ŜTi )	0.121	0.109	0.104	0.107

Table S2 :

 S2 Deletion scores averaged over 2,000 images of ImageNet validation set using ResNet50V2 and Sobol ( ŜTi ). Lower is better.

	Number of samples Deletion scores
	492	0.151
	984	0.140
	1476	0.132
	1968	0.123
	2460	0.121
	2952	0.120
	3444	0.120
	3936	0.121

Table S3 :

 S3 Insertion scores, obtained on 2,000 images from ImageNet validation set. Higher is better. Random consists in inserting randomly among the pixels remaining at each step. The first and second best results are bolded and underlined.

			ResNet50V2 VGG16 EfficientNet MobileNetV2
		Random Baseline (ours)	0.233	0.166	0.115	0.138
		Saliency [1]	0.363	0.303	0.229	0.253
	White box	Guided-Backprop. [3] DeconvNet [2] Grad.-Input [45] Integ.-Grad. [8] SmoothGrad [34]	0.377 0.307 0.194 0.264 0.445	0.242 0.221 0.219 0.237 0.374	0.229 0.229 0.098 0.143 0.299	0.361 0.166 0.126 0.166 0.307
		GradCAM [5]	0.524	0.438	0.393	0.419
	Black box	Occlusion [2] RISE [9] Sobol ( ŜTi ) (ours) Sobol signed ( Ŝ∆ Ti ) (ours)	0.154 0.546 0.370 0.258	0.115 0.484 0.313 0.290	0.152 0.439 0.309 0.204	0.135 0.443 0.331 0.211

* https://www.deel.ai/

Acknowledgments

This work was conducted as part the DEEL * project. It was funded by the Artificial and Natural Intelligence Toulouse Institute (ANITI) grant #ANR19-PI3A-0004. MC was funded by ANR grant VISADEEP (ANR-20-CHIA-0022). TS was funded by ONR (N00014-19-1-2029) and NSF (IIS-1912280). The computing hardware was supported in part by NIH Office of the Director grant #S10OD025181 via the Center for Computation and Visualization.


domain: spi phys

text:
B Fragge

J Sokoloff

O Pascal

O Rouzaud

M Orain

IGNITION OF A LIQUID FUEL BY FOCUSED MICROWAVES

This paper describes the ignition of an ethanol droplet stream via a plasma created in ambient air. For this experiment we use a moderate microwave power of a few hundred watts, applied to a resonant cavity dedicated to this purpose. The first results are presented. The goal is to determine the ignition limits in terms of power and application time, once the plasma is created. This information is the basis for future works including time reversal. The experiments show that with our cavity we are capable of creating a plasma and ignite ethanol droplets. A first estimation of the minimal power necessary to maintain ethanol ignition at an optimal gap of 1.1 mm in the cavity is around 7.3 W. This paper presents an extract of the works that will be published in the European Physics Journal -Applied Physics under the title 'A versatile set-up to study plasma / microwave sources for liquid fuel ignition'.

I.

INTRODUCTION

In order to reach the goal of lower emissions and fuel consumption, there are new, stricter restrictions on the certification of new combustion chambers for aerobic motors, which are difficult to match with the current ignition system, the spark plug [START_REF] Aggarwal | A review of spray ignition phenomena: present status and future research[END_REF]. Besides these restrictions, there are other difficulties with the spark plug: the unknown power introduced into the mixture, energy losses at the wall and most important, the difficulties of ignition at high altitude (low pressure and temperature). These are the main reasons for the demand of a new ignition system and the research activity in this field. This work presents the study of a new ignition system based on microwaves [START_REF] Stockman | Measurements of combustion properties in a microwave enhanced flame[END_REF][START_REF] Ikeda | Microwave enhanced ignition process for fuel mixture at elevated pressure of 1MPa[END_REF] while using time reversal. As shown by Mazières et al. [START_REF] Mazières | Plasma generation using time reversal of microwaves[END_REF], the creation and control of a plasma via time reversal is possible. In section II, we present the preliminary set-up and the tools used for the first experiments. We describe the results obtained so far in section III, before we end with the conclusion and perspectives in section IV.

II. PRELIMINARY WORKS

First of all, the objective for this preliminary work is to design a device that allows to concentrate the electrical field as much as possible. This optimises the chances to create a plasma in air at ambient pressure with only a few hundred watts and to ignite the droplets. To reach this objective, we decided to combine, on one hand, the effect of over-tension in a resonant cavity and, on the other hand, the effect created by initiators placed in the cavity. The initiators are a simple tool to focus the electrical field in between them and to amplify the electric field density in their gap.

The cavity has been designed to resonate around the frequency of f0 = 2.45 GHz. The microwaves are injected with a precision of 100 kHz on a frequency band going from 2.43 GHz to 2.47 GHz. The maximal power of our solidstate Sairem microwave generator Model GMS200W is 200 W. The resonant frequency and the quality factor Q have been determined for each type of initiator with a Vector-Network-Analyser (VNA).

A. Electromagnetic Simulations

We used the program HFSS from ANSYS to design the cavity. This program solves the Maxwell equations with the finite element method. The mesh and its refinement are done automatically until they match the convergence criteria and the Driven Modal solver was used. We simulate the cavity as a box filled with air, including the coaxial connector and the initiators as solid material inside the cavity. The microwaves are injected via a wave port at the connector.

B. The cavity

The cavity has been designed with several degrees of freedom for an easy matching for the different initiators. Firstly, it is possible to change the length of the cavity due to the moveable side plates at its extremities. Secondly, it is possible to regulate the length of penetration of the coaxial connector into the cavity, with which we inject the microwaves. And lastly, the flanges are interchangeable, which allows us to inject the droplet stream along the z-axis, parallel to the initiators or along the y-axis, perpendicular to the initiators (see fig. 1). During the experiments, four types of initiators have been tested: cylinders with a flat head and a spherical head, truncated cones and tips (see fig. 2). The first three are made of aluminium and the tips are made of stainless steel. The gap between the cones and the tips is adjustable between 0 and 5 mm for the aluminium cones and between 0 and ~5.8 mm for the stainless steel tips, respectively. The cylindrical initiators permit a fuel injection along their axis of symmetry as well, conversely to the other two injectors. Table I. shows the possible directions of injection and the cases tested for each initiator, as well as the plasma(s) formed. The empty cavity has been analysed with the VNA at the beginning of the experiments. We measured the resonance frequency f0 and calculated the quality factor Q from the resonance diagram as follows:

𝑄 = ∆ 𝑓(-3 𝑑𝐵) 𝑓 0 (1)
It quantifies the ratio between the energy stored in the cavity and the energy lost through the cavity walls. When designing a cavity, we strive for a high Q to make sure that most of the energy entering the cavity is stored in it and not lost. In table II one can see the highest quality factors possible for each initiator. Our quality factors are 3 times to 6 times higher than for comparable cavities for microwave ignition found in the literature [START_REF] Linkenheil | A novel spark-plug for improved ignition in engines with gasoline direct injection (GDI)[END_REF][START_REF] Pertl | Electromagnetic design of a novel microwave internal combustion engine ignition source, the quarter wave coaxial cavity igniter[END_REF]

C. The monodisperse Injector

The fuel is injected into the cavity by a monodisperse droplet generator (MDG) that creates a monodisperse droplet stream [START_REF] Berglund | Generation of monodisperse aerosol standards[END_REF] in the upward direction. By changing the size of the hole D of the diaphragm at the exit of the injector and playing with the frequency fpiezo that excites the piezoelectric ceramic, we can vary the size of the droplets from 100 m to 300 m. We can determine the droplet diameter according to these parameters with the following equation [START_REF] Berglund | Generation of monodisperse aerosol standards[END_REF]:

𝐷 𝑑 = ( 3𝑣𝐷 2 2𝑓 𝑝𝑖𝑒𝑧𝑜 ) 1 3 (2)
with v the velocity of the droplets. During our experiments, the excitation frequency is held constant at fpiezo = 16 kHz and the velocity is estimated as v = 10 m/s.

Measurements have shown that the calculated value of the droplet diameter is around 5 % lower than the measured one. For the results presented here we used a diaphragm with a hole of D = 150 m and we measured a droplet diameter of Dd = 289 m. These measurements have been conducted with a shadowgraphy set-up and a telecentric lens.

The created droplet stream consists of droplets of a quasiconstant size and regularly spaced from one another, see Fig 3 . The shape and regularity of the droplet stream is controlled via a simple shadowgraphy set-up. The experiments presented here have been done with ethanol. Kerosene will be tested subsequently.

III. RESULTS

The experiments show the possibility of creating a plasma in the cavity filled with air at atmospheric ambient conditions with three out of the four initiators tested, the initiators with a flat head do not work.

However, the creation of plasma is not systematic for a power starting at 100 W. For the cylindrical initiators with a spherical head, the plasma is created on the sharp edge of the hole in the middle and only at one initiator at a time. The plasma is slightly off centre.

Using the stainless steel tips, we can create a plasma arc between the tips.

With the aluminium cones we can create a plasma on one cone, on both cones separately and even an arc between the two cones, depending on the injected power and the gap between the initiators. If we take a look at the reflected power through a coupler and an oscilloscope, we can capture the signature of an instantaneous plasma creation. Figure 3 shows the envelope of the reflected signal with time at 2.45 GHz.

At the moment t1, we start to inject the power, which increases up to its maximum of 200 W at t2. The energy entering the cavity is heating up the walls and causes a dilatation which leads to a decrease of the reflected power. It decreases until the moment t3, where, in one instant, the cavity is matching very well, the reflected power is very low, and most of the energy send by the generator is entering the cavity: a plasma is created. The existence of the plasma leads to a mismatching of the cavity and therefore an increase of the reflected power which remains constant while the plasma is sustained (from the moment t3 on). a. When we inject the droplet stream first and afterwards the microwaves we can observe an immediate plasma creation on both tips/cones without a connexion in between and no combustion. The plasma is deforming until an explosive ignition takes place due to the accumulation of ethanol vapours in the cavity. The flame exits the cavity and propagates to the liquid collector above the cavity.

b. When doing the experiment the other way round, injecting the microwaves first, we create an arc between the tips/cones. When we inject the droplet stream, the ethanol starts burning as soon as it reaches the plasma zone.

Protocol b has been proven to be more secure and more repeatable, and has been used for the following results.

Figure 5 shows the plasma arc between the aluminium cones (a) and the stainless steel tips (d) and the ignition, once ethanol has been injected (b), (e), respectively. We can observe an envelope flame around the droplet stream. The vapour is burning, but the liquid is not completely evaporated since droplets exit the cavity. During the experiments with the aluminium cones, while the microwaves and the droplet stream coexist, we can observe two luminous spheres, one on each cone. As soon as we stop the ethanol stream, the plasma arc between the cones reappears.

With MARCONI power-meters, we continuously measured the incoming and reflected powers Pinc, real and Prefl, respectively, and compared these measurements to the video recorded at the same time. The resulting signature from measurements (see fig. 6) shows the important steps of the experiment, which can be illustrated by the recorded images in fig. 7. The diagram shows the reflection coefficient S11 versus time. The reflection coefficient can be deduced from the power measurements and is defined as follows:

𝑆11 = 10 * log ( 𝑃 𝑟𝑒𝑓𝑙 𝑃 𝑖𝑛𝑐,𝑟𝑒𝑎𝑙 ) (3)
The behaviour of the S11 shows us, how the cavity reacts to changes in the medium inside the cavity. The presence of the ethanol stream modifies the adaptation of the cavity, and in consequence, the reflected power and the S11.

The curve starts with the arc discharge between the tips created, see fig. 7 step 0. We can identify the injection of ethanol by the rapid decrease of the S11, step 1 figures 6 and 7. Once the ignition phase is over, the S11 stays on a quasiconstant level from step 2 on, see figures 6 and 7. Step 0

Step 1

Step 2

Step 3

Step 4 see ethanol burning from time to time, but there is no envelop flame around the droplet train anymore, see figures 6 and 7 step 4. At step 5 (see fig. 6) we can see another decrease in S11, which is due to the liquid jet that is now too weak to cross the cavity and falls back into it. When the jet finally stops and all ethanol is evacuated from the cavity, the S11 increases back on the same level as before, with only the plasma, see step 6 (fig. 6).

We conducted a study to find the minimum power for ethanol ignition depending on gap size and initiator type. For these tests, we created a plasma at 200 W. Afterwards the power was reduced and then the droplet stream injected. If we take into account the losses in the cables, we can determine the power Pinc,real, which arrives at the cavity input. This power is lower than the power Pincoming displayed by the generator.

Table III shows some examples of the experimental conditions and power-meter measurements. The ignition of ethanol with the stainless steel tips at the optimal gap of ~1.1 mm was possible down to Pincoming = 16 W, or a power of Pcavity = 7.3 W that enters the cavity, according to: Pcavity = Pinc, real -Prefl [START_REF] Mazières | Plasma generation using time reversal of microwaves[END_REF] with Prefl the reflected power at the cavity entry. Figure 8 shows the dependence of the minimum power Pcavity on the gap size between the different initiators. It also shows the influence of the initiator material.

If we take the data set of the stainless steel initiators in fig. 8, we can see the tendency of an optimal gap size (minimum ignition energy) around the gap = 1.1 mm. It has been shown by other researchers (e.g. [START_REF] Ballal | Ignition and flame quenching of flowing heterogeneous fuel-air mixtures[END_REF]) that the optimal gap size is called quenching distance dq. At the gap size dq, the heat delivered to the kernel volume for ignition just exceeds the heat losses to the surrounding medium. For gaps smaller than dq the heat losses increase and to overcome them for successful ignition we need more energy, ergo the minimum ignition energy increases. For gaps larger than dq, the kernel volume increases and therefore more energy is needed to heat up the whole volume to ignition temperature. However, if we look at the data for the aluminium cones, we do not observe this tendency. The values of the minimum ignition energy for smaller gaps (≤1.3mm) are all around Pcavity ≈ 10 W. This observation can be due to the geometry of the aluminium cones. As we can see in fig. 9, the initiators are not perfectly truncated cones. As they are screwed into the construction more or less far to get the different gap sizes, it cannot be guaranteed that they face each other in the exact same way for all of the experiments. This little change in geometry can have an important influence on the focused field and as a consequence, on the minimum ignition energy. If we compare now the two data sets, we observe the large difference between the minimum ignition energies at larger gaps. With the stainless steel tips, we need almost two times more energy to ignite the ethanol droplets than with the aluminium cones. This difference can be due to the difference in material and shape of the initiators. On one hand, we have a higher electric conductivity and a larger amount of energy stored in the cavity (higher Q) for the aluminium cones. On the other hand, the presence of sharp edges plays an important role for the intensity of the electric field. Even if the stainless steel tips are sharpened, the edges of the truncated aluminum cones also lead to an increase in electric field density and therefore allow the lower Pcavity needed for ignition.

To determine in which way and to what extend these properties play their role in the presented results, a more exhaustive study is necessary, which is not part of this work.

The diagram in fig. 8 shows the error bars regarding the power measurements as well. We can see that the uncertainties are very low. With our power-meters we can measure the incident and reflected power with a very good precision.

IV. CONCLUSION/PERSPECTIVES

The experiments conducted have shown that we are capable of creating a plasma in air at ambient conditions (~20 °C and 1 bar) with focused microwaves at a reasonable power input and that this plasma is energetic enough to ignite ethanol.

We showed that with the stainless steel tips we could reproduce the relation between the minimum ignition energy and the gap size as seen in the literature, regardless the fact that we use initiators and not electrodes as in the cited studies.

We found the optimum gap for ethanol droplets of 289 m in diameter at a gap ≈ 1.1 mm and a minimum power entering the cavity of Pcavity = 7.3 W.

The next step will be the repetition of these experiments with a droplet stream of kerosene before applying time reversal, coupled with microwaves to the ignition of an aeronautical fuel.

REMERCIEMENTS

Fig. 1 .

 1 Fig. 1. The open cavity with the initiators with spherical head.

Fig. 2 .

 2 Fig. 2. Tested initiators. From left to right : cylindrical with flat head (1), with spherical head (2), truncated aluminium cones (3L, 3R), stainless steel tips (4).

Fig. 3 .

 3 Fig. 3. Example of a droplet stream visualized with shadowgraphy.

Fig. 4 .

 4 Fig. 4. Example of the variation of the reflected power during the creation of plasma We have tested two different protocols during the experiments: a. First the droplet stream, then the microwaves; b. First the microwaves, then the droplet stream.a. When we inject the droplet stream first and afterwards the microwaves we can observe an immediate plasma creation on both tips/cones without a connexion in between and no combustion. The plasma is deforming until an explosive ignition takes place due to the accumulation of ethanol vapours in the cavity. The flame exits the cavity and propagates to the liquid collector above the cavity.b. When doing the experiment the other way round, injecting the microwaves first, we create an arc between the tips/cones. When we inject the droplet stream, the ethanol starts burning as soon as it reaches the plasma zone.Protocol b has been proven to be more secure and more repeatable, and has been used for the following results.Figure5shows the plasma arc between the aluminium cones (a) and the stainless steel tips (d) and the ignition, once ethanol has been injected (b), (e), respectively. We can observe an envelope flame around the droplet stream. The vapour is burning, but the liquid is not completely evaporated since droplets exit the cavity.

Fig. 5 .

 5 Fig. 5. Images of the zone between the tips during ignition tests: Tests with the arc discharge between the aluminium cones at first (a) and the injection/ignition of the droplet stream afterwards (b); arc discharge between the stainless steel tips at first (d) and injection/ignition of the droplet stream afterwards (e).

Fig. 6 .

 6 Fig. 6. Example of the variation of S11 during an ignition test of ethanol.

Fig. 7 .

 7 Fig. 7. Visualization of different steps during ignition tests.

Fig. 8 .

 8 Fig. 8. Minimum ignition power for different initiators for ethanol.

Fig. 9 .

 9 Fig. 9. Aluminium cones in experiment.

TABLE I .

 I POSSIBLE INJECTION DIRECTIONS AND PLASMAS FORMED.

	Initiator	Injection z-axis	Injection y-axis	Plasma
	1	Yes, tested	Yes, not tested	No Plasma
	2	Yes, tested	Yes, tested	1 Sphere
	3	No	Yes, tested	1 sphere, 2 spheres and arc
	4	No	Yes, tested	arc

TABLE II .

 II QUALITY FACTORS FOR DIFFERENT INITIATORS.

		Flat head	Spherical	Aluminium	Stainless
			head	cones	steel tips
	Q	2042	2085	1930	1537

TABLE III .

 III EXAMPLE OF EXPERIMENTAL CONDITIONS WITH PLASMA BETWEEN THE INITIATORS FOR SUCCESSFUL FUEL IGNITION TESTS.

	Initiator	f0	Gap	T	Pinc,real	Prefl
		[GHz]	[mm]	[°C]	[W]	[W]
	Aluminium (3)	2.4570 0.8	19.0 14.1	5.1
	Aluminium (3)	2.4594 1.1	19.3 16.4	6.3
	Aluminium (3)	2.4628 1.8	20.1 20.8	7.2
	Stainless Steel (4)	2.4336 0.7	28.6 59.3	30.8
	Stainless Steel (4)	2.4356 1.1	29.3 11.9	4.6
	Stainless Steel (4)	2.4424 1.8	24.8 65.2	30.3

We thank the STAE for financing this work within the scope of the 'Tremplin' JETTRIM. This work would not have been possible without the help of Geoffroy Illac from ONERA and Olivier Pigaglio from the laboratory Laplace.