Dr Giancarlo Castaman
email: castaman@hemato.ven.it

Alberto Tosetto

Anne Goodeve

Augusto B Federici

Stefan Lethagen

Ulrich Budde

Javier Batlle

Dominique Meyer

Claudine Mazurier

Jenny Goudemand

Jeroen Eikenboom

Reinhard Schneppenheim

Jorgen Ingerslev

David Habart

Frank Hill

Ian Peake

Francesco Rodeghiero

THE IMPACT OF BLEEDING HISTORY, VON WILLEBRAND FACTOR AND PFA-100® ON THE DIAGNOSIS OF TYPE 1 VON WILLEBRAND DISEASE: RESULTS FROM THE EUROPEAN STUDY MCMDM-1VWD

Keywords: Hemostasis, Thrombosis, and Vascular Biology von Willebrand disease -von Willebrand factor -inherited bleeding disorders -PFA-100 closure time -bleeding score

d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The diagnosis of von Willebrand disease (VWD), the most frequent inherited bleeding disorder, is based on the presence of bleeding symptoms and abnormal laboratory tests suggesting a qualitative or quantitative defect of von Willebrand factor (VWF) [START_REF] Castaman | Von Willebrand's disease in the year 2003: towards the complete identification of gene defects for correct diagnosis and treatment[END_REF]. While the use of a standardized bleeding score (BS) has recently gained a wide acceptance as a useful way to assess the clinical severity of the disorder in patients with type 1 VWD [START_REF] Rodeghiero | The discriminant power of bleeding history for the diagnosis of von Willebrand disease type 1: an international, multicenter study[END_REF][START_REF] Tosetto | A quantitative analysis of bleeding symptoms in type 1 von Willebrand disease: results from a multicenter European study (MCMDM-1 VWD)[END_REF], there is a still considerable uncertainty about the optimal use of laboratory tests. In the past, the bleeding time (BT) was considered useful for VWD screening, but nowadays it has lost importance because of its invasive nature and the wide availability of VWF assays to most clinical laboratories. BT has been replaced in many laboratories by the Platelet Function Analyzer (PFA-100™) closure time, a global test that mimics in vitro the BT by recording the time required to stop the flow of the patient' whole blood under high shear stress [START_REF] Kundu | Description of an in vitro platelet function analyzer -PFA-100 TM[END_REF].

While the PFA-100 could be useful for screening severe forms of VWD [START_REF] Fressinaud | Screening for von Willebrand disease with a new analyzer using high shear stress: a study of 60 cases[END_REF], it

is not known whether it could be of value for diagnostic or prognostic purposes in patients with milder forms of VWD in comparison to other laboratory tests. In particular, two questions remain unresolved. Firstly, whether the addition of PFA-100 could improve the diagnostic efficiency of the current approach based on the quantitation of VWF and bleeding severity, particularly in subjects with borderline BS or secondly, in patients with mild VWD, possibly accounting for up to 60-70% of all VWD patients, where VWF level does not completely explain the severity of bleeding as measured by a quantitative BS [START_REF] Tosetto | A quantitative analysis of bleeding symptoms in type 1 von Willebrand disease: results from a multicenter European study (MCMDM-1 VWD)[END_REF]. Thus, it would be useful to know whether abnormal PFA-100 closure times could add more information over VWF measurement to identify patients with a more severe VWD phenotype.

In this study we have evaluated the impact of the PFA-100 by assessing its relationship with BS and VWF level in a population of families with VWD enrolled in a multicenter European study (MCMDM-1VWD). VWD (see also http://www.shef.ac.uk/euvwd/ for further details). Details about the selection of investigated families have been already published in detail [START_REF] Tosetto | A quantitative analysis of bleeding symptoms in type 1 von Willebrand disease: results from a multicenter European study (MCMDM-1 VWD)[END_REF][START_REF] Goodeve | Phenotype and genotype of a cohort of families historically diagnosed with Type 1 von Willebrand Disease in the European study, molecular and clinical markers for the diagnosis and management of Type 1 von Willebrand Disease (MCMDM-1VWD)[END_REF]. Briefly, 154 families from 9 European countries were enrolled, the recruitment criteria being the presence of an index case and at least one other affected family member, both previously diagnosed as having type 1 VWD on the basis of the criteria adopted at each Center. Within the families, subjects were classified by the enrolling Center as the index case (IC, subject who led to investigation) and affected or unaffected family member (AFM and UFM respectively) on the basis of reduced VWF levels and bleeding symptoms at time of investigation of the IC, as previously described [START_REF] Tosetto | A quantitative analysis of bleeding symptoms in type 1 von Willebrand disease: results from a multicenter European study (MCMDM-1 VWD)[END_REF]. On the basis of multimeric and genotypic analysis performed in the MCMDM-1VWD study, subjects from the enrolled families were subsequently divided in three groups: subjects with abnormal multimers (Group 1), subjects with normal multimers and at least one candidate mutation (Group 2) and subjects with normal multimers and no mutation detected (Group 3) [START_REF] Goodeve | Phenotype and genotype of a cohort of families historically diagnosed with Type 1 von Willebrand Disease in the European study, molecular and clinical markers for the diagnosis and management of Type 1 von Willebrand Disease (MCMDM-1VWD)[END_REF]. A total of 1166 healthy controls, defined as subjects that never sought medical attention for a bleeding symptom, were also enrolled.

Laboratory tests. Twenty ml of citrated blood were obtained from all investigated family members and normal controls, and platelet-poor plasma was immediately aliquoted and stored at -80˚C for subsequent measurements. All the subjects were fasting and with no significant low hematocrit or platelet count. Plasma aliquots were sent on dry ice by overnight courier to the central laboratory in Vicenza, where VWF ristocetin cofactor activity (VWF:RCo) and VWF antigen (VWF:Ag) were measured, as previously described [START_REF] Tosetto | A quantitative analysis of bleeding symptoms in type 1 von Willebrand disease: results from a multicenter European study (MCMDM-1 VWD)[END_REF][START_REF] Goodeve | Phenotype and genotype of a cohort of families historically diagnosed with Type 1 von Willebrand Disease in the European study, molecular and clinical markers for the diagnosis and management of Type 1 von Willebrand Disease (MCMDM-1VWD)[END_REF].

PFA-100 closure time was performed in 5 Centers experienced with this test, immediately after Bleeding score. The severity of bleeding symptoms in each investigated subject was assessed by using a standardized bleeding questionnaire, as previously described [START_REF] Tosetto | A quantitative analysis of bleeding symptoms in type 1 von Willebrand disease: results from a multicenter European study (MCMDM-1 VWD)[END_REF].

Statistics. All computations were performed using the Stata software package (College Station, TX, USA). Variables influencing VWF:RCo and VWF:Ag levels were identified using a multiple linear regression model [START_REF] Breiman | Classification and Regression Trees[END_REF][START_REF] Kleinbaum | Applied regression analysis and other multivariable methods[END_REF], using the whole MCMDM-1VWD dataset. Univariate differences in VWF:RCo and VWF:Ag levels for different categories were tested using the Kruskal-Wallis non-parametric test or chi-square statistic, as appropriate. We computed positive and negative predictive values (PPV, NPV) and receiver-operator curves (ROC)

and their associated c-statistics to test the diagnostic capabilities of PFA-100. The c-statistics tests the null hypothesis that two independent tests have a comparable area under the ROC curve, which is a measure of the diagnostic efficiency of a test. One of the advantages of the c-statistics is that several tests may be combined, so that the diagnostic efficiency of a set of tests may be compared. In this study, we tested the hypothesis that the addition of PFA-100 may improve the overall diagnostic efficiency of the usual diagnostic criteria (based on symptoms and VWF measurements). For this purpose, we considered as "VWD affected" a subject fulfilling both the following criteria: 1) diagnosis as affected by the enrolling Center (either IC or AFM) and 2) member of a family showing complete segregation of the VWD phenotype with the VWF locus.. These criteria were the same adopted for a previous study evaluating the predictive power of VWF measurement in this cohort [START_REF] Tosetto | A quantitative analysis of bleeding symptoms in type 1 von Willebrand disease: results from a multicenter European study (MCMDM-1 VWD)[END_REF]. respectively for PFA-100 ADP and VWF:RCo) (Figure 3). PFA-100 EPI and VWF:Ag were discarded from the model, suggesting that most of the variation of bleeding score is predicted by PFA-100 ADP and VWF:RCo alone. The regression model showed that both the third and fourth quartiles of PFA-100 ADP closure times were associated with higher bleeding scores (mean increase in the third and fourth quartiles vs. the first quartile: 2.2 and 2.9 respectively, p=0.02 and p=0.007), even for normal VWF:RCo values (Figure 2). There were no individuals with subtle abnormalities of the multimeric pattern (group 1) showing isolated prolongation of the PFA-100 closure time, vs. 12.3% of group 2 and 13.4% of group 3 (p=0.001).

F
Additional diagnostic value of PFA-100 for the diagnosis of VWD. The NPV and PPV were calculated using the above defined normal ranges in reference limits in patients considered affected using the above mentioned criteria. Assuming a 0.1% prevalence of clinically relevant VWD, both PFA-100 EPI and ADP closure times showed a very high NPV but a very low PPV (100% for both PFA-

DISCUSSION

In the last years, many efforts have been undertaken for a more stringent and objective diagnosis of VWD [START_REF] Sadler | Provisional criteria for the diagnosis of VWD type 1; on behalf of the SSC Subcommittee on von Willebrand factor[END_REF]. These efforts include the evaluation of new diagnostic tests (such as the PFA-100 closure time) but also the re-evaluation of time-honoured procedures such as the bleeding history, and they resulted in new, promising approaches to VWD diagnosis [START_REF] Rodeghiero | The discriminant power of bleeding history for the diagnosis of von Willebrand disease type 1: an international, multicenter study[END_REF][START_REF] Tosetto | A quantitative analysis of bleeding symptoms in type 1 von Willebrand disease: results from a multicenter European study (MCMDM-1 VWD)[END_REF]. The BT has been traditionally used as a challenging test for primary hemostasis, but the test is invasive, is influenced by the operator and by device-dependent reproducibility and does not help in distinguishing among the various types of defects of primary hemostasis [START_REF] Cattaneo | Are the bleeding time and PFA-100 useful in the initial screening of patients with mucocutaneous bleedings of hereditary nature ?[END_REF]. Therefore, the test has been replaced in many laboratories by the Platelet Function Analyzer (PFA-100™) closure time [START_REF] Kundu | Description of an in vitro platelet function analyzer -PFA-100 TM[END_REF]. The two tests were compared in the diagnosis of VWD in several studies with similar design. Wellcharacterized VWD patients underwent both BT and PFA-100 testing, and the sensitivities of the two tests were compared (specificity being fixed for all studies at about 97%) [START_REF] Fressinaud | Screening for von Willebrand disease with a new analyzer using high shear stress: a study of 60 cases[END_REF][START_REF] Cattaneo | Evaluation of the PFA-100 system in the diagnosis and therapeutic monitoring of patients with von Willebrand disease[END_REF][START_REF] Schlammadinger | Comparison of the O'Brien filter test and the PFA-100 platelet analyzer in the laboratory diagnosis of von Willebrand's disease[END_REF][START_REF] Nitu-Whalley | The role of the platelet function analyser (PFA-100) in the characterization of patients with von Willebrand's disease and its relationships with von Willebrand factor and the ABO blood group[END_REF]. Although performed in small series of patients, these studies observed comparable sensitivities, with a pooled estimate around 90% for PFA-100 (92% and 90% for the EPI and ADP cartridges respectively) that was significantly higher than the sensitivity of the bleeding time (61%). The 9 sensitivity of PFA-.100 was even higher in subjects with severe or variant VWD (e.g., type 3 or 2A). On the contrary, the test appears to be somewhat insensitive and of poor utility in subjects with borderline VWF levels [START_REF] Posan | Comparison of PFA-100 testing and bleeding time for detecting platelet hypofunction and von Willebrand disease in clinical practice[END_REF][START_REF] Quiroga | Template bleeding time and PFA-100 have low sensitivity to screen patients with hereditary mucocutaneous hemorrhages: comparative study in 148 patients[END_REF].

The use of PFA-100 for screening purposes was subsequently tested in non-selected series of patients referred for haemostatic evaluation, with reported sensitivities of PFA-100 for type 1 VWD ranging from 61.5% to 71% in the two studies [START_REF] Podda | Usefulness of PFA-100 testing in the diagnostic screening of patients with suspected abnormalities of hemostasis: comparison with the bleeding time[END_REF][START_REF] Quiroga | Template bleeding time and PFA-100 have low sensitivity to screen patients with hereditary mucocutaneous hemorrhages: comparative study in 148 patients[END_REF].

In this study we have evaluated the diagnostic impact of the PFA-100 and its relationship with VWF level and BS in a population of families diagnosed with VWD type 1 in a multicenter

European study (MCMDM-1VWD). Index cases and affected family members had significantly more prolonged PFA-100 closure times compared to unaffected family members (Figure 1 A). The results of PFA-100 were associated with the presence of abnormal multimers and proven linkage with VWF gene mutation (Figure 1 B and 1 C). A strong relationship was found with VWF levels for both cartridges (Figure 2), while both PFA-100 ADP and VWF:RCo were independently associated with bleeding score (Figure 3). The degree of prolongation with PFA-100 ADP correlated with higher bleeding scores, even in presence of normal VWF:RCo values.

Isolated prolongation of the PFA-100 closure time was present in 4.2% of individuals belonging to families in linkage vs. 16.5% of the families not in linkage (p=0.005). This suggests that in families in linkage PFA-100 prolongation is mostly associated with a true VWD, as suggested by the high likelihood of finding a VWF gene mutation [START_REF] Goodeve | Phenotype and genotype of a cohort of families historically diagnosed with Type 1 von Willebrand Disease in the European study, molecular and clinical markers for the diagnosis and management of Type 1 von Willebrand Disease (MCMDM-1VWD)[END_REF], while in families not in linkage other disorders (e.g, mild platelet function disorders) could be present.

Furthermore, it also suggests that the families could have been mainly selected on the basis of their bleeding symptoms. This is also supported by the fact that no individuals with subtle abnormalities of the multimeric pattern and thus almost all with a proven VWF mutation (group 1) had isolated prolongation of the PFA-100 closure time compared to 12.3% of subjects in group 2 It has been demonstrated that the haplotype(s) of platelet glycoprotein polymorphisms influences the bleeding severity of patients with VWD [START_REF] Kunicki | An association of candidate gene haplotypes and bleeding severity in von Willebrand disease (VWD) type 1 pedigrees[END_REF] and furthermore that true inherited platelet function disorders may coexist in patients with VWD , thus complicating the definitive diagnosis of VWD [START_REF] Daly | Identification and characterization of a novel P2Y 12 variant in a patient diagnosed with type 1 von Willebrand disease in the European MCMDM-1VWD study[END_REF]. At present, however, it is not known whether platelet glycoprotein haplotypes can modulate PFA-100 results independently from the presence of reduced VWF levels.

Despite the evidence of an independent role of PFA-100 ADP in influencing the BS and the correlation with VWF levels, the role of PFA-100 in the diagnostic work-up of patients with suspected VWD type 1 remains however uncertain. Assuming a prevalence of clinically significant VWD around 0.1%, the positive predictive value of an abnormal PFA-100 would be around 1%, while the negative predictive value would be 100%. Of course, this figure is biased by the selection criteria and the small number of cases included and results from larger populations of unselected cases should be available for a more reliable estimate. Thus, while in an unselected population a normal PFA-100 would be useful to rule out VWD, it is doubtful whether it could replace the more specific VWF assay in patients with significant mucocutaneous bleeding symptoms. This is also suggested by the findings that adding PFA-100 EPI or ADP closure times does not significantly add to the information given by VWF:RCo.

Thus, until validation of the role of this test in prospective studies becomes available, a careful approach to the patients and their clinical indexes is advised to achieve a sound diagnosis, which could be particularly relevant for the prevention of and the optimal treatment of hemorrhagic events in VWD patients [START_REF] Rodeghiero | How I treat von Willebrand disease[END_REF].

  BLEEDING HISTORY, VON WILLEBRAND FACTOR AND PFA-100® ON THE DIAGNOSIS OF TYPE 1 VON WILLEBRAND DISEASE: RESULTS FROM THE EUROPEAN STUDY MCMDM-1VWD

  Subjects. The "Molecular and Clinical Markers for the Diagnosis and Management of Type 1 von Willebrand disease (MCMDM-1VWD)" Study was a multicenter, EU funded, survey on type 1

  whole blood, according to the manufacturer instructions. A single batch of PFA-100 ADP and epinephrine (EPI) cartridges was supplied free of charge by Dade Behring (Marburg, Germany) and distributed among the Centers performing the assay.

  PFA-100 and VWF levels in families with/without linkage between VWF locus and phenotype or with multimeric abnormalities. Since PFA-100 closure times were associated with bleeding symptoms (as expressed by the bleeding score) independently from VWF levels, we evaluated whether isolated prolongation of PFA-100 was differently distributed in the MCMDM-1VWD families. Isolated prolongation of the PFA-100 was defined as a closure time in the highest two quartiles (hence, above the median value: 124 sec for ADP and 181 sec for EPI cartridges respectively) in the presence of VWF levels in the highest two quartiles (above the median value of 55 IU/dL for VWF:RCo). Isolated prolongation of the PFA-100 closure time was present in 4.2% of individuals belonging to families in linkage vs. 16.5% of the families not showing segregation (p=0.005).

  ADP and 1.34 and 1.16%, respectively). Using a c-logistic model, we subsequently tested whether addition of PFA could add to the diagnosis of VWD. Although PFA-100 EPI and ADP closure times were associated with VWD independently from VWF:RCo levels, neither EPI or ADP added to a model based on measurement of VWF:RCo and Bleeding Score, either measured as a quantitative or qualitative trait (above/below the normal range).

  Figure 1. A) Distribution of the closure times with ADP-and EPI-cartridges among the investigated subjects according to clinical status. Median within each group and the 97.5 percentile of normal reference group are reported. B) Distribution of the closure times with ADP-and EPI-cartridges among the investigated subjects according to presence of an abnormal or normal multimeric pattern. C) Distribution of the closure times with ADP-and EPI-cartridges among the investigated subjects according to linkage status.

Figure 2 .

 2 Figure 2. Relationship of the closure times with ADP-and EPI-cartridges with VWF:RCo and VWF:Ag.

Figure 3 .

 3 Figure 3. Relationship of the closure times with ADP-cartridge and bleeding score and

  Table 1 reports the clinical characteristics of the investigated subjects for whom PFA-100 measurement (either ADP or EPI cartridges) was available. A total of 107/154 originally included index cases, 105/273 affected family members and 71/295 unaffected family members were tested. In addition a control group of 79 healthy subjects was also included. for age and sex, both PFA-100 ADP and VWF:RCo were independently associated with bleeding score with a similar magnitude (t statistics for regression coefficients 2.92 and 2.4

	model, adjusted
	Blood group distribution showed a significant increase of O-blood group subjects in index cases
	o and affected family members. F o
	r r
	P P
	e e r e e r
	R e v i e w R e v i e w
	Relationship of PFA-100 with VWF:RCo and VWF:Ag. Figure 2 shows that by regression analysis
	VWF levels (either VWF:RCo or VWF:Ag) were both strongly related with the PFA-100 closure
	7

6

RESULTS

Enrolled subjects and laboratory data.

Normal range in control subjects. After excluding 3 and 5 outlier observations for EPI and ADP cartridges, respectively (defined as observation lying outside 3 standard deviations from mean), the upper 97.5 percentiles were 183.8 and 122 seconds for EPI and ADP cartridges.

PFA-100 in the MCMDM-1VWD families according to clinical status, multimeric pattern and linkage analysis. Figure

1

A reports the distribution of closure times in investigated subjects according to clinical status. Median values were similarly prolonged in index cases and affected family members, while median values of normal controls overlapped those of unaffected family members, even though some abnormal results were observed in the latter group. Subjects with abnormal multimers had on average significantly prolonged PFA-100 results compared to subjects with normal multimers with and without a VWF gene mutation (P < 0.001 for both EPI and ADP cartridges)(Figure

1 B

). Subjects belonging to families with proven linkage had a more prolonged mean PFA-100 values compared to subjects with no or uninformative linkage (P < 0.001 for both EPI and ADP cartridges) (Figure

1 C

).

times (either with ADP or EPI), with a non-linear progression

Relationship of bleeding score with PFA-100 and VWF levels. In a multiple stepwise regression

Table I .

 I Clinical parameters in enrolled subjects, as classified by the participating Centers (index case, IC; affected family member, AFM; unaffected family members, UFM) tests differences between IC and AFM as defined by enrolling Centers † tests differences between controls and UFM as defined by enrolling Centers

				IC	AFM		UFM	Controls
						P *
				(n=107)	(n=105)		(n=71)	(n=79)	P †
	Females (%)			73 (68.2)	57 (54.2)	0.03	39 (54.9)	52 (65.8)	0.17
	Mean age, years (range)			35.9 (1-80)	34.2 (3-85)	0.50	39.1 (9 -75)	41.2 (14 -78)	0.41
	Blood group O (%)			71 (66.3)	64 (60.9)	0.41	33 (46.4)	23 (29.1)	0.02
	Mean VWF:RCo, IU/dL (±SD) F o Mean VWF:Ag, IU/dL (±SD)	33.7 (25.6) 36.5 (22.1)	33.1 (30.6) 36.4 (25.7)	0.87 0.77	90.2 (30.7) 98.1 (32.2)	90.8 (26.4) 96.7 (29.3)	0.89 0.97
	Mean FVIII:C, IU/dL (±SD)	r	50.3 (25.7)	53.4 (33.4)	0.47	109.7 (54.1)	(not applic.)	-
	Group 1 (Abnormal multimers)	P 43 (40.2 %)	56 (53.3 %)		0 (-)	-
	Group 2 (NM, mutation) Group 3 (NM, no mutation)		e 37 (34.5 %) e r 27 (25.3 %)	20 (19.2 %) 29 (27.6 %)	0.03	8 (11.2 %) 63 (88.8 %)	--
	The regression model showed that both the third and fourth quartiles of PFA-100 ADP closure times were associated with higher bleeding R e v i e w
	scores (mean increase in the third and fourth quartiles vs. the first quartile: 2.2 and 2.9
	respectively, p=0.02 and p=0.007).
				14

*

British Journal of Haematology

Supported by: The European Community under the Fifth Framework Programme (QLG1-CT-2000-00387) and by Dade Behring.


Karen Fort
email: karen.fort@inist.fr.

Gilles Adda
email: gilles.adda@limsi.fr.

Kevin Bretonnel Cohen
email: kevin.cohen@gmail.com

Karën Fort

Amazon Mechanical Turk: Gold Mine or Coal Mine ?

diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

What is MTurk? What is it not?

MTurk is an on-line crowdsourcing, microworking 1 system which enables elementary tasks to be performed by a huge number of on-line people. Ideally, these tasks are meant to be solved by computers, but they still remain out of computational reach (for instance, the translation of an English sentence into Urdu). MTurk is composed of two populations: the Requesters, who launch the tasks to be completed, and the Turkers, who complete these tasks. Requesters create the so-called "HITs" (Human Intelligence Tasks), which are elementary components of complex tasks. The art of the requesters is to split complex tasks into basic steps and to fix a reward, usually very low (for instance MTurk should therefore not be considered as a game with a purpose, unlike Phrase Detectives [START_REF] Chamberlain | Phrase Detectives: a Web-based Collaborative Annotation Game[END_REF] in which the gain is not emphasized (only the best contributors gain a prize) or, for French, JeuxDeMots ("Play On Words") [START_REF] Lafourcade | Making people play for lexical acquisition[END_REF], which does not offer any prize.

MTurk is not a game or a social network, it is an unregulated labor marketplace: a system which deliberately does not pay fair wages, does not pay due taxes, and provides no protections for workers.

Why are we concerned?

Since its introduction in 2005, there has been a steadily growing use of MTurk in building or validating NLP resources, and most of the main scientific conferences in our field include papers involving MTurk. Figure 1 was created by automatically searching the proceedings of some of the main speech and language processing conferences, as well as some smaller events specializing in linguistic resources, using the quoted phrase "Mechanical Turk". We then manually checked the retrieved articles, source by source, to identify those which really make use of MTurk, ignoring those which simply talk about it. (For example, in the LREC 2010 proceedings, eight articles talk about MTurk, but only five used it, and in 2008, out of two papers citing MTurk, only one used it.) The present journal, Computational Linguistics (CL), appears in the bar chart with a zero count, as none of the articles published in it so far mention MTurk. All of the other sources contained at least one article per year using MTurk. The total number of publications varies from year to year, since e.g. conferences may accept different numbers of papers each year, and some conferences, such as LREC, occur only every two years.

We performed another, less detailed, search, this time in the whole ACL Anthology (not source by source), using the same quoted phrase "Mechanical Turk" on November 5, 2010. We examined the hits manually, and out of the 124 resulting hits, 86 were papers in which the authors actually used MTurk as part of their research methodology. Interestingly, we noticed that at least one paper that we know to have used MTurk, namely [START_REF] Biadsy | An unsupervised approach to biography production using Wikipedia[END_REF], was not returned by the search. The published version of this paper does not explicitly mention MTurk, but the corresponding presentation at the conference indicated that MTurk was used. This is some evidence that use of MTurk may be under-reported. It should be noted that these results include a specialized workshop, the NAACL-HLT 2010 Workshop on Amazon Mechanical Turk (35 papers), the existence of which is, in itself, strong evidence of the importance of the use of MTurk in the domain.

A vast majority of papers present small to medium size experiments where the authors have been able to produce linguistic resources or perform evaluations at a very low cost; at least for transcription and translation, the quality is sufficient to train and evaluate statistical translation/transcription systems (Callison-Burch and Dredze 2010; Marge, Banerjee, and Rudnicky 2010). However, some of these papers bring to light language resource quality problems. For example, [START_REF] Tratz | A taxonomy, dataset, and classifier for automatic noun compound interpretation[END_REF] note that the user interface limitations constitute "The first and most significant drawback" of MTurk, as, in their context of annotating noun compound relations using a large taxonomy, "it is impossible to force each Turker to label every data point without putting all the terms onto a single web page, which is highly impractical for a large taxonomy. Some Turkers may label every compound, but most do not." They also note that "while we requested that Turkers only work on our task if English was their first language, we had no method of enforcing this." Finally, they note that "Turker annotation quality varies considerably". Another important point is made in [START_REF] Bhardwaj | Anveshan: A tool for analysis of multiple annotators' labeling behavior[END_REF], where it is shown that, for their task of word sense disambiguation, a small number of trained annotators are superior to a larger number of untrained Turkers. On that point, their results contradict that of [START_REF] Snow | Cheap and fast -but is it good? evaluating non-expert annotations for natural language tasks[END_REF], whose task was much simpler (the number of senses per word was 3 for the latter, versus 9.5 for the former). The difficulty of having Turkers perform complex tasks also appears in [START_REF] Gillick | Non-expert evaluation of summarization systems is risky[END_REF], an article from the proceedings of the NAACL-HLT 2010 Workshop on Amazon Mechanical Turk, in which non-expert evaluation of summarization systems is proved to be "not able to recover system rankings derived from experts". Even more interestingly, [START_REF] Wais | Towards building a high-quality workforce with mechanical turk[END_REF] show that standard machine learning techniques (in their case, a Naïve Bayes classifier) can outperfom the Turkers on a categorization task (classifying businesses into Automotive, Health, Real Estate, etc). Therefore, in some cases, NLP tools already do better than MTurk. Finally, as we said earlier, the vast majority of papers present only small or medium size experiments. This can be explained by the fact that, at least according to (Ipeirotis 2010a), submitting large jobs in MTurk results in low quality and unpredictable completion time.

Who are the Turkers?

Many people conceive of MTurk as a transposition of Grid Computing to humans, thus making it possible to benefit from humans' "spare cycles" to develop a virtual computer of unlimited power. The assumption is that there is no inconvenience for humans (as it is not a real work), and the power comes from the myriad. This a fiction.

Let us look first at how many Turkers are performing the HITs. This is a quite difficult task, because Amazon does not give access to many figures about them. We know that over 500k people are registered as Turkers in the MTurk system. But how many Turkers are really performing HITs? To evaluate this, we combined two different sources of information. First, we have access to some surveys about the demographics of the Turkers (Ipeirotis 2010b;[START_REF] Ross | Who are the turkers? worker demographics in amazon mechanical turk[END_REF][START_REF] Ross | Who are the crowdworkers?: shifting demographics in mechanical turk[END_REF]. These surveys may have a bias over the real population of Turkers, as some Turkers may be reluctant to respond to surveys. Because the results of these surveys are quite consistent, and the surveys are usually easy to complete, not particularly boring, and paid above the usual rate, we may assume that this bias is minor, and accept what they say as a good picture of the population of Turkers. In these surveys we see a lot of interesting things. For instance, there is a growing number of people from India: there were below 10% in 2008, above 33% in early 2010, and they represented about 50% of the Turkers in May2 . Even if these surveys show that the populations from India and the US are quite different, we may take as an approximation that they have about the same reasons to perform HITs in Mturk, and produce about the same activity. We looked at how many HITs the 1,000 Turkers who completed the survey in (Ipeirotis 2010b) claim to perform each week: between 138,654 and 395,106 HITs per week. 3 The second source of information comes from the Mechanical Turk Tracker4 : according to it, 700,000 HITs are performed each week. But the tracker system neither keeps track of the HITs which are completed in less than one hour, nor is able to quantify the fact that the same HIT can be completed by multiple workers and in fact should be, according to regular users like Callison-Burch and Dredze (2010). Asking the authors of (Ipeirotis 2010b), and the creator of the Mechanical Turk Tracker (who are in fact the same person), they (he) suggested that we should multiply the number given by the tracker by 1.7 × 5 to take into account these two factors5 , resulting in the (conjectural) total number of 5,950,000 HITs. Taking the two data points6 , we are able to hypothesize that the real number of Turkers is between 15,059 and 42,912. However, from the surveys, we have access to another figure: 80% of the HITs are performed by the 20% most active Turkers [START_REF] Deneme | How many turkers are there[END_REF], who spend more than 15 hours per week in the MTurk system [START_REF] Adda | Language resources and amazon mechanical turk: legal, ethical and other issues[END_REF], which is consistent with the Pareto principle which says that 80% of the effects come from 20% of the causes. We may therefore say that 80% of the HITs are performed by 3,011 to 8,582 Turkers. These figures represent 0.6 to 1.7% of the registered Turkers, which in turn is in accord with with the "90-9-1" rule7 valid in the Internet culture.

Another important question is if activity in MTurk should be considered as labor or something else (hobby, volunteer work,. . . ). The observed mean hourly wages for performing jobs in the MTurk system is below $2 ($1.25 according to [START_REF] Ross | Who are the turkers? worker demographics in amazon mechanical turk[END_REF])). Because they accept such low rewards, a common assumption is that Turkers are US students or stay-at-home mothers who have plenty of leisure time and are happy to fill their recreation time by making some extra money. According to recent studies in the social sciences (Ipeirotis 2010b;[START_REF] Ross | Who are the crowdworkers?: shifting demographics in mechanical turk[END_REF], it is quite true that a majority (60%) of Turkers think that MTurk is a fruitful way to spend free time and getting some cash; but they are only 20% (5% of the India Turkers) who say that they use it to kill time. And these studies also show that 20% (30% of the India people) declare that they use MTurk "to make basic ends meet". From these answers, we find that money is an important motivation for a majority of the Turkers (20% use MTurk as their primary source of income, and 50% as their secondary source of income), and leisure is important for only a minority (30%). We cannot conclude from these studies that the activity in MTurk should be considered as labor for all the Turkers, but at least for the minority (20%) for whom MTurk represents a primary source of income.8 Moreover, using the survey in (Ipeirotis 2010b), we find that this minority is performing more that 1/3rd of all the HITs.

What are the issues with MTurk?

The very low wages (below $2 an hour) are a first issue, but the use of Mechanical Turk raises other ethical issues as well. The position of many prototypical Turkers would be considered ethically unacceptable in major first-world countries. Denied even the basic workplace right of collective bargaining (unionization), this community has no recourse to any channels for redress of employer wrongdoing, let alone the normal ones available to any typical worker in the United States and many other developed nations (e.g. class action lawsuits, other lawsuits, and complaints to government agencies) while simultaneously being subjected to egregious vulnerabilities, including the fact that they have no guarantee of payment for work properly performed.

Legal issues surrounding the use of MTurk have also been encountered. At least one university legal department was sufficiently concerned that Turkers working for several months would claim employee status and demand health and other benefits that they refused to allow grant funds to be expended on MTurk. A small number of universities have insisted on institutional review board approval for MTurk experiments (Institutional review boards in US universities are independent bodies that review proposed experiments for legal and ethical issues).

Is MTurk the future of linguistic resource development?

The implicit belief that the very low cost of MTurk derives from the fact that incentivizing casual hobbyists requires only minimal payment is a mirage: once you admit that a majority of Turkers are not considering MTurk as a hobby, but as a primary or a secondary source of income, and that 1/3rd of the HITs are performed by Turkers who need MTurk to make basic ends meet, you then have to admit that MTurk is, at least for them, a labor marketplace. Moreover, the frequent assumption that the low rewards are a result of the classical law of supply-and-demand (large numbers of Turkers means more supply of labor and therefore lower acceptable salaries) is false. Firstly, we do not observe that there are too many Turkers. In fact, there are not enough Turkers. This can be observed through the difficulty to find Turkers with certain abilities (for instance understanding a specific language [START_REF] Novotney | Cheap, fast and good enough: automatic speech recognition with non-expert transcription[END_REF]), and in the difficulty to perform very large HIT groups (Ipeirotis 2010a). This is not surprising, as we have seen that the number of active Turkers is not that big. Secondly, the low cost is a result of the requesters' view of the relation between quality and reward: many articles (see for instance [START_REF] Marge | Using the amazon mechanical turk for transcription of spoken language[END_REF]) relate that there is no correlation between the reward and the final quality. The reason is that increasing the price is believed to attract spammers (i.e. Turkers who cheat, not really performing the job, but using robots or answering randomly), and they are numerous in the MTurk system, because of an inadequate worker reputation system. 9 We obtain here a schema which is very close to what the 2001 economics Nobel prize winner George Akerlof calls "the market for lemons", where asymmetric information in a market results in "the bad driving out the good". He takes the market for used cars as an example [START_REF] Akerlof | The market for 'lemons': Quality uncertainty and the market mechanism[END_REF], where owners of good cars (here, good workers) will not place their cars on the used car market, because of the existence of many cars in bad shape (here, the spammers), which encourage the buyer (here, the requester) to offer a low price (here, the reward) because he does not know the exact value of the car. After some time, the good workers leave the market because they are not able to earn enough money given the work done (and sometimes they are not even paid), which in turn decreases the quality. At the moment, the system is stable in terms of the number of Turkers, because good workers are replaced by naive workers.

Amazon's attitude towards reputational issues has been passive. It maintains itself to be a neutral clearinghouse for labor, in which all else is the responsibility of the two consenting parties. This attitude has led to an explosion of micro-crowdsourcing startups, which observed the MTurk flaws and tried to overcome them.10 Some of them could become serious alternatives to MTurk (TheQuill 2010), like Samasource11 , which offers at least a fair wage to workers, who in turn are clearly identified on the Web site, with their resume. But others are even worse than MTurk, ethically speaking. MTurk is ethically questionable enough; as a scientific community with ethical responsibilities we should seek to minimize the existence of even less ethical alternatives to it.

What's next?

If we persist in claiming that with MTurk we are now able to produce any linguistic resource or perform any manual evaluation of output at a very low cost, funding agencies will come to expect it. It is predictable that in assessing projects involving linguistic resource production or manual evaluation of output, funding agencies will prefer projects which propose to produce 10 or 100 times more data for the same amount of money. MTurk costs will then become the standard costs, and it will be very difficult to obtain funding for a project involving linguistic resource production at any level that would allow for more traditional, non-crowdsourced resource construction methodologies. Therefore, our community's use of MTurk not only supports a workplace model that is unfair and open to abuses of a variety of sorts, but also creates a de facto standard for the development of linguistic resources that may have long-term funding consequences.

Non-exploitative methods for decreasing the cost of linguistic resource development exist. They include semi-automatic processing, better methodologies and tools, games with a purpose, as well as microworking websites (like Samasource) that guarantee workers minimum payment levels. We encourage the computational linguistics and NLP communities to keep these alternatives in mind when planning experiments. If a microworking system is considered desirable by the ACL and ISCA communities, then we also suggest that they explore the creation and use of a linguistically specialized special-purpose microworking alternative to MTurk that both ensures linguistic quality and holds itself to the highest ethical standards of employer/employee relationships. Through our work as grant evaluators and recipients, we should also encourage funding bodies to require institutional review board approval for crowdsourced experiments and to insist on adherence to fair labor practices in such work.

  Figure 1Evolution of MTurk usage in NLP publications

http://blog.crowdflower.com/2010/05/amazon-mechanical-turk-survey/

The two figures come from the fact that each Turker gave a range of activity rather than an average number of HITs.

this system keeps tracks of all the HITs posted on Mturk, each hour http://mturk-tracker.com.

Personal communication in the comments of http://behind-the-enemy-lines.blogspot.com/ 2010/03/new-demographics-of-mechanical-turk.html, reporting that the tracker is missing ∼ 70% of the posted HITs, which are posted and completed within less than one hour, and a 5x factor for the unobserved HIT redundancy.

1,000 Turkers perform between 138,654 and 395,106 HITs per week, and the total number of HITs in the Mturk system is about 5.95M HITs per week.

http://en.wikipedia.org/wiki/1\%25_rule_(Internet_culture)

And even for the 50% who are utilizing MTurk as a secondary source of income.

For more details, see http://behind-the-enemy-lines.blogspot.com/2010/10/ be-top-mechanical-turk-worker-you-need.html

. . . for instance, Agent Anything, Clickworker, CloudCrowd, CrowdFlower, DoMyWork, JobBoy, LiveWork, Microtask, microWorkers, MiniFreelance, MiniJobz, MinuteWorkers, MyEasyTask, MyMicroJob, OpTask, RapidWorkers, Samasource, ShortTask, SimpleWorkers, SmartSheet, ...

http://www.samasource.org

Acknowledgments

We would like to thank Sophie Rosset, Joseph Mariani, Panos Ipeirotis, Eduard Hovy, and Robert Dale for their suggestions and encouragements. Any remaining errors are our own.


C Lazzaroni
email: claudia.lazzaroni@lpp.polytechnique.fr

P Chabert

A Rousseau

N Sadeghi

Excitation structure in a Micro Hollow Cathode Discharge in the normal regime at medium argon pressure

A microplasma is generated in the microhole (400 µm diameter) of a molybdenum-aluminamolybdenum sandwich (MHCD type) at medium pressure in pure argon. Imaging and emission spectroscopy have been used to study the light emission mechanisms in the microdischarge. We find that emission intensities of both argon atom and argon ion lines present sharp peaks located near the cathode, and that the position of these peaks moves toward the cathode with increasing gas pressure. An ionizing-sheath model has been developed to calculate the evolution of the sheath thickness as a function of the gas pressure. This model includes the ratio between the ion flux coming from the bulk of the plasma and the ion flux created in the sheath and also accounts for the gas heating in the microhole. The variation of the calculated ionizing sheath thickness as a function of pressure agrees well with the variation of the measured peak positions. The case of a sheath free of ionization has also been considered. However, the comparison between experiments and the various theories shows that a significant fraction of the ionization occurs in the sheath.

Introduction

The study of microplasma began to develop in the middle of the 90's in the USA [1], before undergoing an important expansion thanks to the experimental and theoretical works that have been performed in the past few years [2, 3, 4, 5 and 6]. The interest of microplasmas is that they can be generated at medium and high pressure (atmospheric pressure) with a very low applied voltage or injected power compared with others non-thermal plasmas such as dielectric barrier discharges or corona. They have applications in various fields such as surface treatment [7], sterilization [8], light sources [9] or microjets [10]. A particular microplasma configuration is the hollow cathode type geometry where a hole is drilled through an electrode-dielectric-electrode sandwich structure. This device is particularly stable. At very low current (typically <0.1 mA in our device), the plasma is located inside the hole and the voltage increases with the current: this is the abnormal regime. However, despite a DC driving source, a self-pulsed regime appears for intermediate discharge currents (typically ≤0.5 mA time averaged current for our device) with frequency of several tens of kilohertz [11]. This regime corresponds to a periodic plasma expansion on the cathode surface [12]. At higher currents, the plasma expands on the cathode backside surface and the voltage becomes relatively independent of the discharge current: this is the normal regime.

In this paper, we study the excitation mechanisms of the MHCD structure during the normal regime thanks to the observed radial profiles of argon atom and argon ion emission line intensities. A theoretical calculation of the sheath thickness is proposed to help a better understanding of the experimental results.

Experimental set-up

The microplasma device shown in figure 1 is similar to the one developed by Schoenbach [1]. It is a metal-dielectric-metal sandwich (microhollow cathode type). The dielectric, constituted of a 250 µm alumina foil, separates the two molybdenum electrodes which are both L=150 µm thick foils. A R=200 µm radius hole is drilled through the three foils with a laser beam, which are subsequently glued to form the sandwich structure. A ballast resistor, R b =440kΩ is located on the cathode side and a measurement resistor, R m =100Ω is located on the anode side. The discharge voltage is measured on the cathode side with a high voltage probe and the discharge current is deduced from the voltage drop across R m . These two signals are recorded on a 350MHz oscilloscope (Lecroy 6030A).

FIGURE 1. Experimental set-up.

Experiments reported in this paper are run in stable DC discharge conditions at constant argon flow of 100 sccm. Argon gas pressure ranges from 30 to 200 Torr and the discharge current is fixed at 1 mA. Optical emission spectroscopy measurements are performed to investigate the plasma excitation structure of the micro-discharge and also to evaluate the gas temperature across the radius of the hole. The image of the MHCD, viewed from the anode side, is formed with an achromatic 12.5 mm diameter, f=30 mm, lens (with an enlargement factor of about 8) on the entrance slit of a grating spectrograph. To record argon atomic and ionic lines in the 427 nm spectral region, a 2 m focal length spectrograph working at the third diffraction order of the 1200 g/mm grating is used. To record the nitrogen first positive emission around 670 nm, from which the gas temperature is deduced, a 1 m focal length spectrograph working at the first diffraction order of the 1200 g/mm grating is used. In both cases, a CCD camera, with pixels size 16 x16 µm 2 , placed on the exit plane of the spectrographs captures the horizontally dispersed images at different spectral positions. Therefore, with fully open entrance slit, images of the MHCD observed at different wavelengths are formed at different horizontal positions of the CCD. Each pixel of these images corresponds to an area of about 2.0µm×2.0µm inside the MHCD. Considering the spherical aberrations of the whole optical system, the spatial resolution along the diameter of the hole is estimated to be about 5 µm. The spectral dispersion of the 2 m spectrograph is 1.325 pm/pixel at 427.5 nm (around argon lines) and that of the 1 m spectrograph is 11.39 pm/pixel at 670 nm (N 2 emission).

Experimental results

Spatial distribution of Ar and Ar + lines emission intensities

Figure 2 shows the CCD image of the 400 µm diameter MHCD (100 Torr, 1 mA) recorded with fully open entrance slit. The spectrograph is set to form images from both Ar (5p[3/2] 1 -4s[3/2] 1 ) line at 427.217 nm (left), and Ar + (4p' 2 P 3/2 -4s' 2 D 5/2 ) line at 427.752 nm (right) on the CCD surface. As seen in this figure, the intensity of the Ar + emission line has an annular shape, with its maximum intensity near the cathode surface, whereas the intensity of the Ar emission line shows a pronounced maximum at the center of the MHCD hole. -0,2 -0,1 0,0 0,1 0,2 0 The observed light from the micro-hole comes from the following mechanisms: The first two reactions are direct electron impact excitations from the ground state argon atoms to the observed excited states of Ar and Ar + , respectively. In the plasma bulk, Ar + density is at least 10 5 times smaller than the Ar density. Due to the acceleration and flux conservation of ions, the density ratio will even be much larger in the sheath. The peak cross section for the excitation of the upper level of ion line is about 10 -21 m -2 at 40 eV [13]. The cross section for the excitation of this level from the ground state of ion can be at maximum 10 times larger than this value, with a threshold of about 20 eV. Considering the high energy of beam electrons inside the sheath, the small differences in cross sections and excitation threshold cannot compensate more than five orders of magnitude in densities and consequently the excitation of Ar + line from the ground state of ion will be negligible. R 3 is the ground state Ar + ion production from the ground state of atom and considering the high pressure domain of the MHCD, these Ar + ions are rapidly converted into Ar 2 + ions, with a rate coefficient for the reaction (R 4 ) k 4 =2.7 x 10 -43 m 6 .s -1 [14]. The last two reactions are electron-ion recombinations involving Ar + and Ar 2 + ions. In our MHCD plasmas, with an electron density n e ≅10 20 m -3 , and electron temperature T e ≅1 eV, the recombination coefficients of Ar + ions [15] a 5 ≅5.6 x 10 -39 T e (eV) -9/2 n e ≅ 5 x 10 -19 m 3 .s -1 is much smaller than that of Ar 2 + ions [16] a 6 ≅8.5 x 10 -13 (T e (kT)/300) -2/3 ≅ 7 x 10 -14 m 3 .s -1 . In conclusion, under our experimental conditions the dominant e-ion recombination mechanism that contributes to the production of excited Ar atoms is through R 4 and R 6 channels. The maximum of the emission intensity of atomic line at the MHCD centre observed in Fig. 3 for p≥100 Torr comes from excited Ar atoms produced by electron-ion recombination mechanism and as expected the maximum of the intensity is located at the central region of the MHCD, where the charged particle densities are maximum [START_REF] Makasheva | Topic 10, 28 th ICPIG[END_REF].

           + → + + → + + → + + → + + → + + → + - + - - + + + - + - - + - - - ) ( * ) ( * 2 ) ( 2 ) ( 2 ) ( 2
) ( *
At 100, 150 and 200 Torr, the peaks observed for the two lines near walls of the MHCD may be related to the position of the sheath edge. Indeed, electrons emitted from the cathode are accelerated within the sheath, where the electric field is large, and release this energy away from the cathode. The exact location of the maximum of emission and ionization may only be calculated from kinetic theory. Since ionization in the sheath is required to sustain a DC discharge, we expect ionization and excitation in the sheath. However, the maximum of excitation may be located after the sheath edge, as we shall see in the next section. Since the threshold energy for the electronic excitation of the upper level of the 427.752 nm Ar + line ( R 2 ) is about 37 eV, i.e. much larger than that for the excitation of the 427,217 nm atomic line (R 1 ), we expect the maximum of Ar + line to come first. Indeed, we experimentally observe that the maximum of Ar line emission arises 10 to 20 µm further away from the cathode compared to the ionic line. At lower pressures (50 Torr and below), the sheath size is no longer negligible compared to the hole radius and thus the maxima from both sides approach the centre and form a single maximum (hollow cathode effect). However, for Ar + line a maximum around r= 100 µm at 50 Torr and a shoulder around r=50 µm at 30 Torr are also visible in figure 3. Also, we observe that when increasing the neutral gas pressure, the maxima near the cathode of both ionic and atomic lines move towards the cathode, which suggests that the sheath size shrinks. To verify this hypothesis and quantify the effect, we perform an analytical calculation of the sheath size. We shall see that the model requires the knowledge of the neutral gas temperature. This temperature has been measured and the results are presented in the next section.

Measurement of the gas temperature

The gas temperature is deduced from the population distribution in rotational level of N 2 (B 3 П g , ν=4 and 5) by recording the emission spectra of the first positive nitrogen system N 2 (B 3 П g ) N 2 (A 3 Σ u ). A small amount (less than 1%) of nitrogen has been added to the feedstock gas and the recorded emission spectra of the 4-1 and 5-2 bands have been fitted with a simulated spectra (home made code using the spectroscopic constants of [START_REF] Roux | [END_REF]) in order to obtain the rotational temperature. As an example, we show on figure 4, the recorded spectra from the emission at the MHCD axis for a pressure of 100 Torr and a current of 1 mA. The best simulated spectra for a gas temperature of 470 K, also shown on figure 4, do not include the 6-3 band whose rotational lines start down from the 665 nm region. The rotational temperature is supposed to be equal to the translational gas temperature because the rotational population of this excited state reaches thermal equilibrium before its radiative deexcitation. This is due to the long radiative life time of the N 2 (B 3 П g ) state (a few µs [19]) compared to the large collision frequency at the high argon gas pressure in our experiments. The radial dependence of the gas temperature, obtained by fitting the observed spectra at different radial positions, is relatively constant along the MHCD diameter (± 20K). This is probably because the gas heating is localized near the MHCD walls and that some of the heat transfer occurs along the axial direction. At 100 Torr and 1 mA current, the gas temperature is about 470K±10K and it is only weakly dependent on gas pressure. However, as expected, the gas temperature increases with the discharge current.

Calculation of the sheath thickness

Case of an ionizing sheath

We start our analysis by considering an ionizing sheath for which the analytical calculation of the sheath size is based on the well-known physics of the cathode fall in DC discharges (see for example [START_REF] Lieberman | Principle of Plasma Discharges and Materials Processing[END_REF] chapter 6). In accordance with our experimental observations, we will assume that electron-ion recombination is a negligible process inside the sheath and we do not account for it in the charged particles balance. Indeed, in the sheath, the velocity of both particles is very large compared to the thermal velocities and therefore the recombination rate is much smaller there. Besides, the electron density is too low inside the sheath to have a signification recombination process.

We consider a cylindrical geometry, with the parameters shown on figure 5: Electrons emitted from the cathode at r = R are accelerated by an electric field, ionizing the neutral gas background. The increase of the radial electron flux Γ e from the cathode to the bulk is given by the following equation:

e e r dr r d r Γ - = Γ ) ( ) ( 1 α (1)
where we have assumed a one-dimentional cylindrical geometry, and where ) (r α is the first Townsend coefficient and represents the inverse of an « ionization » mean free path.

This equation is integrated between R-d and R to obtain:

            - Γ = - Γ ∫ - R d R e e dr r d R R R d R ) ( exp ) ( ) ( α (2)
Writing the continuity of total charge, the electron current ) ( d R I eleaving the sheath edge at r=R-d minus the electron current ) (R I e emitted from the cathode, is equal to the ion current ) (R I i striking the cathode at r = R minus the ion current ) ( d R I i -entering the sheath at r = R-d such that the fluxes obey the following equation:

) ( ) ( ) ( ) ( ) ( ) ( R R d R d R d R d R R R e e i i Γ - - Γ - = - Γ - - Γ (3) Moreover, ) ( ) ( R R i e Γ = Γ γ
, where γ is the secondary emission coefficient of the cathode material. The ion current entering the sheath is considered to be a fraction (to be estimated) of the ion current at the cathode:

) ( ) ( ) ( R R d R d R i i Γ = - Γ - ξ
. This fraction ξ is supposed to be smaller than 0.5 in order to keep an ionizing sheath. Indeed, when 0 = ξ , the flux entering the sheath is negligible compared to the flux produced in the sheath (by ionization). This is the typical regime of the cathode fall in a DC discharge tube. On the other hand, when 1 = ξ , the flux produced in the sheath is negligible and this model breaks down. This case, typical of low-pressure wave-excited discharges where the sheath is usually free of ionization, is considered in the next section.

Expressing ) ( d R e - Γ as a function of ) (R e Γ
leads to the following integral form:

r 0 R d sheath bulk         + - = ∫ - 1 1 ln ) ( γ ξ α R d R dr r (4)
At this point, we have a relation between ) (r α and d, the sheath thickness that we wish to calculate. We then need an expression of ) (r α in order to calculate the integral (4). The coefficient α is a complicated function of the pressure and of the accelerating field which is very difficult to calculate self-consistently. However, α might be expected to be expressed in the form [21]:

) exp( E p B A p β β α - = with ) ( 300 K T = β ( 5
)
where p is the pressure measured experimentally and E the electric field. The factor β accounts for higher gas temperature, and therefore lower neutral gas density, inside the micro-hole. The constants A and B are determined experimentally for each gas and found to be roughly constant over a restricted range of E/p for any given gas [START_REF] Lieberman | Principle of Plasma Discharges and Materials Processing[END_REF]. an argon gas, A = 11.5 cm -1 .Torr -1 and B = 176V.cm -1 .Torr -1 , for a range of E/p of 100-600 V.cm -1 .Torr -1 [START_REF] Lieberman | Principle of Plasma Discharges and Materials Processing[END_REF]. The next step is to determine the radial evolution of the electric field E within the sheath. Here we use the approximation of a high-voltage matrix sheath, that is uniform ion density and electron density essentially equal to zero. This approximation gives in Cartesian coordinates, a linear field variation with the distance from the cathode. This variation has been confirmed experimentally by the direct measurement of E [START_REF] Debontride | [END_REF] or by observing the deflecting of a electron beam projected across the sheath, the deflection being proportional to the electric field [START_REF] Cobine | Gaseous Conductors Dover[END_REF]. Using the matrix sheath approximation and the Poisson's equation in cylindrical coordinates, the expression of the electric field E as a function of r is:

        - - - = r d R r d Rd R E r E 2 2 0 ) ( 2 ) ( (6)
where 0 E is the value of the electric field at the cathode (r=R). Due to the cylindrical geometry, the electric field does not vary linearly in space, unless in the case d<<R. Using the relation between the electric field and the electric potential V,

V E ∇ - = r r
, and integrating it between R-d to R, we find the relation between 0 E and the cathode sheath voltage c V :

c V R d R d R d R d R d Rd E - - + - - = ln ) ( 2 ) 2 ( 2 2 2 2 0 (7)
We finally substitute the expression of α as a function of c V in equation ( 4) to obtain:

∫ -             - -       - - + - - =         + - R d R C dr r d R r R d R d R d R d V p B p A 2 2 ) ( 1 ln ) ( 2 ) 2 ( 2 exp 1 1 ln β β γ ξ (8)
This integral equation is solved numerically to obtain the sheath thickness for each neutral gas pressure, with a secondary emission coefficient of γ=0.07 [4], a cathode sheath voltage V C of 250 V and a radius of the MHCD of 200 µm.

A plasma transport model is required to evaluate the parameter ξ, which represents the fraction of the flux produced in the plasma to the flux produced within the sheath. In this work, we keep ξ as a parameter and evaluate its influence on the calculated sheath thickness d. The theoretical values of the sheath thickness are compared with the experimental results on figure 6 for three values of ξ between 0 and 0.5: We first see that the sheath size decays when ξ increases. However, we note that for ξ<0.5, the influence on the calculated sheath thickness remains weak. We further observe that the maxima of both emission lines are located just after the sheath edge whatever the value of ξ (with the exception of the case at 30 Torr, ξ = 0). Moreover, the evolution of the sheath thickness with the pressure follows the same trends as that of the maxima of the emission lines.

The above model does not allow the possibility of a sheath free of ionization to be investigated. Indeed, letting, ξ = 1 would result in a sheath thickness equal to zero. In the next section, we develop a model appropriate for this case.

Case of a non ionizing sheath

The calculation of the sheath size in a non-ionizing sheath can be performed in a self-consistent fashion. The model is known as the collisional Child-Law, and is described in text books [see for instance 18, page 185]. It consists of the following set of equations:

         = = = = ) ( ) ( 0 ) ( ) ( 0 d en divE c divJ b u en J a E u i i i i i i i ε µ (9)
The first equation (9a) indicates that the ion motion is limited by collisions with neutrals. The ion velocity u i is therefore a function of the electric field E and of the ion mobility µ i . In the sheath, the reduced electric field is relatively high (E/p>10 3 V.cm -1 .Torr -1 ), such that the mobility is not a constant but varies with the ion fluid velocity [20, page 144]:

i i i i u m e π λ µ 2 =
, where e is the elementary charge, m i the ion mass and λ i the ion-neutral mean free path. The second equation (9b) is simply the definition of the current density J i , where n i the ion density. The third equation (9c) is a consequence of the assumption of non-ionizing sheath, and the last one (9d) is the Maxwell-Gauss law.

These four equations may be combined and integrated twice, in cylindrical coordinates, between R-d and R to obtain the electric field:

3 2 2 3 0 ) ) ( 1 ( ) 1 ( 2 ) (       - - + = r d R S R I e m r E C d i i ε γ λ π (10)
To obtain this expression we have used the following boundary conditions,

     = - + = 0 ) ( ) 1 ( ) ( d R E S I R J C d i γ ( 11
)
where I d is the discharge current and S c the surface on which the discharge current is collected at the cathode. Note that since the plasma expands on the cathode side, this surface is not equal to the cathode surface inside the hole. This is an important aspect of the following discussion. Finally, the electric field is further integrated between R-d to R, to obtain the following integral equation:

dr r d R S R I e m V R d R C d i i d ∫ -       - - + = 3 2 2 3 0 ) ) ( 1 ( ) 1 ( 2 ε γ λ π (12)
Here we have assumed that the potential drop across the sheath is equal to the discharge voltage V d . This equation is solved to obtain the evolution of the sheath thickness with the neutral gas pressure. The discharge voltage V d and the discharge current I d were maintained at 250 V and 1 mA respectively. The ion-neutral mean free path for charge exchange is taken to be λ i = 0.3 µm at 100 Torr [20, page 80]. We note that Ar 2 + ions may also represent a significant fraction of the ion current and that consequently λ i may be somewhat slightly different from our estimation.

An other issue is the determination of S C , which is not independent of pressure due to the cathodic plasma expansion. To estimate the evolution of this parameter with pressure, we have measured the size of the region emitting light, noted xR, on the cathode backside. Remembering that the cathode thickness is L=150µm, the surface S C then reads ( )

1 2 2 2 - + = x R RL S c π π (13)
This provides an upper value for S C since J i is not uniform but may decay from the centre of the hole towards the edge of the emitting region.

Figure 7 shows the result of the calculation for x=1 (not including the backside expansion) and x determined experimentally for each pressure (x>1). The sheath size calculated with x=1 is near the maxima of the emission lines. However, we note that the trend with pressure does not agree well with experimental results. When x is a function of pressure, the sheath size is considerably larger, but the trend with pressure is in better agreement with our observations. However, these results are in contradiction with the model hypothesis. Indeed, the maximum of ionization must be near the maximum of the emission intensity of Ar line, which implies that ionization should take place within the sheath. This model seems inappropriate and consequently a large fraction of ionization is likely to occur within the sheath (as described in the previous model of section 4.1).

Conclusions

In this paper, we have used spectrally resolved imaging of the emission of the MHCD to study the spatial dependence of the excitation mechanism inside the discharge. The Micro Hollow Cathode Discharge was operated in the normal glow regime where the plasma does not stay confined inside the hole but also expands on the cathode backside. In addition, we proposed two sheath models to calculate the position of the sheath edge. The first model considers a fully ionizing sheath with a nonself consistent electric field, and the second considers a non-ionizing sheath with variable mobility and self-consistent calculation of the electric field. Our two main conclusions are:

(i) Both ionic and atomic lines present sharp electron impact excitation peaks near the cathode and the position of these peaks move towards the cathode as the pressure increases, in very good agreement with the position of the sheath edge calculated from the ionizing sheath model. The sheath edge calculated with the non-ionizing sheath model does not coincide with the position of the emission peaks, which suggests that a significant fraction of the ionization occurs in the sheath and consequently the ion flux produced in the sheath represents a significant fraction of the total ion flux at the cathode surface. (ii)

In addition to the sharp peaks near the plasma edge, the Ar emission line exhibits a dome-like profile, with a maximum in the discharge centre. This was attributed to the recombination of electrons with the molecular (Ar 2

+

) ions, a process that dominates in the discharge centre were the densities are maximal.

FIGURE 2 .

 2 FIGURE 2. CCD imaging of Ar 427.217 nm (left) and Ar + 427.752 nm (right) lines in the 400 µm diameter hole, at 100 Torr argon pressure and 1mA discharge current.

FIGURE 3 .

 3 FIGURE 3. Radial profiles of the emission intensity of Ar 427,217 nm and Ar + 427,752 nm lines in the 400 µm diameter hole at 1 mA and 30, 50, 100, 150 and 200 Torr argon pressure.

FIGURE 4 .

 4 FIGURE 4. Experimental (black point) and simulated (red line) emission spectra in the 400 µm diameter hole for a pressure of 100 Torr and a 1 mA discharge current.

FIGURE 5 .

 5 FIGURE 5. Schematic structure of the MHCD with a cylindrical cathode of radius R, separated from the plasma (in black) by an annular sheath of thickness d.

FIGURE 6 .

 6 FIGURE 6. Pressure dependence of the distance from the cathode of the electron excitation maxima of atomic (black open circles) and ionic (red open stars) lines, and of the calculated sheath thickness for different values of ξ, in the 400 µm diameter hole at 1mA.

FIGURE 7 .

 7 FIGURE 7. Position of the maxima (evaluated from the cathode) due to electron-impact excitation of the atomic line (black open circles) and of the ionic line (red open stars), as a function of pressure. The lines give the sheath thickness calculated for a non-ionizing sheath for x=1 (blue dotted line) and for x=f(P) (blue line). The hole diameter is 400 µm and the discharge current is 1mA.