Impact of Discharge Current rate of High-current Low-inductance Vacuum Spark on Submicron size Structure in Electrode Surface Area

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Physics Procedia 71 ( 2015 ) 160 – 164

Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi: 10.1016/j.phpro.2015.08.340

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18th Conference on Plasma-Surface Interactions, PSI 2015, 5-6 February 2015, Moscow, Russian

Federation and the 1st Conference on Plasma and Laser Research and Technologies, PLRT 2015,

18-20 February 2015

Impact of discharge current rate of high-current low-inductance

vacuum spark on submicron size structure in electrode surface area

S.A. Sarantsev*, Ya.M. Dvoyeglazov, I.F. Raevskiy

National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe shosse 31, 115409, Moscow, Russia

Abstract

This paper deals with the results of studying the impact of discharge current rate on electrode surface area of high-current low-inductance vacuum spark. Iron electrodes were utilized for research. It was discovered that the size of periodic structure cells on a cathode surface decreased from 600 nm (63 kA) to 150 nm (180 kA) as the discharge current rate grew along with switching to the micropinching mode. The discharge current rate exerted no significant impact on the size of structure elements on anode surface (the structure size at all currents was ~ 400 nm).

Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)

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Keywords: High-current low-inductance vacuum spark; submicron structure; electrode surface; micropinch; Z-pinch.

1. Introduction

High-current low-inductance vacuum spark (HCLIVS) [Kuznetsov et al. (2014), Bashutin et al. (2013), Astrakhantsev et al. (1995)] belongs to Z-pinch discharges [Vikhrev et al. (2012), Vikhrev et al. (2007)]. The difference of this discharge type from a classic Z-pinch or plasma focus (PF) [Baronova et al. (2012)] lies in the fact that substance forming plasma column is represented by an electrode material. Thus, the physical processes on

* Corresponding author. Tel.: +7-495-788-5699 add. 9319; fax: +7-499-324-2111. E-mail address: [email protected]

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electrode surface can significantly influence di most of HCLIVS researches barely study these interelectrode space is poorly investigated. The of HCLIVS from the moment of discharge initi HCLIVS attributes, including thermonuclear pl simple and relatively inexpensive design, this f technological applications and other purposes.

This paper presents the results of studying th formed by the HCLIVS plasma flows.

2. Experimental unit

Experiments were conducted on the Pion Research University MEPhI with HCLIVS-bas The key unit parameters are listed in Table 1.

Electrode system of the Pion unit features a p in diameter with a 3 mm axial aperture. The approximately 5 mm. The discharge is initia material of the discharge initiation system is cer

Table 1. Key Parameters of Pion Facility. Charge voltage

Max. discharge current Stored energy Interelectrode distance Discharge period

Discharge initiation (trigger) Electrode material Vacuum chamber pressure

Fig. 1. Layout

ischarge formation and its development dynamics. Unfor processes. The relation between these processes and pro erefore, it is impossible to fully describe formation and d iation and up to the moment of micropinch disintegration lasma parameters (ne > 10

21

cm-3, T ~ keV), long operatio fact highly impedes the possibility of using the HCLIVS he periodic submicron structure in the electrode surface ar

n unit at the Plasma Physics Department of the Natio ed plasma source. Fig. 1 shows the layout of the interelec pointed anode 3 mm in diameter and a flat cylindrical cat

electrodes are made of Fe. The distance between the e ated from the erosional plasma sources. Plasma formin

ramic. 5÷15 kV 45÷180 kA 0.15÷1.35 kJ 5 mm 5.5 ȝs

Discharge on the of ceramic insulator sur

Fe

10-6_Torr

t of Interelectrode Space of the Pion Unit.

tunately, the cesses in the development . Despite the onal life, and for potential rea, which is onal Nuclear ctrode space. hode 22 mm electrodes is ng dielectric rface

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3. Impact of discharge current on periodic su The cathode surface undergoes significant ch gradually sealed, the bead is formed around it, plasma flows. This edging is a melt pushed out o

Areas 1 (aperture after partial sealing) and 3 the process of a sausage-type instability develop out of the contact area by the flowing plasma p sprays directed from the center towards the perip The cathode surface was studied using a r periodic submicron structure with ~150÷600 nm flows (areas 1 and 2). The size of cells substan configuration of the initiation system (trigger), cathode after 500 discharges (3(b)).

Fig. 2. Sectional View of a Cathode before the Experiment ( edging, D – peripheral contour.

(a)

Fig. 3. Surface of a New Cathode and a Cathode aft

It was found that during the stable microp conditions of no micropinching or its presence

ubmicron structure size in the electrode surface area hanges as the discharge rate grows (Fig. 2). The cathode

and a sharp edging appears outside the area of direct c of the central area by plasma pressure.

(bead) in Fig. 3(b) have direct contact with plasma flow pment in HCLIVS. Area 4 (edging) appears when the me pressure. Due to the fast cooling down, the melt sets an pheral contour.

aster electronic microscopy. The novel discovery show m cells (Fig. 4) appeared in the areas of direct contact w ntially depends on the discharge conditions (discharge c and the electrode material). Fig. 3 shows a new cathode

(left) and after a Few Hundreds of Discharges (right). A – aperture, B –

(b)

fter 500 Discharges. 1 – aperture, 2 – bead, 3 – edging, 4 – peripheral co

pinching of a cathode the structural cells size was sma e in few discharges. Figures 5 and 6 demonstrate this ef

e aperture is contact with ws formed in elt is pushed d forms the wed that the with plasma current rate, (3(a)) and a bead, C – ntour. ller than in ffect. Fig. 5

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shows a graph of dependency of micropinch dependency is valid for the configuration of dependency of the structural cells size on the d are valid for the area 2 (Fig. 3(b)). The cathod ignition direction and in the ignition direction (F

Fig. 4. Periodic Structure on a Cathode

Fig. 5. Probability of micropinch occurrence depending on interelectrode space.

Thus, the structural elements size on a catho grows (during the transition to the stable m influences the structural cells size. Most likel towards the trigger during the development of H

The following experiment was conducted to by the micropinch presence rather than the dis (15 kV, max. discharge current ~180 kA, iron trigger was adjusted to substantially reduce the created a micropinch at 20% of discharges in ~150 to ~600 nm. However, the area of the peri Considering the obtained results, we can sta influence the cathode surface. When a micropi surface is smaller than in case of no micropinc

occurrence in HCLIVS on voltage in the interelectrode electrode system and trigger shown in Fig. 1. Fig. 6 discharge current rate (voltage in the interelectrode spac des were analyzed in two perpendicular directions: trans Fig. 1 – transverse to the figure view and along the figure

e Surface at the Aperture Bottom (left) and in the Bead Area (right).

n voltage in the Fig. 6. Dependency of structural elements size in c area on the discharge current rate.

de decreases from ~ 500 nm to ~ 150 nm as the discharge micropinching). Fig. 6 also demonstrates that the trig

ly, this happens because the sausage-type instability ge HCLIVS configured as shown in Fig. 1.

o confirm the assumption that the structural cells size wa charge current rate. In the conditions of high micropinch electrodes, and 5 mm interelectrode distance), the config micropinch probability discharge by discharge. This kind a series. In such conditions, the structural cells size inc iodic structure covered most part of the cathode.

ate that the structural cells size reflects the intensity of p inch occurs in the discharge, the contact area of plasma ch in HCLIVS (this fact was also confirmed by shadow p

e space. The displays the ce). The data

sverse to the e view). cathode surface e current rate ger location ets displaced as influenced h probability guration of a d of a trigger creased from plasma flows and cathode photos of the

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HCLIVS discharge at the Pion unit). Since the d current rate per unit of a cathode surface (curre higher, so the rate of impact on the cathode formation of micropinch, which result in high-en Upon studying the anode surface, we did no discharge changed conditions (changed dischar cases (fig. 7). The difference occurred only in t discharge current dropped. Under the relatively very point of anode (~ 1 mm2 ); under the high mm2). This fact also confirms the assumption rather than its rate.

Fig. 7. Per

4. Conclusion

The novel discovery is that the submicron str contacting with plasma flows. The structural ce discharge development conditions. This size hi structural cells size decreases as the discharge structural cells size on anode is approximately 4

References

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Vikhrev, V.V., Korolev, V.D. 2007. Neutron generation from Vikhrev, V.V., Mironenko-Marenkov, A.D. 2012. On the sp

discharge current rate is equal in both cases, it can be sta ent density) differs. In case of micropinching, the curren is higher, too. In addition, the acceleration processes nergy ion fluxes, also intensify the impact.

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riodic structure on the anode surface.

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