Ph.D Julia Rousseau

M.D Rolf F Barth

Ph.D Melvin L Moeschberger

Ph.D Hélène Elleaume
email: h.elleaume@esrf.fr

Efficacy of intracerebral delivery of Carboplatin in combination with photon irradiation for treatment of F98 glioma-bearing rats

Keywords: F98 rat glioma, carboplatin, intracerebral delivery, osmotic pumps, radiotherapy

Efficacy of radiotherapy combined with prolonged

Introduction

Despite multimodality treatment strategies that combine surgery, chemotherapy, and radiotherapy, high grade gliomas are almost uniformly fatal [START_REF] Behin | Primary brain tumours in adults[END_REF]. One of the most important advances in the treatment of glioblastoma multiforme (GBM) over the past 20 years has been the introduction of temozolomide in combination with photon irradiation, followed by repetitive cycles of temozolomide. Although a significant subset of patients were survivors at 24 months, the overall increase in median lifespan of GBM patients was only 2.5 months [START_REF] Stupp | Chemoradiotherapy in malignant glioma: standard of care and future directions[END_REF] indicating that there still is an urgent need to develop innovative therapeutic approaches to treat brain tumors. Experimental types of radiation therapy based on diverse physical principles, such as boron neutron capture therapy (BNCT) [START_REF] Barth | Boron neutron capture therapy of cancer: current status and future prospects[END_REF] and stereotactic synchrotron radiotherapy (SSR) [START_REF] Adam | Prolonged survival of Fischer rats bearing F98 glioma after iodine-enhanced synchrotron stereotactic radiotherapy[END_REF][START_REF] Biston | Cure of Fisher rats bearing radioresistant F98 glioma treated with cis-platinum and irradiated with monochromatic synchrotron Xrays[END_REF][START_REF] Rousseau | Enhanced survival and cure of F98 gliomabearing rats following intracerebral delivery of carboplatin in combination with photon irradiation[END_REF], are among those under investigation. Recently, we have reported on the therapeutic efficacy of intratumoral (i.t.) administration of carboplatin by convection-enhanced delivery (CED), followed by irradiation with either 80keV synchrotron X-rays (SSR) or 6MV (LINAC) photons for the treatment of the invariably fatal F98 rat glioma [START_REF] Rousseau | Enhanced survival and cure of F98 gliomabearing rats following intracerebral delivery of carboplatin in combination with photon irradiation[END_REF]. These studies showed that either type of chemo-radiotherapy produced significant prolongations of survival times of F98 glioma-bearing rats with subsets of long-term (>200 days) survivors [START_REF] Rousseau | Enhanced survival and cure of F98 gliomabearing rats following intracerebral delivery of carboplatin in combination with photon irradiation[END_REF]. Similar results were obtained with i.t. delivery of cisplatin by CED in combination with either synchrotron X-rays or 6MV photon irradiations 1 .

Cisplatin and carboplatin are highly effective anti-cancer drugs that have been used clinically to treat various malignancies [START_REF] Kelland | The resurgence of platinum-based cancer chemotherapy[END_REF]. The formation of platinum adducts with DNA can interfere with the repair of radiation-induced damage, which could explain the synergistic interaction between either cisplatin or carboplatin and ionizing radiation [START_REF] Nias | Radiation and platinum drug interaction[END_REF]. The clinical use of systematically administered platinated drugs to treat patients with brain tumors has been Rousseau 5 limited, by their systemic toxicity, but also by their poor ability to penetrate an intact bloodbrain barrier (BBB) in areas where there are microinvasive deposits of tumor [START_REF] Muldoon | Chemotherapy delivery issues in central nervous system malignancy: a reality check[END_REF]. Several approaches have been proposed to bypass the BBB and deliver anticancer drugs directly to the tumor, thereby increasing tumor drug concentrations and reducing the systemic toxicity.

These methods include CED of drugs via catheters placed into the tumor [START_REF] Bobo | Convection-enhanced delivery of macromolecules in the brain[END_REF], or surgical implantation of drug-loaded polymer matrices in the brain [START_REF] Brem | Polymers to treat brain tumours[END_REF][START_REF] Olivi | Interstitial delivery of carboplatin via biodegradable polymers is effective against experimental glioma in the rat[END_REF][START_REF] Emerich | Injectable chemotherapeutic microspheres and glioma II: enhanced survival following implantation into deep inoperable tumors[END_REF][START_REF] Sheleg | Local chemotherapy with cisplatindepot for glioblastoma multiforme[END_REF]. At least with chemosensitive tumors, there has been a clear relationship between the therapeutic response and increasing both the concentration and duration of exposure to carboplatin [START_REF] Olivi | Interstitial delivery of carboplatin via biodegradable polymers is effective against experimental glioma in the rat[END_REF][START_REF] Emerich | Injectable chemotherapeutic microspheres and glioma II: enhanced survival following implantation into deep inoperable tumors[END_REF] or cisplatin [START_REF] Yapp | Cisplatin delivery by biodegradable polymer implant is superior to systemic delivery by osmotic pump or i.p. injection in tumorbearing mice[END_REF]. Using the RG2 model implanted in rats, Emerich et al. have shown that sustained delivery of carboplatin with biodegradable microspheres placed into the tumor produced significantly greater improvements in survival than equipotent bolus injections [START_REF] Emerich | Injectable chemotherapeutic microspheres and glioma II: enhanced survival following implantation into deep inoperable tumors[END_REF]. Similarly, Carson et al. have shown that carboplatin, delivered by osmotic-pumps into the brainstems of rats bearing F98-gliomas, significantly enhanced their survival [START_REF] Carson | New approach to tumor therapy for inoperable areas of the brain: chronic intraparenchymal drug delivery[END_REF][START_REF] Guarnieri | Flexible versus rigid catheters for chronic administration of exogenous agents into central nervous system tissues[END_REF][START_REF] Wu | Section on tumors: Young Investigator Award: Local release of carboplatin via an Alzet mini-osmotic pump prolongs survival in a rat brainstem tumor model[END_REF]. In the present study, we have used carboplatin because it has been shown to be tumoricidal against gliomas both in vitro and in vivo [START_REF] Rousseau | Enhanced survival and cure of F98 gliomabearing rats following intracerebral delivery of carboplatin in combination with photon irradiation[END_REF][START_REF] Kelland | The resurgence of platinum-based cancer chemotherapy[END_REF][START_REF] Olivi | Interstitial delivery of carboplatin via biodegradable polymers is effective against experimental glioma in the rat[END_REF][START_REF] Knox | Mechanism of cytotoxicity of anticancer platinum drugs: evidence that cis-diamminedichloroplatinum(II) and cis-diammine-(1,1-cyclobutanedicarboxylato)platinum(II) differ only in the kinetics of their interaction with DNA[END_REF] and it was less neurotoxic than cisplatin [START_REF] Olivi | Direct delivery of platinum-based antineoplastics to the central nervous system: a toxicity and ultrastructural study[END_REF] when delivered i.c. to rats and monkeys [START_REF] Olivi | Interstitial delivery of carboplatin via biodegradable polymers is effective against experimental glioma in the rat[END_REF][START_REF] Carson | New approach to tumor therapy for inoperable areas of the brain: chronic intraparenchymal drug delivery[END_REF][START_REF] Olivi | Direct delivery of platinum-based antineoplastics to the central nervous system: a toxicity and ultrastructural study[END_REF][START_REF] Degen | Safety and efficacy of convectionenhanced delivery of gemcitabine or carboplatin in a malignant glioma model in rats[END_REF][START_REF] Strege | Toxicity and cerebrospinal fluid levels of carboplatin chronically infused into the brainstem of a primate[END_REF]. As described in detail below, we show that a 6-day i.t. infusion of carboplatin using osmotic pumps, combined with SSR was highly effective in prolonging survival and even curing F98 glioma-bearing rats.

Materials and Methods

Tumor model.

The F98 rat glioma simulates human GBMs in several important ways [START_REF] Tzeng | Phenotype and functional activity of tumorinfiltrating lymphocytes isolated from immunogenic and nonimmunogenic rat brain tumors[END_REF], including its infiltrative pattern of growth within the brain and lack of a curative response to a variety of therapeutic modalities, except for BNCT [START_REF] Barth | Boron neutron capture therapy of brain tumors: enhanced survival and cure following blood-brain barrier disruption and intracarotid injection of sodium borocaptate and boronophenylalanine[END_REF] and our recent studies using i.c. delivery of cisor carboplatin combined with either synchrotron or megavoltage X-irradiation [START_REF] Biston | Cure of Fisher rats bearing radioresistant F98 glioma treated with cis-platinum and irradiated with monochromatic synchrotron Xrays[END_REF][START_REF] Rousseau | Enhanced survival and cure of F98 gliomabearing rats following intracerebral delivery of carboplatin in combination with photon irradiation[END_REF]. F98 Rousseau 6 glioma cells (CRL-2397, American-Type-Culture-Collection, Manassus, VA) were cultured in Dulbecco's Modified Eagle's Medium (DMEM), supplemented with 10% fetal calf serum.

For tumor cells implantation rats were anesthetized with isoflurane, followed by an intraperitoneal injection of ketamine (60 mg/kg body weight [b.w.]) and xylazine (7 mg/kg b.w.) The animals' eyes were coated with an ocular lubricant to prevent keratitis. A 10µL suspension of either 1,000 or 4,000 F98 cells in serum-free DMEM containing 1% agarose (gelling temperature <30°C), was injected stereotactically into the right caudate nucleus using a syringe pump (KDS310, GENEQ Inc., Montréal, Canada). This was mounted on a stereotactic headframe (David-Kopf-Instruments, Tujunga, California) and a Hamilton syringe (702N) was secured to the pump. The cells were injected over 16sec. via a 22-gauge needle, which was inserted 3.5mm to the right of the bregma to a depth of 5.5mm from the skull and then withdrawn to its target depth of 5.0mm. Following implantation, the needle was left in place for 2 minutes and then withdrawn slowly. The hole in the calvarium was filled with bone wax and the operative field was cleaned with betadine before closure of the scalp incision. All operative procedures and animal care were carried out in conformity with the guidelines of the French Government (decree 87-848, 1987, license 7593 and A38071).

Chemotherapy.

Carboplatin (M.W. 371.25; Faulding-Pharmaceuticals S.A, Asnières, France) was diluted in 5% dextrose (Laboratoire-Aguettant, Lyon, France) to obtain a final concentration of 0.5mg/mL. ALZET TM osmotic pumps (model #2001) and brain infusion kits (model #2, rigid stainless-steel cannula, 5mm, 28-gauge) (Charles-River Laboratories, L'Abresles, France) were assembled and filled with carboplatin. The pumps were stored in the dark at 37°C for 24h prior to their use in a sterile solution of 0.9% saline. Seven days after tumor implantation the animals were anesthetized and the scalp re-opened. The bone wax was removed with a needle and the infusion cannula was advanced to a depth of 5mm through the hole made for tumor cell inoculation. The brain infusion kit was fixed in place with surgical glue and the pump was implanted in a subcutaneous pocket, in the mid-scapular region with a sufficient amount of catheter tubing to permit free motion of the animal's head and neck. The pumps were left in place from days 7 to 13, during which time the animals received an infusion of 144µL of either carboplatin (72µg, 194 nmol) or 5% dextrose, delivered at a flow rate of 1µL/h over 6 days, following which the pumps were removed.

X-irradiations.

Stereotactic synchrotron radiotherapy was carried out at the ESRF as previously described [START_REF] Adam | Prolonged survival of Fischer rats bearing F98 glioma after iodine-enhanced synchrotron stereotactic radiotherapy[END_REF][START_REF] Biston | Cure of Fisher rats bearing radioresistant F98 glioma treated with cis-platinum and irradiated with monochromatic synchrotron Xrays[END_REF]. A single 15 Gy dose was delivered on day 14 after tumor implantation, using monochromatic 78.8keV X-rays (80eV energy-bandwidth) [START_REF] Elleaume | Instrumentation of the ESRF medical imaging facility[END_REF]. The right cerebral hemisphere was centred on the rotation axis of the irradiation system and the beam was shaped to 10mm in width and 1mm in height. The dose was delivered while the rat was being rotated, and translating upwards between each of the adjacent 360º arcs so that the irradiated target volume encompassed a cylinder 10mm in diameter and 13 to 15mm in height. The Xray dose rate of 0.3Gy/s was determined using an ion chamber (Unidos, PTW, Freiburg, Germany). Contrast-enhanced imaging was performed by synchrotron computed tomography (SRCT), 15 to 18 days after implantation to confirm the presence of tumor.

Experimental groups.

Initially we evaluated the safety and efficacy of chemotherapy and radiotherapy treatments administered independently. In this study, 4,000 F98 cells were implanted into the brains of male Fischer rats weighing 230-260 g (Charles-River Laboratories, L'Abresles, France). The animals were stratified into four groups, and treated as indicated following tumor (day 0) implantation: Group 1: Untreated controls; Group 2 received a 6-day infusion of carboplatin (144µL containing 72µg) beginning on day 7; Group 3 received a single fraction of 15 Gy on day 14; Group 4 (excipient control) received a 6-day infusion of 5% dextrose alone, which had been used for the dilution of carboplatin, beginning on day 7, in combination with a single 15 Gy dose of X-rays on day 14 (Table 1).

In the second study, we evaluated the combination of chemotherapy and X-irradiation.

The tumors were established by implanting 1,000 F98 cells i.c. The animals were randomized into 4 experimental groups, as follows: Group 5: Untreated-controls; Group 6: Chemotherapy alone; Group 7: X-irradiation alone; Group 8: Chemotherapy in combination with Xirradiation, 24 h after completion of infusion (Table 2).

Monitoring of clinical status and histopathology.

After therapy, the animals were weighed 3X per week and their clinical status was monitored until the end of the study (200 d. and 195 d., in the first and second experiment, respectively). Those animals showing a combination of sustained weight loss, ataxia and periorbital hemorrhage were euthanized by intracardiac injection of Dolethal (150 mg/kg; Vetoquinol, Lure, France). The brains were fixed in 10% buffered formalin, and cut coronally at the level of the optic chiasm and 2mm anterior and posterior to it. Slices were embedded in paraffin, sectioned at 4µm, stained with hematoxylin and eosin (H&E), and examined microscopically to assess histopathologic changes. The tumor size index (TSI) was determined from H&E stained coronal sections, using a semi-quantitative grading scale ranging from 0 to 4, as follows: 0, no tumor; 1, very small (<1mm); 2, small (~1-3 mm); 3, large (~4-7 mm); and 4, massive (>8mm).

Statistical evaluation of survival data.

Kaplan-Meier survival curves were plotted for each group. Differences between groups were assessed for statistical significance by means of the log-rank test (JMP®, SAS-Institute- Rousseau 9 Inc., Cary, NC) or in the case of crossing survival curves, a modification of the Cox proportional hazards model was used to allow for non-proportional hazards [START_REF] Klein | Survival analysis techniques for censored and truncated data[END_REF]. Minimal statistical significance was defined at p<0.05. Those rats still alive at the end of the study were euthanized. The mean survival times (MST), standard errors (SE), and median survival times (MeST) were calculated using the euthanization date for rats that were still alive at the end of the study (censored data). Percent increased lifespan (%ILS) was determined relative to MST or MeST of untreated controls as previously described [START_REF] Rousseau | Enhanced survival and cure of F98 gliomabearing rats following intracerebral delivery of carboplatin in combination with photon irradiation[END_REF]. In those instances where censored data were used to calculate the MST, the "true" MST actually would have been larger. Although there are methods that can be used to estimate the "true" MST, this would have required a number of assumptions. Therefore, we have chosen to record the arithmetic mean even though this underestimated the true mean.

Rousseau 10

Results

Therapeutic response following chemotherapy or X-irradiation alone.

Surgery for pump implantation was well tolerated. Although the animals lost ≤10% of their body weight within 7 days following initiation of chemotherapy, it subsequently was regained and clinically the animals appeared to be normal. Survival data and Kaplan-Meier plots for animals that received chemotherapy alone or X-irradiation alone are shown in Table 1 and Fig. 1, respectively. The survival plots of all treatment groups were significantly different from the untreated controls (p<0.001). The MST±SE of untreated controls was 22±1 d. post tumor implantation compared to 38±2 d. for rats that received irradiation alone, and the MST of the latter was not significantly different (p=0.51) from animals that received infusion of 5% dextrose followed by X-irradiation (34±2 d.). Rats that received carboplatin alone had a MeST of 82 d. and a censored MST of 84±18 d. The corresponding %ILS relative to the median was 273%, there was one cured animal in this group.

Therapeutic response following chemo-radiotherapy.

Survival data following chemo-radiotherapy are summarized in Table 2 and Kaplan-Meier survival plots are shown in Fig. 2. The survival plots of all treatment groups were significantly different from that of the untreated controls (p<0.0015). Untreated rats had a MST of 23±1 d. compared with 44±3 d. for X-irradiated animals. Rats which had received carboplatin alone, had a MeST of 30 d. and a censored MST of 69±20 d., with 3 rats surviving >195 days, at which time the study was terminated. Animals that had received carboplatin, followed by X-irradiation, had a MST of >142±21 d. and a MeST of >195 d., with 6 of 11 rats (55%) alive at the end of the study. This was significantly different from irradiated animals (p=0.0014) or those that received carboplatin alone (p=0.0472). The corresponding %ILSs, based on the MeSTs, were 25%, 85% and 713% (censored) for Rousseau 11 carboplatin alone, radiotherapy alone or chemo-radiotherapy, respectively. It is noteworthy that in the chemo-radiotherapy group, 8 of 11 rats (73%) were still alive at day 100.

Neuropathologic evaluation.

In the first experiment (Table 1), rats that received 5% dextrose as an excipient control, followed by X-irradiation, all had large to massive tumors (TSI=3.13), the histology of which was similar to that of untreated animals, and to those that received X-irradiation alone. Six out of 9 animals that received carboplatin had large to massive tumors with central necrotic zone (TSI=2.44). One animal in this group had no evidence of tumor at the end of the study and was regarded as cured (Fig. 3B).

In the second experiment (Table 2), five of 13 rats that received carboplatin alone, showed no macro-or microscopic evidence of tumor (TSI=1.69), although two brains showed marked dilatation of the lateral ventricles. Microscopic findings in the brains of cured animals included focal accumulations of macrophages with some intracellular dystrophic deposits of calcium, occasional giant cells and microscopic porencephalic cysts (Figs. 3C,3D). There was moderate flattening of choroidal epithelial cells lining the lateral ventricle of the tumor bearing cerebral hemisphere in the brains of cured animals suggesting that this may have been due to a drug effect. Four rats that had been treated with 15Gy had tumors, some of which were large (TSI=3.25). The brains of the 4 other rats from this group were not available for neuropathologic evaluation. Rats that had received carboplatin in combination with X-irradiation had the best response to therapy (TSI=0.63; Table 2 and Fig. 2). A total of 7 out of 11 animals had no microscopic evidence of residual tumor. Several of the cured rats' brains showed right ventricular dilatation, suggesting that they may have had partial aqueductal obstruction at some point in time, and several others had microscopic porencephalic cysts that were surrounded by a light scattering of hemosiderin-laden macrophages. There was mild to moderate flattening of choroidal epithelial cells lining the Rousseau 12 lateral ventricles of the tumor bearing cerebral hemisphere of some animals suggesting a drug radiation related effect [START_REF] Perry | Cancer therapy-associated CNS neuropathology: an update and review of the literature[END_REF]. In contrast, untreated control animals had large (TSI=3.4) infiltrative tumors, which invaded both gray and white matter.

Discussion

In the present study, we have evaluated the therapeutic efficacy of prolonged i.c. infusions of carboplatin in combination with photon irradiation. First, we showed that infusions of carboplatin alone, and infusions of 5% dextrose in combination with X-irradiation, were well tolerated. As expected, the Kaplan-Meier survival plots of animals that had received 5% dextrose (vehicle for carboplatin infusions) in combination with X-irradiation was not statistically different from that of X-irradiated rats (p=0.51) and all of these animals died within <50 days following tumor implantation. The censored MST of 9 animals that had received carboplatin alone was 84±18 d. with one long-term survivor. In the second study, the group, which had received the same chemotherapeutic regimen, had a censored MST of 69±20 d. and 3 of 13 rats were long-term survivors (Table 2). Chemotherapy alone improved the survival of F98 glioma-bearing rats, and this was further enhanced by the addition of Xirradiation. The combination of treatments was the most effective protocol with 55% longterm survivors and 73% of the rats alive at 100 days.

Our results demonstrate the therapeutic efficacy of i.c. administration of carboplatin in combination with X-irradiation for the treatment of F98 glioma-bearing rats. Comparable survival data have been obtained by us that confirm those reported here. With similar treatment conditions, F98 glioma-bearing rats that had received prolonged infusion of carboplatin in combination with X-irradiation had MST>117±9 d. (5/12 rats were still alive 200 d. after tumor implantation) compared to 22±2 d. for untreated animals2 . Rousseau 13 In comparison to that which we previously have reported [START_REF] Rousseau | Enhanced survival and cure of F98 gliomabearing rats following intracerebral delivery of carboplatin in combination with photon irradiation[END_REF], we observed that the initiation of treatment 7 d. rather than 13 d. following tumor implantation, combined with prolonged infusion of carboplatin further improved survival (MeST of >195 d. versus 60d., previously). However, a direct comparison between these studies is difficult because delivery and schedule of irradiations were different: 1×15 Gy versus 3×8 Gy and 72µg of carboplatin delivered over 6 days versus 20µg of carboplatin over 40 minutes. Chemotherapy for the short-term infusions (6) was initiated 13d. after tumor implantation at which time the tumors had a volume of ~20-25mm 3 , as determined by SRCT or MRI [START_REF] Yang | Development of a syngeneic rat brain tumor model expressing EGFRvIII and its use for molecular targeting studies with monoclonal antibody L8A4[END_REF], whereas in the present study, it was started on day 7 at a point in time that the tumor would not have been detectable by SRCT or MRI. Therefore, it was not unexpected that the MSTs were longer since it is well recognized that the earlier treatment is initiated, the better the therapeutic response. This is supported by our own recent experimental data showing that if treatment with carboplatin, followed by 6MV X-irradiation was delayed until d. 17, at which time the tumor volume was ~60-80 mm 3 , the prolongation in MSTs were significantly less than those observed in animals whose treatment was started on d. 13 following implantation 3 . Firm conclusions can not be drawn on the fundamental mechanism of effects and further studies will be carried out to evaluate the importance of these various parameters.

To the best of our knowledge, only two clinical reports describe the direct i.c. infusion of platinated drugs for glioma treatment [START_REF] Sheleg | Local chemotherapy with cisplatindepot for glioblastoma multiforme[END_REF][START_REF] Bouvier | Direct delivery of medication into a brain tumor through multiple chronically implanted catheters[END_REF]. Using 68 catheters, Bouvier et al. infused 8.2mg of cisplatin over 10 days to treat a patient with recurrent malignant glioma. There were no adverse side effects associated with, chemotherapy, catheter implantation or removal.

Although chemotherapy halted progression of the tumor, it recurred, and the patient died 6 months later [START_REF] Bouvier | Direct delivery of medication into a brain tumor through multiple chronically implanted catheters[END_REF]. In the other study, Sheleg et al. administered cisplatin, incorporated into biodegradable polymers, after subtotal resection of the GBM. Twenty polymer discs loaded Rousseau 14 with cisplatin (1 mg/cm 2 ) were placed in the tumor bed. Two to three weeks after surgery, the patients received radiotherapy (60Gy). This combined treatment significantly increased the MST compared to that observed in patients who had received radiotherapy alone (427 versus 211 d., respectively) [START_REF] Sheleg | Local chemotherapy with cisplatindepot for glioblastoma multiforme[END_REF].

A variety of systems has been proposed for the treatment of malignant brain tumors to deliver anti-cancer drugs locally in a sustained-released manner (11, 12, 15-18, 20, 30). Our chemotherapy results are in good agreement with those reported by Olivi et al. [START_REF] Olivi | Interstitial delivery of carboplatin via biodegradable polymers is effective against experimental glioma in the rat[END_REF] using carboplatin-loaded biodegradable polymers for the treatment of the F98-glioma. They reported that 5% carboplatin-loaded polymer (~0.5mg of drug), released over 8 to 13d., produced a threefold increase in MeST compared to controls. Systemic administration of carboplatin also significantly prolonged survival, but this was less effective than i.c. delivery [START_REF] Olivi | Interstitial delivery of carboplatin via biodegradable polymers is effective against experimental glioma in the rat[END_REF]. Therapeutic efficacy was dose-related with an increasing number of early deaths associated with escalating doses of the drug up to 0.5mg [START_REF] Olivi | Interstitial delivery of carboplatin via biodegradable polymers is effective against experimental glioma in the rat[END_REF]. In our study, the amount of carboplatin administered (72µg) was well below that used by Olivi et al. [START_REF] Olivi | Interstitial delivery of carboplatin via biodegradable polymers is effective against experimental glioma in the rat[END_REF]; it was well tolerated and resulted in 11% to 23% long-term survivors for chemotherapy alone (Tables 1 and2). Carson et al. have reported that the infusion of carboplatin over a 7-day period with ALZET TM pumps at a dose of 0.1mg doubled the MST of Fischer rats bearing brainstem implants of either the F98-glioma or 9L gliosarcoma [START_REF] Carson | New approach to tumor therapy for inoperable areas of the brain: chronic intraparenchymal drug delivery[END_REF]. It is noteworthy that the brainstem was not damaged by direct infusion of 200µL of carboplatin (0.5mg/mL) at a flow rate of 1µL/h, and that there was no neurotoxicity until a lethal dose (≥1.0mg/mL) was administered [START_REF] Wu | Section on tumors: Young Investigator Award: Local release of carboplatin via an Alzet mini-osmotic pump prolongs survival in a rat brainstem tumor model[END_REF]. Carboplatin also has been infused into the brainstem of monkeys over extended periods of time, and dose-dependent neurotoxicity only was observed with doses >400µg [START_REF] Strege | Toxicity and cerebrospinal fluid levels of carboplatin chronically infused into the brainstem of a primate[END_REF][START_REF] Storm | A surgical technique for safely placing a drug delivery catheter into the pons of primates: preliminary results of carboplatin infusion[END_REF]. Using atomic absorption spectroscopy, Guarnieri et al. have mapped the distribution of platinum after i.c. infusion of carboplatin using ALZET TM pumps into F98 glioma-bearing rats, with delivery parameters similar to those that we used. Platinum Rousseau 15 concentrations were maximal in brain sections corresponding to the infusion site with diminished amounts (1 to 5µg/g tissue) in sections that were 3mm from the point of infusion [START_REF] Guarnieri | Flexible versus rigid catheters for chronic administration of exogenous agents into central nervous system tissues[END_REF][START_REF] Khan | Infusion rates and drug distribution in brain tumor models in rats[END_REF]. However, this distribution pattern was observed in only half of the brains infused with rigid catheters such as the ones we have used. In contrast, the use of flexible catheters resulted in the expected drug distribution in all of the rats, in both normal and tumor-bearing animals. It was suggested that rigid catheters might result in backflow of the drug into the subdural space along the catheter path [START_REF] Guarnieri | Flexible versus rigid catheters for chronic administration of exogenous agents into central nervous system tissues[END_REF].

In conclusion, our data have demonstrated that intratumoral infusion of carboplatin, followed by either synchrotron, as in the present study, or as previously reported (6) 6MV LINAC photon irradiation, is highly effective for the treatment of F98 glioma bearing rats.

Studies to further improve this chemo-radiotherapeutic approach are in progress and will focus on further optimizing the drug delivery and radiation parameters, and on striking a balance between neurotoxicity and therapeutic efficacy.

Figure Legends

Fig. 1. Kaplan-Meier survival curves for F98 glioma-bearing rats after chemotherapy or radiotherapy. The origin of the x-axis corresponds to tumor implantation. Group 1: untreated (×), Group 2: carboplatin ( ), Group 3: X-irradiation alone (-); and Group 4: 5% dextrose (vehicle) followed by X-irradiation ( ). The survivals of all treatment groups were significantly different from the untreated controls (p<0.001). The survival of the animals that received 5% dextrose infusion (excipient), followed by X-irradiation was not significantly different (p=0.51) from animals that received X-irradiation alone. Fig. 2. Kaplan-Meier survival curves for F98 glioma-bearing rats after chemo-radiotherapy. The origin of the x-axis corresponds to tumor implantation. Group 5: untreated (×); Group 6: X-irradiation (-); Group 7: carboplatin alone ( ), or Group 8: in combination with X-irradiation ( ).The survivals of all treatment groups were significantly different from the untreated controls (p<0.0015). The survival of the animals that received carboplatin, followed by X-irradiation was significantly different from X-irradiated animals (p=0.0014) or those that received carboplatin alone (p=0.0472).

Table legends

Table 1. Survival times of F98 glioma-bearing rats following chemotherapy or radiotherapy alone. Table 2. Survival times of F98 glioma-bearing rats following chemotherapy with or without X-irradiation.

Fig. 3 .

 3 Fig. 3. Histology. A. F98 glioma in an untreated rat. The tumor invaded the gray and white matter and extended into the leptomeningeal space.

Rousseau, J., Barth, R.F. and Elleaume, H. Unpublished data

Rousseau, J., Barth, R.F. and Elleaume, H. Unpublished data.

Acknowledgments: We thank the European Synchrotron Radiation Facility for providing beam time; Mrs. Massart, Mr. Brochard, Dr. Nemoz for help during the experiments; Mr. Dallery for animal care; Dr. Charlety for providing carboplatin; Ms. Martz for secretarial assistance and Mr. Scully for photographic assistance. R.F.B. acknowledges the support of N.I.H. grant 1-R01-CA09845 and the Department of Pathology and a Research Investment Fund grant from The Ohio State University College of Medicine.

Conflicts of interest.

There are no conflicts of interest.

Rousseau 27


Nicolas Coppey

Laure Noé

Jean-Charles Dupin

Pascal Puech

Hugues Vergnes

Marc Monthioux

Brigitte Caussat

SILICON NANOPARTICLE/CARBON NANOTUBE COMPOSITES FOR LI-ION BATTERY ANODES

Introduction

In the context of high demand for energy in mobile devices, many studies deal with the lithium-ion cell technology. Because of its larger uptake ability of Li, silicon was suggested for being used (combined with a conductive material) instead of graphite, currently the most frequently used anode material for combining good electrical conductivity and the ability to host Li as intercalated ions between the graphenes [START_REF] Armand | Building Better Batteries[END_REF][START_REF] Kasavajjula | Nano-and bulk-siliconbased insertion anodes for lithium-ion secondary cells[END_REF]. However, charge/discharge cycles induce fatal structural damages to bulk Si, as a consequence of successive swellings then shrinkings of the Si lattice [START_REF] Liu | Size-Dependent Fracture of Silicon Nanoparticles During Lithiation[END_REF]. Hence bulk graphite and/or Si anode materials could be replaced by nanostructured composites combining mechanical strength, electrical conductivity, and high charge capacity. Carbon nanotubes mixed with silicon nanoparticles appear as a promising material [START_REF] Zhu | Synthesis of carbon nanotube composites with size-controlled silicon nanoparticles[END_REF][START_REF] Wang | Nanostructured Hybrid Silicon/Carbon Nanotube Heterostructures: Reversible High-Capacity Lithium-Ion Anodes[END_REF][START_REF] Martin | Chemical Coupling of Carbon Nanotubes and Silicon Nanoparticles for Improved Negative Electrode Performance in Lithium-Ion Batteries[END_REF].

Here, multi-walled carbon nanotubes Graphistrength® were used as substrate for the Fluidized Bed Chemical Vapour Deposition (FBCVD) of silicon from silane (SiH4). The paper describes the synthesis and characterisation of the nanostructured composite material obtained, as well as the results of the electrochemical tests (involving chargedischarge cycles) carried-out to analyze their behavior as Liion cell anodes.

Experimental

The experimental equipment was presented previously [START_REF] Coppey | Fluidized Bed Chemical Vapor Deposition of Silicon on Carbon Nanotubes for Li-Ion Batteries[END_REF]. It consisted first in a vertical cylindrical stainless steel column, 0.083 m ID and 1 m high. At the bottom of the column, a perforated steel plate provided a homogeneous distribution of the gas phase and its flange was water-cooled to avoid any premature decomposition of silane. At the exhaust, an HEPA13 cartridge allowed capturing the elutriated particles. The reactor was externally heated by a two-zone electrical furnace and its wall temperatures were controlled by two thermocouples connected to PID regulators. Several thermocouples were also placed into a vertical tube of 6 mm in diameter inside the reactor to monitor the axial profile of temperature. Experiments were performed at atmospheric pressure. A differential fast response pressure sensor (LPX5480, DRUCK) measured the total pressure drop across the bed. A DasyLab® system enabled the on-line acquisition of the differential pressure and FB temperatures. N50 electronic grade silane SiH4 and Alpha1 high purity nitrogen (from AIR LIQUIDE) were supplied to the bottom of the bed through a ball rotameter (GT1355, BROOKS INSTRUMENTS) and a mass flow meter (FC7710CD, AERA), respectively.

Multi-walled carbon nanotubes Graphistrength® from ARKEMA (called CNTs in the following), were used without any purification treatment. They are entangled in coarsely spherical balls (CNT balls), with a Sauter diameter of 338 microns in the average, which could be fluidized homogeneously, with excellent contact between the solid and gas phases, according to preliminary experiments. Nanotubes walls are made of 10-15 concentric tubes of graphene with a mean external diameter of 12 nm. The BET specific surface area of CNT balls is 233 m 2 /g. The deposition conditions were chosen in order to exalt gaseous species diffusion through the porosity of the CNT balls. 100 g of CNT were used per run. For all experiments, we chose to keep constant the reactor temperature at 500 °C, the total gas flow rate and the inlet SiH4 concentration at 10 vol. %; the mass of deposited silicon was then controlled by the run durations. They were chosen in order to cover the 10-70 wt. % range of deposited silicon, according to literature results about C/Si anode materials [START_REF] Wang | Nanostructured Hybrid Silicon/Carbon Nanotube Heterostructures: Reversible High-Capacity Lithium-Ion Anodes[END_REF][START_REF] Martin | Chemical Coupling of Carbon Nanotubes and Silicon Nanoparticles for Improved Negative Electrode Performance in Lithium-Ion Batteries[END_REF].

The laser scattering size analyses of CNT balls after airdispersion under an over pressure of 0.1 bar were performed with a Malvern MasterSizer2000 setup. The specific surface area of the CNT was obtained by the Brunauer-Emmet-Teller (BET) method, using a physisorption analyser MICROMERITICS ASAP2010, after 2 hours of degassing at 200 °C. The mass of Si deposited was measured by bed weighing and by ICP-AES chemical analysis, on a THERMO-FISHER iCAP6300. The powder morphology was observed by Transmission Electron Microscopy (TEM) on a PHILIPS CM30 on which X-photon energy dispersive spectroscopy (EDAX), with a 150 kV accelerating voltage, was also performed. Raman spectra were recorded at room temperature using the MicroRaman instrument from RENISHAW INC. The scattered light was collected in backscattering geometry using a microscope objective x100 to focus the laser beam (514 nm) on the sample. X-ray photoelectron spectroscopy was performed on a KRATOS Axis Ultra 165, with aluminium X-ray source.

Electrochemical tests were conducted with a prototype half-cell using a 15 cm 3 PTFE bescher filled with 5 cm 3 of electrolyte, closed with a lid supporting working and counter electrodes plunged into the electrolyte. A copper mesh was folded to receive the as-synthesized CNT-Si balls and was connected to the working electrode plot. The counter electrode was a 1 cm 2 lithium foil, and 1 M LiPF6 in ethylene carbonate (EC) / diethyle carbonate (DEC) (1:1 vol. ratio) was employed as the electrolyte. The cell was assembled and tested in an argon-filled glovebox (Compact 21, MBRAUN,[H2O]

Results and Discussion

In previous studies [START_REF] Coppey | Fluidized Bed Chemical Vapor Deposition of Silicon on Carbon Nanotubes for Li-Ion Batteries[END_REF], silicon revealed to be deposited as nanoparticles (NPs), uniformly distributed inside the CNT balls. The nucleation occurred randomly with a possible preference for the morphological defects of the tubes. NPs are mostly individualized, suggesting that the reactivity of gaseous precursors is low with respect to nucleation. Moreover, the mean NP diameter tends to increase from 11 to 36 nm with the mass of deposited Si within the 10-70 wt. % range.

From high-resolution transmission electron microscopy (HRTEM), no crystalline structure could be observed for the Si NPs (Fig. 1). This result was wished because amorphous silicon could reduce the event of irreversible loss that usually occurs at the first lithiation-delithiation cycle for crystalline silicon [START_REF] Obrovac | Reversible Cycling of Crystalline Silicon Powder[END_REF]. It can be explained by the quite low deposition temperature used (500 °C).

From X-photon energy dispersive spectroscopy (X-EDS) line scan spectra of a Si NP (Fig. 2), the O signal follows the Si signal, indicating that the whole volume of the NP is affected by an oxidation event. The bulk O/Si atomic ratio is estimated at xxxxx. Oxygen atoms come from the contact with ambient and they have diffused until the centre of the NP. A carbon signal is also detected, however with a contribution from the surrounding nanotubes with an extent which cannot be quantified. Raman spectroscopy (at 514 nm wavelength) results seem to confirm that Si NPs are slightly oxidized, and indicate the presence of hydrogen due to the silane pyrolysis generated C-H or Si-H bonds, as revealed by a strong luminescence (results not shown).

From X-ray photoelectron spectroscopy (XPS), silicon appeared in 3 different chemical environments (Fig. 3): mostly in Si° with 2p 3/2 and 2p 1/2 peaks at 99.4 eV and 100.0 eV respectively, and SiO2 at 103.5 eV and 104.2 eV. Chemical C-Si bonds, scanned at 101.2 eV and 102.0 eV, were also found, probably between the surface of the Si NPs and that of the CNTs, respectively. Within the surface layer analysed by XPS -depth penetration is about 2 nm in such a material -the Si/O atomic ratio is estimated at ~0.9 by integrating the peak areas. This possible surface enrichment in O could not be seen by X-EDS because of the electron probe size used (~10 nm). The whole NP material can then be described as a Si(Cx)OyHz amorphous material (with x, y, and z << 1).

Several identical CNT samples loaded with 30 wt. % of silicon were tested as Li-ion battery anode. Reproducible results were obtained. From the voltage profile (Fig. 4), a plateau beginning around 250 mV vs Li + /Li can be observed, corresponding to the lithiation of amorphous silicon and, in the case of the first discharge, to the formation of a solid electrolyte interface (SEI) [START_REF] Chen | Hybrid silicon-carbon nanostructured composites as superior anodes for lithium ion batteries[END_REF]. This phenomenon led to an irreversible capacity, added to the first discharge reversible capacity (total of 3187 mAh/g). From the charging voltage profile, a modulation in slope around 0.5 V vs Li + /Li can be attributed to the delithiation reaction of amorphous LiSix.

From the charge-discharge characteristics (Fig. 5), a mean capacity loss of 2 % per cycle is observed from the 2 nd to 26 th cycle, which could be attributed to the mechanical destruction of Si NPs due to the huge volume change during lithiation, in spite they are not crystalline.

A sample loaded with 70 wt. % of silicon was also tested without any significant result at this time. The formation of an unstable SEI layer was suspected to block the diffusion of Li + ions into the Si Nps.

Conclusions

Thanks to their amorphous structure and their individualized, nanosized morphology, the Si nanoparticles deposited by Fluidized Bed CVD on the multi-walled CNTs from CNT balls are able to be loaded with lithium with excellent specific capacity on the very first charge-discharge cycles. Si/CNT composite balls, used as-synthesized in Cu mesh-pack electrode, ensure good diffusion and uptake of Li+ and simple but efficient electrical contact. However, further studies are needed -actually in progress -to fix the problem of the degradation of the electrode, probably due to the formation of an unstable SEI and the mechanical instability of the NPs, by adjusting the composition of the electrolyte and the current density.

  5 ppm). Constant-current charge-discharge cycles were conducted with a potentiostat RADIOMETER ANALYTICAL at C/10 rate, considering the theoretical specific capacity of silicon (pure silicon: 3579 mAh/g), between 1.2 and 0.05 V vs Li + /Li.

Figure 3 :Figure 1 :

 31 Figure 3: XPS binding energy

Figure 2 :

 2 Figure 2: X-EDS line scan spectra of a 40 nm-large Si nanoparticle. The pseudo-Gaussian shape of the concentration curves reflects the progression of the ~10 nm large probe across the ~45 nm large NP.

Figure 4 :

 4 Figure 4: Voltage profile of a 30 wt % Si-containing sample, cycled at 20 mA/g current density. Specific capacity values are balanced with Si mass.

Figure 5 :

 5 Figure 5: Charge-discharge characteristics of a 30 wt. % Si-containing sample, cycled at 20 mA/g current density.

Acknowledgments

The authors acknowledge Arkema for supplying the Graphistrength® carbon nanotubes, M. Molinier and E. Prévot from LGC/INPT for technical support, C. Rey Rouch, M.L. De Solan and G. Raimbeaux for their help in the numerous characterizations in the Process and Analysis Service at LGC/INPT.


Hidir Duzkaya
email: hduzkaya@gazi.edu.tr

Abderrahmane Beroual
email: abderrahmane.beroual@ec-lyon.fr

Influence of Fullerene Nanoparticles on AC and LI Breakdown Voltages of Natural Ester

Keywords: natural ester, fullerene, AC breakdown, lightning impulse breakdown, Weibull distribution

This study aims to investigate the AC and lightning impulse (LI) breakdown voltage performance of natural ester-based nanofluids enriched with fullerene (C60) nanoparticles, which stand out with their long-term stability and superior dielectric properties. The AC and LI breakdown voltages of these natural ester-based nanofluids with five different concentrations of fullerene (C60) are measured 30 times and the compatibility of these measurements with the statistical distribution is checked with the Anderson-Darling test. The AC breakdown voltage performances of 0.3 g/L and 0.4 g/L C60 nanofluids are 5.1% and 7.8% better, respectively, than natural ester. The LI breakdown voltage performance of 0.1 g/L C60 nanofluid is 8.2% better than natural ester. After accepting the conformity of these measurements with the statistical distribution, AC and LI withstand voltages are calculated at 1%, 10% and 50% breakdown voltage probabilities. The withstand voltages of both AC and LI nanofluids are better than natural ester in almost all samples at 1% breakdown voltage probability. The results show that natural ester-based C60 nanofluids can meet the electrical strength requirements of power system equipment with better AC and LI breakdown voltage performance.

I. INTRODUCTION

Insulating fluids are used in many power system equipment applications such as power transformers, tap changers, cables and capacitors [START_REF] Fofana | 50 years in the development of insulating liquids[END_REF]. These fluids, which are also defined as transformer oil due to their widespread use in power transformers, increase the electrical and thermal performance of these equipment. Some of the main functions of these fluids are protecting the solid insulation material against moisture and air, increasing the dielectric strength, protecting against corrosion and cooling [START_REF] Rao | Alternative dielectric fluids for transformer insulation system: Progress, challenges, and future prospects[END_REF].

Mineral oils have conventionally been used in the insulating fluid industry for over a hundred years [START_REF] Fofana | 50 years in the development of insulating liquids[END_REF]. These oils have key advantages such as relatively good dielectric and cooling performance, low cost, availability and compatibility with solid insulating material in transformers [START_REF] Duzkaya | Statistical Analysis of AC Dielectric Strength of Natural Ester-Based ZnO Nanofluids[END_REF]. The main shortcomings of mineral oils are relatively low flash/fire points, low moisture tolerance, corrosive sulfide decomposition products and low biodegradability [START_REF] Khaled | Statistical investigation of AC dielectric strength of natural ester oil-based Fe3O4, Al2O3, and SiO2 nano-fluids[END_REF]. These thermal, electrical and environmental disadvantages make the search for alternatives a necessity for the billion-liter insulating fluid industry [START_REF] Fofana | 50 years in the development of insulating liquids[END_REF]. Alternative dielectric fluids need to meet requirements such as increase fire safety, improve heat transfer and dielectric strength, increase the service life of power system equipment, be environmentally friendly and sustainability [START_REF] Mahanta | Electrical insulating liquid: A review[END_REF].

Although there are synthetic esters and high molecular weight hydrocarbons among the alternatives of mineral oils, natural esters stand out in terms of critical parameters such as high dielectric strength, environmental impact and availability [START_REF] Rao | Alternative dielectric fluids for transformer insulation system: Progress, challenges, and future prospects[END_REF]. Natural esters are produced from vegetable seeds such as olive, canola, coconut, rapeseed, sunflower and palm [START_REF] Fofana | 50 years in the development of insulating liquids[END_REF]. The main advantages of these esters over mineral oils are almost twice the AC dielectric strength and flash/fire points, complete biodegradability and high moisture tolerance [START_REF] Duzkaya | Statistical Analysis of AC Dielectric Strength of Natural Ester-Based ZnO Nanofluids[END_REF]. These advantages of natural esters have been exploited since the early 1990s as binary mixtures with mineral and synthetic oils for potential more compact designs and retro-filling applications of power transformers [START_REF] Fofana | Challenge of mixed insulating liquids for use in high-voltage transformers. 1. Investigation of mixed liquids[END_REF][START_REF] Beroual | Comparative study of breakdown voltage of mineral, synthetic and natural oils and based mineral oil mixtures under AC and DC voltages[END_REF].

The disadvantages of natural esters include high pour temperatures, oxidation instability, increased dissipation factor at high temperatures, and high viscosity compared to mineral oils [START_REF] Duzkaya | Statistical Analysis of AC Dielectric Strength of Natural Ester-Based ZnO Nanofluids[END_REF][START_REF] Tenbohlen | Application of vegetable oil-based insulating fluids to hermetically sealed power transformers[END_REF]. These disadvantages can cause the electrical and thermal limits determined by the standards to be exceeded in applications where natural esters are used [START_REF] Fofana | Challenge of mixed insulating liquids for use in high-voltage transformers. 1. Investigation of mixed liquids[END_REF][START_REF] Tenbohlen | Application of vegetable oil-based insulating fluids to hermetically sealed power transformers[END_REF].

Nanoparticles have been used to improve the thermal and electrical characteristics of fluids since the 1990s. The thermal properties of these nanoparticle-added nanofluids such as conductivity, heat transfer, diffusivity and convective coefficient can be improved compared to the base fluid [START_REF] Rafiq | A review on properties, opportunities, and challenges of transformer oil-based nanofluids[END_REF]. The dielectric strength of these nanofluids can also improve depending on the nanoparticle and the base fluid [START_REF] Khaled | AC dielectric strength ofmineral oil-based Fe3O4 and Al2O3 nanofluids[END_REF]. Nanoparticles such as Al2O3, Fe3O4, SiO2, TiO2 and ZnO are commonly used to improve the electrical and thermal properties of transformer oils. The AC and positive impulse breakdown voltages of these nanoparticles-added nanofluids can increase up to 50% depending on the type, concentration and shape of the nanoparticle and the type of fluid [START_REF] Rafiq | A review on properties, opportunities, and challenges of transformer oil-based nanofluids[END_REF][START_REF] Khaled | AC dielectric strength ofmineral oil-based Fe3O4 and Al2O3 nanofluids[END_REF][START_REF] Chen | A promising nano-insulating-oil for industrial application: Electrical properties and modification mechanism[END_REF].

Research on fullerene-added nanofluids is a relatively new topic in the literature. Nadolny and Dombek [START_REF] Chen | A promising nano-insulating-oil for industrial application: Electrical properties and modification mechanism[END_REF] determined that the heat transfer factor of 0.1 g/L C60-added synthetic ester-based nanofluid could be improved by 10% and other important parameters did not change compared to the base fluid. Chen et al [2019] reported that the AC, positive and negative LI breakdown voltages of mineral oil-based fullerene nanofluid increased by 18%, 7.5% and 8.3%, respectively, compared to base oil. The AC and LI breakdown voltages of natural ester-based fullerene nanofluids increase by up to 10% compared to pure natural ester in spherical electrode geometry [START_REF] Beroual | AC and impulse breakdown voltages of natural ester based fullerene nanofluids[END_REF].

This study aims to investigate the AC and LI breakdown stress measurements of natural ester-based fullerene nanofluids at five different concentrations. The compatibility of these measurements with the statistical distribution is checked with the Anderson-Darling test and the Weibull distribution function is calculated for all samples. The withstand voltages of natural esters and nanofluids at 1%, 10% and 50% breakdown voltage probabilities are determined by using these distribution functions.

II. EXPERIMENTAL PROCEDURE

A. Preparation of Nanofluids

In this study, Midel eN1204 natural ester, which is a mixture of rapeseed and canola oils, is the base fluid. The fullerene nanoparticle has a purity of 98% and an average particle size of 21 nm. There are one-and two-step methods in the literature for the preparation of nanofluids. The one-step method is based on the principle of simultaneous synthesis and mixing of nanoparticles in a fluid [START_REF] Ahmad | State-of-Art in Nano-Based Dielectric Oil: A Review[END_REF]. Although more homogeneous nanofluids are produced with this method, it is not preferred in transformer oils due to its cost and incompatibility with large-scale industrial production. The two-step method, which is preferred in the preparation of transformer oil-based nanofluids, is based on the principle of producing and mixing the fluid and nanoparticle separately, see Fig. 1. Natural ester-based nanofluids with five different fullerene nanoparticle concentrations between 0.05 g/L and 0.4 g/L are prepared for this study. The detailed procedure followed during the preparation of these nanofluids can be found in the authors' previous work [START_REF] Beroual | AC and impulse breakdown voltages of natural ester based fullerene nanofluids[END_REF].

B. Measurement Procedure

AC breakdown voltages of natural ester and fullerene nanofluids are measured using the BAUR DTA 100C measuring system according to the IEC60156 standard [START_REF]Insulating Liquids-Determination of the Breakdown Voltage at Power Frequency-Test Method; International Electrotechnical Commission (IEC) Standard[END_REF]. LI breakdown voltages are measured using a two-stage Marx generator (200 kV-2 kJ) producing standard impulse voltage characteristics with 1.2/50 µs. For each sample, the AC and LI breakdown voltages are measured 30 times, consisting of 5 sets of 6 measurements. Horizontally positioned spherical electrodes with a diameter of 12.5 mm are used for these measurements. The electrode gap is 2.5 mm and 1.0 mm for AC and LI breakdown voltage measurements, respectively. The rate of rise of the voltage is set as 2 kV/s in AC breakdown voltage measurements. The up-and-down method procedure is followed to be able to measure LI breakdown voltages [START_REF] Primo | Investigation of the lightning impulse breakdown voltage of mineral oil based Fe3O4 nanofluids[END_REF]. These breakdown voltage measurements and the experimental procedures before/after the measurements are detailed in the authors' previous work [START_REF] Beroual | AC and impulse breakdown voltages of natural ester based fullerene nanofluids[END_REF].

III. RESULTS AND DISCUSSIONS

The main characteristics of AC and LI breakdown voltage measurements of natural ester and fullerene nanofluids are discussed in Table 1.

In AC breakdown voltage measurements, the mean and standard deviation data of the samples are calculated. The average AC breakdown voltage of nanofluids with 0.05 g/L, 0.1 g/L and 0.2 g/L fullerene concentrations is lower than the base fluid. This characteristic change can be observed in lowconcentration nanoparticle-added natural ester-based nanofluids. This reduction rate was observed around 15% in natural ester-based nanofluids using nanoparticles such as Fe3O4, Al2O3 and SiO2 [START_REF] Khaled | Statistical investigation of AC dielectric strength of natural ester oil-based Fe3O4, Al2O3, and SiO2 nano-fluids[END_REF][START_REF] Khaled | DC breakdown voltage of natural ester oilbased Fe3O4, Al2O3, and SiO2 nanofluids[END_REF]. The AC breakdown voltage strength of nanofluids added with 0.3 g/L and 0.4 g/L C60 increased by 5.1% and 7.8%, respectively, compared to natural esters. The increase in breakdown voltage with C60 concentration can be explained by the nanoparticle's strong electronegativity, photon absorption ability and high static dielectric constant [START_REF] Chen | A promising nano-insulating-oil for industrial application: Electrical properties and modification mechanism[END_REF][START_REF] Nadolny | Electro-insulating nanofluids based on synthetic ester and TiO2 or C60 nanoparticles in power transformer[END_REF].

The LI breakdown voltage performance of nanofluid added with 0.1 g/L C60 is 8.2% better than natural ester, see Table 1. Similarly, the LI breakdown voltage of the samples with 0.05 g/L, 0.2 g/L and 0.3 g/L concentrations are also better than the natural ester. This performance decreases dramatically in nanofluid added with 0.4 g/L C60. The increase in LI breakdown voltage in nanofluids with lower concentration of nanoparticles can be explained by electron trapping and tunneling mechanisms [START_REF] Wang | Electronic Properties of Typical Molecules and the Discharge Mechanism of Vegetable and Mineral Insulating Oils[END_REF]. The superior electrical properties of C60 nanoparticles capture and slow down the electrons between the electrodes as the streamer develops [START_REF] Chen | A promising nano-insulating-oil for industrial application: Electrical properties and modification mechanism[END_REF]. In nanofluid samples with more than 0.2 g/L C60 concentration, the percolation threshold is exceeded and C60 nanoparticles form a conductive layer with the tunneling mechanism [START_REF] Chen | A promising nano-insulating-oil for industrial application: Electrical properties and modification mechanism[END_REF][START_REF] Nadolny | Electro-insulating nanofluids based on synthetic ester and TiO2 or C60 nanoparticles in power transformer[END_REF]. The dramatic decrease in LI breakdown voltage performance after samples with 0.2 g/L C60 concentration can be explained by exceeding the percolation threshold.

The velocity of LI breakdown voltages of nanofluids is higher than that of natural ester. The increase in this velocity in low-concentration nanofluids can be explained by the formation of a more energetic streamer mechanism with increasing trap charge in fullerene nanofluids [START_REF] Nadolny | Electro-insulating nanofluids based on synthetic ester and TiO2 or C60 nanoparticles in power transformer[END_REF][START_REF] Beroual | AC and impulse breakdown voltages of natural ester based fullerene nanofluids[END_REF]. The main reason for the increase in velocity in highly concentrated nanofluids is the development of the tunneling mechanism as a result of the decreasing distance between the nanoparticles and the faster movement of electrons in the streamer channel [START_REF] Chen | A promising nano-insulating-oil for industrial application: Electrical properties and modification mechanism[END_REF][START_REF] Beroual | AC and impulse breakdown voltages of natural ester based fullerene nanofluids[END_REF].

In order to calculate withstand voltages in different breakdown voltages of AC and LI breakdown voltages, the conformity of these measurements with statistical distribution should be tested. The hypothesis of whether the distribution of these measurements fits the 5% significance level (α= 0.05) is determined using the Anderson-Darling test. The Anderson-Darling normality test is very sensitive to the distribution of data outside the mean and is a frequently used method for measurement data [START_REF] Duzkaya | Statistical Analysis of AC Dielectric Strength of Natural Ester-Based ZnO Nanofluids[END_REF][START_REF] Khaled | Statistical investigation of AC dielectric strength of natural ester oil-based Fe3O4, Al2O3, and SiO2 nano-fluids[END_REF].

W and p-value parameters are used to test the statistical distribution conformity hypothesis of this test; W being the coefficient of concordance and the p-value is defined as the probability of error in the statistical law compliance test of the measurement data. In the Anderson-Darling test, W must be lower than 1.5786 in order for the hypothesis to be accepted in the 0.05 significance level [START_REF] Anderson | A test of goodness of fit[END_REF]. The hypothesis is accepted when this p-value is greater than the confidence interval 0.05 [START_REF] Duzkaya | Statistical Analysis of AC Dielectric Strength of Natural Ester-Based ZnO Nanofluids[END_REF]. According to Table 2 andTable 3, both W and p-value meet the limit values defined for the hypothesis to be accepted by the Anderson-Darling test. These results show that both AC and LI breakdown voltage measurements fit the statistical distribution for all samples and withstand voltages can be calculated at different breakdown voltage probabilities.

AC withstand voltages at 1%, 10% and 50% breakdown voltage probabilities are shown in Table 4. The best performance in all AC breakdown voltage probabilities is observed in nanofluid added to 0.4 g/L C60. The withstand voltages of all nanofluid samples are better than natural ester at 1% AC breakdown voltage probability. This performance increase of 0.3 g/L and 0.4 g/L C60 added nanofluids is quite remarkable as 16.0% and 22.3%, respectively. Compared to natural ester, the lowest withstand voltage performance in all breakdown voltage possibilities is 0.05 g/L C60 added nanofluid. The increase in withstand voltage performance of nanofluids at low breakdown voltage possibilities is due to the fact that the standard deviation of natural ester's breakdown voltage measurements is larger than nanofluids, see Table 1. Since the large standard deviation indicates that the measurements are distributed over a wider voltage range, the withstand voltage limit is reduced at a 1% breakdown voltage probability.

The withstand voltages of natural esters and nanofluids in 1%, 10% and 50% breakdown voltage probabilities of LI breakdown voltage measurements are given in Table 5. At 1% breakdown voltage probability, the best LI withstand voltage performance compared to natural ester is 6.8% in 0.05 g/L C60 added nanofluid. The of nanofluid added with 0.1 g/L C60 is 5.2% better than natural ester at this puncture stress probability. The LI withstand voltages of the nanofluid added with 0.1 g/L C60 at 10% and 50% breakdown voltage probabilities are 7.5% and 7.9% better, respectively, than the base fluid. The worst LI withstand voltage performance in all rupture voltage possibilities is observed in the nanofluid with 0.4 g/L C60.

The withstand voltages at 1% and 10% breakdown voltage possibilities are an important parameter in determining the safety limits in the design and operation of power system equipment [START_REF] Martin | Statistical analysis of the AC breakdown voltages of ester based transformer oils[END_REF]. These withstand voltages show a remarkable improvement over natural ester in both AC and LI breakdown voltage possibilities.

IV. CONCLUSIONS

The main findings of this study, which investigated the AC and LI breakdown voltage characteristics of natural esterbased C60 nanofluids, are as follows,

• AC breakdown voltages of nanofluids added with 0.3 g/L and 0.4 g/L C60 are 5.1% and 7.8%, respectively, compared to natural ester.

• LI breakdown voltage performance of nanofluids is better than natural esters in all samples except 0.4 g/L C60 added nanofluid. The best LI breakdown voltage performance is in 0.1 g/L C60 added nanofluid with 5.9% increase.

• At 1% breakdown voltage probability, the AC withstand voltage performance of nanofluids added to 0.3 g/L and 0.4 g/L C60 is 16.0% and 22.3% better, respectively, than natural ester. All nanofluid samples outperform natural ester at withstand voltages at this breakdown voltage probability.

• 0.05 g/L and 0.1 g/L C60 added nanofluids have better LI withstand voltages than natural ester in all breakdown voltage possibilities. This difference is 68% 52%, respectively, at 1% breakdown voltage probability.

• The LI breakdown voltage velocities of nanofluids are higher than that of natural esters. It is thought that the main reason for this phenomenon is the trap charge in low-concentration nanofluids and the tunneling mechanism in high-concentration nanofluids.

• The AC and LI breakdown voltage measurements and withstand voltages of nanofluids outperform natural ester, revealing their potential to be used in high voltage equipment such as power transformers.

Fig. 1 .

 1 Fig.1. Schematic diagram of two-step method for preparation of NFs[START_REF] Duzkaya | Statistical Analysis of AC Dielectric Strength of Natural Ester-Based ZnO Nanofluids[END_REF]

TABLE I .

 I MAIN CHARACTERISTICS OF AC AND LI BDVS[START_REF] Beroual | AC and impulse breakdown voltages of natural ester based fullerene nanofluids[END_REF]

	Base Oil	Solution	Nanofluid			NE	0.05 g/L	0.1 g/L	0.2 g/L	0.3 g/L	0.4 g/L
	Stirrer	NP	Ultrasonication Oleic Acid	AC BDV	Mean (kV) Std. Dev. (σ) Increment (%)	66.7 5.9	58.3 3.3 -12.6	62.6 4.1 -6.1	63.2 4.0 -5.2	70.1 5.0 5.1	71.9 5.6 7.8
					Mean (kV)	75.7	78.9	81.9	79.7	78.0	67.3
				LI BDV	Std. Dev. (σ) Increment (%) BDV Time (µs)	5.3 3.39	4.3 4.2 1.75	5.9 8.2 2.82	5.6 5.3 2.34	6.9 3.3 1.61	3.5 -11.1 2.44
						(km/s)	0.29	0.58	0.35	0.43	0.62	0.41
				TABLE II.	ANDERSON-DARLING TEST OF CONFORMITY TO WEIBULL
						DISTRIBUTION OF AC BDVS
						W		p-value		Conformity of Weibull Distribution
				NE		0.5449		0.1611			Accept
				0.05 g/L	0.2381		0.7620			Accept
				0.1 g/L	0.3660		0.4361			Accept
				0.2 g/L	0.3114		0.5328			Accept
				0.3 g/L	0.2018		0.8676			Accept
				0.4 g/L	0.2934		0.5784			Accept
				TABLE III.	ANDERSON-DARLING TEST OF CONFORMITY TO WEIBULL
						DISTRIBUTION OF LI BDVS OF NE AND NFS
						W		p-value		Conformity of Weibull Distribution
				NE		0.3512		0.4716			Accept
				0.05 g/L	0.6760		0.0698			Accept
				0.1 g/L	0.5982		0.1099			Accept
				0.2 g/L	0.5769		0.1334			Accept
				0.3 g/L	0.4078		0.3485			Accept
				0.4 g/L	0.4374		0.2964			Accept

TABLE III .

 III AC WITHSTAND VOLTAGES AT DIFFERENT BREAKDOWN PROBABILITIES DEDUCED FROM WEIBULL DISTRIBUTION

	BDV Probability (%)	NE AC BDV (kV)	Concentration of NFs	AC BDV (kV)	Increment (%)
			0.05 g/L	48.14	1.7
			0.1 g/L	50.46	6.6
	1	47.32	0.2 g/L	52.84	11.7
			0.3 g/L	54.88	16.0
			0.4 g/L	57.88	22.3
			0.05 g/L	53.90	-6.2
			0.1 g/L	56.95	-0.9
	10	57.46	0.2 g/L	58.50	1.8
			0.3 g/L	63.10	9.8
			0.4 g/L	65.37	13.8
			0.05 g/L	59.07	-13.07
			0.1 g/L	62.80	-7.6
	50	67.95	0.2 g/L	63.47	-6.6
			0.3 g/L	70.61	3.9
			0.4 g/L	72.35	6.5
	TABLE V.	LI WITHSTAND VOLTAGES AT DIFFERENT BREAKDOWN
	PROBABILITIES DEDUCED FROM WEIBULL DISTRIBUTION
	BDV Probability (%)	NE LI BDV (kV)	Concentration of NFs	LI BDV (kV)	Increment (%)
			0.05 g/L	64.82	6.8
			0.1 g/L	63.83	5.2
	1	60.68	0.2 g/L	61.80	1.8
			0.3 g/L	60.58	-0.2
			0.4 g/L	56.72	-6.5
			0.05 g/L	72.60	6.0
			0.1 g/L	73.64	7.5
	10	68.50	0.2 g/L	71.69	5.0
			0.3 g/L	69.50	1.5
			0.4 g/L	62.17	-9.2
			0.05 g/L	79.49	3.8
			0.1 g/L	82.62	7.9
	50	76.56	0.2 g/L	80.78	5.5
			0.3 g/L	78.57	2.6
			0.4 g/L	66.81	-12.7


Abderrahmane Beroual

Similarities Between Creeping Discharges Propagating at Solid/Liquid and Solid/Gas Interfaces: Variation Laws of the Stopping Length

Keywords: Similarities Between Creeping Discharges Propagating at Solid/Liquid creeping discharges, solid/liquid interface, solid/gas interface, stopping length, variation laws of stopping length

I. INTRODUCTION

From electrical insulation viewpoint, the triple junction constitutes the weakest point in any high voltage component or equipment (bushings, transformer windings, switchgears, insulators, gas circuit breaker, gas insulating lines, cable ends and connectors, power capacitors …). Indeed, when the applied voltage, and thence the electric field, reaches a critical value, partial discharges (PDs) can be initiated in the vicinity of this zone. The increase of the voltage leads these PDs to develop and to transform into surface discharges (creeping discharges) which, by propagating over the insulators surface, can lead to flashover. This results in partial or total deterioration of the insulation system requiring the repair of equipment or even their replacement. To avoid such situations, the designers of high voltage apparatus have to know the characteristics of these discharges and their evolution (morphology and length) depending on the physical and geometrical properties of solid insulator, nature of fluid (liquid or gas), pressure, the electrodes geometry and the voltage (waveform, amplitude and application time). Therefore, the knowledge of the optical and electrical parameters of these discharges and their evolution during the operation of high voltage switchgear is of great interest for industrial applications in order to minimize maintenance requirements and reduce the risk of failure. Numerous studies have been devoted to the characterization of this type of discharge versus various parameters as well as to the mechanisms involving in their development . Thus, the shape of these discharges and the variation of their stopping length Lf (maximum radial extension) and of the associated current and electric charge as a function of the physical and geometric properties of the solid and fluid constituents of the insulation system, the waveform of voltage and pressure, have been studied [START_REF] Bedoui | Creeping Discharge in Transformer Oil under DC Voltage[END_REF][START_REF] Kebbabi | Influence of solid insulating thickness on the morophology of creeping discharges propagating over solid/liquid interfaces -Fractal analysis[END_REF][START_REF] Kebbabi | Influence of the properties of materials and the hydrostatic pressure on creepage discharge characteristics over solid/liquid interfaces[END_REF][START_REF] Kebbabi | Optical and Electrical Investigations on Creeping Discharges over Solid/Liquid Interface under Lightning Impulse Voltage[END_REF][START_REF] Beroual | Influence of the Voltage Waveform and Hydrostatic Pressure on Morphology of Final Length of Discharges Propagating over Solid/Liquid Interfaces[END_REF][START_REF] Beroual | Influence of hydrostatic pressure on morphology and final length of creeping discharges over solid/liquid interfaces under impulse voltages[END_REF][START_REF] Beroual | Characterization of Surface Discharges Propagating over Pressboard in Mineral and Vegetable Oils and their Fractal Analysis[END_REF][START_REF] Reffas | Comparison of dielectric properties of olive oil, mineral oil, and other natural and synthetic ester liquids under AC and lightning impulse stresses[END_REF][START_REF] Beroual | Effect of Micro-fillers in PTFE Insulators on the Characteristics of Surface Discharges in Presence of SF6, CO2 and SF6-CO2 Mixture[END_REF][START_REF] Beroual | Investigation on Creeping Discharges Propagating over Epoxy Resin and Glass Insulators in Presence of Different Gases and Mixtures[END_REF][START_REF] Sadaoui | DC Creeping Discharges over Insulating Surfaces in Different Gases and Mixtures[END_REF][START_REF] Sadaoui | AC Creeping Discharges Propagating over Solid/Gas Interfaces[END_REF][START_REF] Sadaoui | Surface Discharges over Insulators of various materials in presence of SF6, CO2, N2 and SF6-CO2 and SF6-N2 Mixtures under DC voltage[END_REF]. The influence of capacitive phenomena on the shape and length of these discharges has been also investigated [START_REF] Beroual | Influence of Capacitive Effects on the Characteristics of Creeping Discharges Propagating over Solid/Liquid Interfaces under Impulse Voltages[END_REF][START_REF] Beroual | Evidencing of the capacitive effects on the development of creeping discharges at solid/fluid interfaces[END_REF]. This paper deals with the existing similarities between the parameters characterizing the surface discharges propagating radially at solid/liquid and solid/gas interfaces basing on the experimental outcomes acquired by my group on this subject the last twenty years. It is mainly concerned with the stopping length of this type of discharges and the highlighting variation laws of this characteristic parameter as a function of voltage and pressure. These are very useful for the design and sizing of mixed insulation systems.

II. EXPERIMENT

The experimental set-up used for the characterization of creeping discharges is practically the same for the two types of immersion fluids: liquid and gas. It has been described in numerous already published papers [START_REF] Kebbabi | Influence of solid insulating thickness on the morophology of creeping discharges propagating over solid/liquid interfaces -Fractal analysis[END_REF][START_REF] Beroual | Characterization of Surface Discharges Propagating over Pressboard in Mineral and Vegetable Oils and their Fractal Analysis[END_REF][START_REF] Reffas | Comparison of dielectric properties of olive oil, mineral oil, and other natural and synthetic ester liquids under AC and lightning impulse stresses[END_REF][START_REF] Beroual | Effect of Micro-fillers in PTFE Insulators on the Characteristics of Surface Discharges in Presence of SF6, CO2 and SF6-CO2 Mixture[END_REF]. The only difference in the experimental setup is the hydrostatic pressure application system in the case of solid/liquid interfaces and the gas admission and control system in the case of solid/gas interfaces. For the continuity, we recall here its main components. It comprises a cylindrical test cell containing the insulation system (solid/liquid or solid/gas) and a point-plane electrode system (Figure 1); a voltage source (DC, AC or lightning impulse voltage); a visualization/photography and recording system as well as a current measurement probe. The upper cover of test cell is of Plexiglass (transparent material); it serves as a support of the point high voltage electrode and for the observation and monitoring of discharges. The bottom cover of brass serves as a ground electrode (plane electrode). The point electrode of tungsten has a radius of curvature rp of 10 µm.

The samples of the considered solid insulators are circular with thicknesses ranging from 2 to 20mm and dielectric constants varying between 2 and 5; they are inserted between the electrodes so that the tip is placed perpendicularly at their center to allow the radial propagation of the discharges. The investigated immersing liquids are mineral oil, natural and synthetic ester oils; and the gases are SF6, N2, CO2 and their mixtures.

The visualization and recording system of discharges consists of a high SVGA resolution CCD camera connected to a computer through a video acquisition card (Meteor-II/Multi-Channel). The creeping discharges at solid/liquid and solid/gas interfaces present several similarities in their shape, the associated current and electrical charge. These characteristic parameters as well as their length depend on the type of fluid (liquid or gas), the type of insulator and its thickness, dielectric constants of both constituents of insulation system, applied voltage waveform and pressure. Figure 2 shows two examples of discharges propagating on an insulating solid surface, immersed in a liquid and a gas, respectively. In both cases, the discharges propagate radially. It is also observed that the associated current waveforms with these discharges consist of primary currents corresponding to the main discharges followed by secondary currents of opposite sign to that of the main discharge whatever the type of interface (solid/liquid or solid/gas interface) [START_REF] Kebbabi | Optical and Electrical Investigations on Creeping Discharges over Solid/Liquid Interface under Lightning Impulse Voltage[END_REF][START_REF] Beroual | Effect of Micro-fillers in PTFE Insulators on the Characteristics of Surface Discharges in Presence of SF6, CO2 and SF6-CO2 Mixture[END_REF]. As the currents, the measured total electrical charge associated to these discharges is also influenced by the applied voltage, the thickness and the dielectric constant of solid materials; it increases with the applied voltage and/or the dielectric constant of insulator and/or when the thickness of insulator is reduced [START_REF] Kebbabi | Influence of solid insulating thickness on the morophology of creeping discharges propagating over solid/liquid interfaces -Fractal analysis[END_REF][START_REF] Beroual | Influence of the Voltage Waveform and Hydrostatic Pressure on Morphology of Final Length of Discharges Propagating over Solid/Liquid Interfaces[END_REF][START_REF] Beroual | Effect of Micro-fillers in PTFE Insulators on the Characteristics of Surface Discharges in Presence of SF6, CO2 and SF6-CO2 Mixture[END_REF].

The stopping length of creeping discharges Lf is of a great interest for the sizing of insulating systems. Thus, the knowledge of the variation laws of Lf versus the experimental parameters is fundamental.

IV. STOPPING LENGTH OF CREEPING DISCHARGES AND VARIATION LAWS

A. Solid/liquid interface

Influence of voltage

The stopping length of surface discharges varies quasilinearly with voltage whatever the voltage waveform [START_REF] Kebbabi | Optical and Electrical Investigations on Creeping Discharges over Solid/Liquid Interface under Lightning Impulse Voltage[END_REF][START_REF] Beroual | Influence of the Voltage Waveform and Hydrostatic Pressure on Morphology of Final Length of Discharges Propagating over Solid/Liquid Interfaces[END_REF]. Figures 3 to 5 give some examples for solid/liquid interfaces for DC, AC and lightning impulse voltage. Therefore, the Lf=f(U) characteristics can be described by the following relationship

(1) where kv is the slope of straight lines. It is a constant that depends on both materials constituting the insulation system (solid and liquid). U0 is the threshold voltage over which the discharge is initiated. It can be deduced from extrapolation at Lf= 0.

Influence of hydrostatic pressure

By plotting the stopping length of creeping discharges Lf versus the hydrostatic pressure P for different values of the voltage U, we observe that for a given U, Lf decreases quasilinearly when P is increased [START_REF] Beroual | Influence of hydrostatic pressure on morphology and final length of creeping discharges over solid/liquid interfaces under impulse voltages[END_REF]. Lf is all the shorter as the pressure is high. And, for a given voltage and thence a given shape of discharge, there is a pressure for which the discharge disappears. Figure 6 gives an example of the variation of Lf of negative creeping discharges propagating over Bakelite immersed in mineral oil under negative lightning impulse voltage for different crest values of the voltage. So, from these characteristics, one can deduce the disappearance pressure of discharges Pdisp by extrapolation at Lf = 0 mm at a given voltage. Thus, the Lf=f (P) characteristics can be described by the following relationship [START_REF] Atten | Streamer propagation over a liquid/solid interface[END_REF] where kp is the slope of straight lines. As kv, kp is a constant that depends on both materials constituting the insulation system.

B. Creeping discharges over solid/gas interface

Influence of voltage

As for the solid/liquid interfaces, Lf varies quasi-linearly with the voltage, whatever the voltage waveform [START_REF] Sadaoui | DC Creeping Discharges over Insulating Surfaces in Different Gases and Mixtures[END_REF][START_REF] Sadaoui | AC Creeping Discharges Propagating over Solid/Gas Interfaces[END_REF]. Thus, for a given pressure, the Lf=f(U) characteristics can be described by a similar relationship as for liquid/solid interface.

(3)

Influence of gas pressure

As for solid/liquid interface, we also observe that for a given voltage (U), Lf decreases quasi-linearly with increasing the gas pressure P [START_REF] Sadaoui | AC Creeping Discharges Propagating over Solid/Gas Interfaces[END_REF]. Lf is all the shorter as the pressure is high. And, there is a pressure for which the discharge disappears. One can thus deduce the disappearance pressure of discharges Pdisp by extrapolation at Lf = 0 mm. Figure 9 gives an example of the variation of Lf for three types of insulators under AC voltage.

V. CONCLUSION

In addition to the highlighting the similarities between the shape, associated current and electrical charge of creeping discharges propagating over insulators immersed in gases and liquids submitted to different voltage waveforms, it is shown that there is a linear relationship between the stopping length of creeping discharges Lf propagating at both types of interfaces (solid/liquid and solid/gas) and the applied voltage as well as between Lf and pressure. This is very useful for the dimensioning of solid/fluid insulating systems.

Fig. 1 .

 1 Fig. 1. Scheme of the test cell III. EXPERIMENTAL RESULTS AND SIMILARITIES

Fig 2 :

 2 Fig 2: Photographs of discharges propagating over (a) Bakelite (phenoplast resine) sample of 2 mm thickness immersed in mineral oil, under negative lightning impulse voltage the crest value of which is Ucrest=28 kV; and (b) Glass sample of 2 mm thickness immersed in SF6 at an effective pressure of 0.1 MPa under positive DC voltage of 30 kV.

Fig. 3 .

 3 Fig. 3. Stopping length of creeping discharges propagating over three type of insulators of 5 mm thickness immersed in mineral oil versus AC voltage. The dielectric constant (εr) of insulators are 2.9 for Polycarbonate, 5 for glass and 4.8 for Bakelite.

Fig 4 .Fig. 5 .

 45 Fig 4. Stopping length of creeping discharges propagating over Bakelite sample of 5 and 10mm thickness immersed in mineral oil versus negative and positive DC voltage.

Fig. 6 .

 6 Fig. 6. Variation of the stopping length the negative creeping discharge propagating over Bakelite immersed in mineral oil under negative lightning impulse voltage for different crest values of the voltage.

  Figures 7 and 8 give examples of the variation of Lf versus the voltage, for different types of insulators, immersed (a) in SF6 at 0.3 MPa under AC voltage; and (b) in CO2 at 0.3 MPa under DC voltage.

Fig. 7 .

 7 Fig. 7. Stopping length of creeping discharges versus the voltage, under AC; the insulators (glass, Bakelite and epoxy resin) being immersed in SF6 at 0.3 MPa.

Fig. 8 .

 8 Fig. 8. Stopping length of creeping discharges versus the voltage, under DC; the insulators (glass and Bakelite) being immersed in CO2 at 0.3 MPa.

Fig. 9 .

 9 Fig. 9. Stopping length of creeping discharges versus the pressure for glass, Bakelite and epoxy resin samples of 2 mm thickness in N2 under AC voltage; U= 14 kV. It thus possible to write a similar relationship as for solid/liquid interface