Some Results of Researches of Powerful Pulse Discharges in Dense Gas Media

HAL Id: hal-00003134

https://hal.archives-ouvertes.fr/hal-00003134

Preprint submitted on 22 Oct 2004

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Some Results of Researches of Powerful Pulse Discharges in Dense Gas Media

Philip Rutberg, A. Bogomaz, A. Budin, V. Kolikov, M. Pinchuk

To cite this version:

Philip Rutberg, A. Bogomaz, A. Budin, V. Kolikov, M. Pinchuk. Some Results of Researches of

Powerful Pulse Discharges in Dense Gas Media. 2004. �hal-00003134�

Some Results of Researches of Powerful Pulse Discharges in Dense Gas Media

P.G.Rutberg, A.A.Bogomaz, A.V.Budin, V.A.Kolikov, M.E.Pinchuk

Institute of Problems of Electrophysics of the Russian Academy of Sciences 191186, Russia, Saint-Petersburg, Dvortsovaya emb., 18

E-mail rutberg@iperas.spb.su

Abstract

The paper represents the results of researches of high pressure and super-high pressure discharges at comparatively high level of introduced energy. A range of parameters of such discharges:

Rate of current rise dJ/dt - (108-1012) A/s Amplitude of current J

-(0.1-2.0) MA Time of discharge (0.005-2) ms.

Initial pressure of the working gas

P

- (0.1-40) MPa.

Energy introduced into gas (0.05-10) MJ

Working gases - hydrogen, helium, nitrogen, air and argon.

The kind of heat transfer between the arc and environmental gas and temperature of the discharge channel vary depending on these conditions and discharge geometry. There can be a turbulent heat transfer from the arc channel to gas, Joule heating of gas at arc movement along the electrodes, gas heating by shock waves. Thus electrode erosion are of considerable importance. The temperature of the discharge channel depending on the above-mentioned conditions varies from 7·10

К to 5·10

К.

Introduction

Investigation results of a high-current discharge in various gas at initial pressure of 0.1-40 MPa have shown that the gas density has strong influence on the arc properties [1-3]. At above- mentioned parameters the discharges have increased the energy content and increased stability.

The challenge has been both in determination of discharge channel characteristics, and research of heat transfer process between the arc and surrounding gas. Thus the principal concern has been given to discharges in light gases - hydrogen and helium, which were used in powerful electric discharge launchers.

High density of energy in the discharge channel due to which the stream of energy of ~10

W/sm

makes inevitable the formation of erosion plasma jets from the electrodes. As it will be shown below, such jets significantly influence on the discharge properties as a whole and on its heat transfer with the surrounding gas. This sharply distinguishes them from the discharges with lower current density, where jets are not of primary importance. At dJ/dt ~ 10

-10

A/s the discharge channel generates shock waves influencing on heat exchange. The rate of energy introduction, type of gas and design of the discharge chamber have the greatest influence on characteristics of the discharge with above-mentioned parameters.

Some most interesting investigation results of such discharges are represented below.

Designs of dense plasma generators

The diagram of the design of a dense plasma generator used at investigation of the discharge with rate of current rise of dJ/dt ~10

A/s and initial pressure 0.1-4.0 MPa, is shown in Fig.1. The discharge was initiated by the explosion of a tungsten wire stretched between the cylindrical cathode and anode copper insert. The internal diameter of the discharge chamber is ~100 mm.

Current amplitude is 200 kA at discharge time of ~1 ms.

Fig. 1. Pulse plasma torch IMPP-1: 1 - cathode, 2 - insulation, 3 – initiating wire, 4 - anode, 5 - diaphragm.

In a rate range of current rise dJ/dt - (0.6 – 1.8)·10

A/s the discharges in hydrogen and nitrogen were investigated at initial pressure of 5-40 MPa. The current amplitude reaches 2 MA at discharge time of ~500 µs. Researches were carried out in the high pressure chamber of a powerful electric discharge generator of dense plasma. The schematic diagram of the generator is shown in Fig. 2a. The final pressure magnitude is close to the ultimate strength of the construction, and it does not allow to weaken the construction of the chamber by a peephole and to carry out optical measurements at full-scale experiment. Therefore a diagnostic chamber with peepholes in two variants of a design (Fig. 2) for more detailed researches of the discharge physics have been created.

In the first variant the geometry of the electric discharge launcher (Fig. 2а) has been retained. In the second variant the researches were carried out with symmetrical axial electrodes (Fig. 2b) that excluded arc movement along the electrodes.

2 3 4 5

1

а) б)

Fig. 2. Diagnostic discharge chambers with various geometry of the electrode system: a) coaxial; 1 - insulation, 2 – seat of the pressure gauge, 3 – initiating wire, 4 - cathode, 5 - peephole, 6 - membrane, 7 – exhaust unit; b) axial; 1 - cathode, 2 - initiating wire, 3 – pressure gauge, 4 - peephole, 5 - anode.

The design diagram of the discharge chamber for the discharge investigation in air at initial

atmospheric pressure and in helium at initial pressure up to 15 MPa at rate of current rise dJ/dt -

6·10

A/s is shown in Fig.3. Maximal discharge current amplitude is ~600 kA. Time of the

discharge current half-period is 6-8 mks. The discharge was initiated by shorting of the discharge

gap by the plasma jet from the injector located in the anode. The piezoelectric pressure gauge with a

rod was placed in the cathode. Pressure on the discharge axis has been measured by this pressure

gauge. The second piezoelectric gauge was located on a wall of the discharge chamber.

Fig. 3. Discharge chamber for the investigation of the fast discharge: 1 – pressure gauges, 2 – Rogowski belt, 3 - peephole, 4 – plasma injector.

Methods of diagnostics

Basic difficulties of arc investigation in similar systems with currents of hundreds and thousands kiloamperes at high and ultrahigh pressure first of all are connected with high density of plasma. Such plasma has a high factor of radiation absorption. In this case there is a situation when traditional optical methods of diagnostics give the information basically about transition and external areas of the arc. High thermal and radiation streams practically exclude an opportunity of contact diagnostics of dense plasma of such discharges. Besides, significant technical requirements imposed upon the diagnostic methods are connected with a high level of the electromagnetic, acoustic and thermal disturbances.

High-speed photographing of the discharge channel both in a frame-by-frame mode and in photo streak mode was used in our researches. Brightness temperature of the discharge channel was determined and its spectrum was photographed. The factor of plasma optical absorption was deter- mined by the plasma translucence. The scheme for speed shadow investigations was developed. The design diagrams of optical measurements are shown in fig. 4-7.

1 2 3 4 5 6 7 8 9 10 7 11 12 13 15

I II III IV V VI

14

Fig. 4. Schematic diagram of the shadow installation: I- high-speed camera ZLV-2, II- matching lens, III- receiving

part of the shadow installation, IV- discharge chamber, V- collimator part of the shadow installation, VI- lighting

source; 1 - high-speed camera ZLV-2, 2 – color filters, 3 – the second component of the matching lens, 4 - visualizing

stop, 5 – the first component of matching lens, 6 – receiving lens, 7 - peepholes, 8 - arc, 9 – cathode, 10 - anode, 11 –

collimator lens, 12 – beam cleaning device, 13 -condenser, 14 - electrodynamic shutter, 15 - argon laser LGN-402.

1

5

4 6 7

3

9 8 2

Fig. 5. Diagram for measurement of brightness temperature: 1 - high-speed camera ZLV-2, 2 – neutral filters, wideband green filter, interference filter 5500 Å, 3 - mirror, 4,8 - peepholes, 5 - arc, 6 - cathode, 7 - anode, 9 – etalon capillary brightness source of Podmoshensky

1 2 3 4 5 6 7 8 9 10 7 12

I II III IV

1 1

1 3

Fig. 6. Laser measurement of optic absorption: I- high-speed camera ZLV-2, II- matching optic, III- discharge chamber, IV- lighting source; 1 - high-speed camera ZLV-2, 2 - color filters, 3 – the third component of the matching optic system, 4 - stop, 5 - the second component of the matching optic system, 6 - the first component of the matching optic system, 7 – peepholes, 8 - arc, 9 - cathode, 10 - anode, 11 - electrodynamic shutter, 12 - argon laser LGN-402, 13 – focusing plane.

III

1 2 3 4 5 6 7 8 9 10 7

I II 13 IV

11 12

V

Fig.7. Measurement of optic absorption by capillary discharge: I- high-speed camera ZLV-2, II- matching optic, III- discharge chamber, IV- matching lens, V- lighting source; 1 - high-speed camera ZLV-2, 2 - color filters, 3 - the third component of the matching optic system, 4 - stop, 5 - the second component of the matching optic system, 6 - the first component of the matching optic system, 7 - peepholes, 8 - arc, 9 - cathode, 10 - anode, 11 - matching lens, 12 - etalon capillary brightness source of Podmoshensky, 13 - focusing plane.

A number of the important data, such as the power put into the discharge volume, dynamics

of the discharge channel movement, formation of shock and acoustic waves, can be obtained by use

of pulse pressure transducers. For pressure measurement alongside with use of serial piezoelectric

transducers T-500, T-6000 we developed a piezoelectric transducer (Fig. 8) with a rod having a

high level of noise immunity and the time resolution of 0.6µs. At pressure measurement on the

discharge axis the piezoelectric transducer with transmitting quartz or ceramic rods was put into the

cathode. Loading on the tourmaline piezoelectric cell decreased due to acoustic mismatch of rods

and reduction of a frontal rod by rubber. The transducer was calibrated by a shock-wave method. At

comparison of transducer indications to other measurements the delay, connected with the time of pressure impulse distribution along the frontal rod of the transducer, was taken into account.

Fig. 8. Diagram of the design of the pulse pressure transducer: 1 - tourmaline piezoelectric cell; 2 - seal; 3 – sound adsorber; 4 - insulation; 5 – cable header

Investigation results

At rate of current increase of dJ/dt - (1–3)·10

А/s the discharges in hydrogen, helium,

nitrogen and argon were investigated at initial pressure of 0.1-4 MPa [4]. The condenser battery,

power supply, inductive storages and shock generators with accumulated energy from 100 kJ up to

10 MJ [5-6] served as a power source. Photo-streaks of the discharge glow in various gases (Fig. 9)

are obtained. The image of photo-streak corresponds to the middle area between the anode and

cathode. Temperatures of the discharge channel and rate of its expansion |(7-15) 10

К and (0.8-

6.0)·10

m/s accordingly (fig. 10 and table 1) have been determined. At estimation of heat exchange

mechanisms between the arc and ambient gas it was found that for hydrogen the gas heating takes

place due to turbulent heat transfer. Modes of burning at which with reduction of the distance

between the electrodes, growth of amplitude of pressure fluctuations of gas were observed during

heating.

Ar

He

H

Fig. 9. Photo-streaks of the discharge channel expansion in hydrogen, helium, argon

Fig. 10. Temperature of the discharge channel in different gases

Table. 1. Rate of the discharge channel expansion in different gases. Measurement of the expansion rate on: I – glow intensity, P – pressure transducer indications.

V

, m/s Gas Р

,atm

I Р

Н

1.8

6 10 12.5

- 152 210 -

175 150 200 250

Не 1.3

10 15 21

600 244 164 190

- 200

- -

Ar 1.9

5.4 9.6 20

192 161 97 76

280 - - -

Occurrence of voltage fluctuations and fluctuations of pressure in the chamber [6]

corresponds to the found out turbulent mode at burning of hydrogen arc, that results in growth of gas heating efficiency with reduction of the distance between the electrodes (Fig. 11). Such hydrogen arc behavior has been registered at amplitude of the discharge current of 80 kA, distances between the anode and cathode of 5-30 cm, magnitude of the initial pressure of hydrogen 1.6 MPa and time of the first half-cycle of the discharge current 2 ms

l = 29 cm l = 9 см

Fig.11. Increasing of pressure amplitude of oscillations at the discharge in hydrogen at decrease of the distance between the electrodes

The discharge in hydrogen differs from the discharges in other gases by significantly higher sums of near electrode drops. They were obtained by extrapolation for zero of the discharge gap length and are equal to U = ~1kV (Fig. 12) [6].

Fig. 12. Dependence of voltage drop at the arc on the discharge gap length 1 – hydrogen 110 kA, 2 – helium, 3 – nitrogen, 4 – argon, 5 – wire vapors, 2-5 – 150-180 kA.

With the increase of the atomic gas number, in which the discharge takes place, the degree of

the discharge turbulence decreases and plays less part in heat exchange between the arc and gas. So

at photo-streaks the discharge glow for helium can still be seen, and for argon the channel expanses symmetrically without ejections.

Growth of the reduction pressure oscillation is observed with decrease of the distance between the electrodes. The occurrence of such oscillations is connected with formation of electrode jets and results in better heat exchange. For example, we have found the increasing of efficiency of energy transfer from the arc to gas with decrease of the distance between electrodes. Interaction of jets from the electrodes is observed in high-speed photos of the discharge in the diagnostic chamber.

Electrode jets, found at investigations in the diagnostic chamber, are also present in the chamber shown in Fig. 1. They are one of the reasons of turbulence occurrence. The additional reason of turbulence occurrence is interaction of electrode jets and initiation of a plasma plate (see Fig. 17, 80 µs). The other reason causing the efficient gas mixing in the chamber volume can be its inflow by erosion electrode jets Fig. 13 [7].

260µs

280 µs

Fig. 13. Shadow picture of gas movement in the discharge volume and corresponding distribution of density in g/cm

] (tungsten electrodes 6 mm diameter, interelectrode gap 12 mm, gas air, anode is at the left, cathode is at the right, J

110 kA).

Recent investigations carried out in the diagnostic chamber have shown that [8, 9] high voltage drops near the electrodes (see Fig.12) in hydrogen are also stipulated by the jets appearing at high energy densities near the electrodes.

Lets estimate various mechanisms of heat conductivity allowing energy transfer from the arc to gas, which experimental value reaches 5 MJ.

First of all our interest is in the mechanism of energy transfer from the discharge to gas corresponding to the case of a burning mode in hydrogen. Density of normal atoms in the discharge zone is about 10

-10

cm

, in the volume, which is not occupied by the discharge, the density is approximately one order higher. As soon as the zone of the discharge occupies less than half of the chamber volume, then the basic mass of gas is concentrated outside the discharge zone.

Radiant energy W

= σ T

St, where σ = 5.668·10

erg/cm

·s·grad

− Stefan-Boltzmann

constant, S − average area of the discharge surface, t − time of energy transfer. For hydrogen on the

assumption of that plasma radiates as a black body with the surface temperature ~10

К, the power

of radiation will be ~6·10

W/cm

. For typical case, corresponding to the initial pressure 10 atm.,

value S=250cm

, t ≈ 1ms, and, hence, W

≈ 1.5·10

J. Thus the energy removed as much as possible at the expense of energy radiation along the whole area of the discharge surface cannot exceed 10%

of the total energy. It is necessary to note that in reality the discharge radiates as a black body only over the range of wave-length to 5.000Å, that means that the radiant energy is rather less.

Energy, dissipating in sound vibrations, can be estimated as follows. Average on area the density of the sound energy is W

=0.5 ρ v

=0.5(p

)

/ ρ (v

)

, where v

− amplitude of sound velocity, p

− amplitude of pressure in a sound wave, ρ − gas density, v

− sound velocity in gas.

In the case been considered (p

~50 atm., v

~5.10

cm/s) W

≈ 5 J/cm

, and if one considers that oscillations take place in the chamber volume, occupied by the discharge, then the total energy, accumulated in oscillations, will be no more than 3·10

J, and make value of about percents from the total discharge energy.

Obviously, the basic mechanism responsible for heat transfer is turbulent heat transfer. Really, the power transferred by such way is equal to

W

=κ

∇TSt.

Strong turbulence can be supported by jets going from the zone of burning to the cold gas.

Reynolds number necessary for the effective turbulence heat exchange should be 10

-10

. In our case Reynolds numbers, calculated by the experimental data, were about 10

-10

.

Factor of turbulence heat exchange κ

=ρC

D

, where D

≈l

v − factor of turbulence diffusion;

l

− characteristic dimension corresponding to the discharge radius; v − average rate of a heterogeneity propagation. At the discharge surface temperature of 10

К we will obtain: D

~4·10

cm

/s (l

≅4 cm, v≅10

cm/s), κ

=0.85·10

·4·10

≅10

W/cm grad. Hence, the power of thermal stream is q

=κ

∇T=10

·10

≈10

W/cm.

Taking into account the average surface of the discharge and time of heat transfer, the turbulence transfer can completely provide the required energy transfer to gas (1.5·10

-5·10

J).

At the discharge in the argon, where turbulence degree is the lowermost, despite the fact that the surface discharge temperature is higher than in hydrogen, the average mass gas temperature in the volume is rather lower, that testifies about the prevailing influence of the turbulence transfer. It is also confirmed by the direct measurements of the arc jet parameters that means that the heat exchange is worse than at the developed turbulence.

Investigations of near electrode processes in the discharges with current strength of 10

-10

А show that the cathode and anode jets are one of the reasons of the turbulence formation.

Gas movement was investigated by shadow methods in a specially created diagnostic discharge chamber. It was found that the direction of the gas movement at the walls is opposite to the gas movement in the jets.

In the range dJ/dt (0.6 – 1.8)·10

А/s [3, 10, 11] the discharges in hydrogen and nitrogen at initial pressures of 5-40 MPa. The investigations were carried out both at operation of the powerful electric discharge plasma generator and in a specially developed diagnostic chamber (Fig. 2а).

The amount of the discharge currents reaches 1.6 MA, and the power input in the discharge

volume is 2 MJ. A very difficult problem of the discharge initiation and support of hydrogen arc

burning was solved at initial pressure of 20-40 MPa (Fig. 14). After initiating the arc is pushed out

from the gap by gas-kinetic and magnetic pressure and moving along the electrodes heats up the gas in

the chamber. The mode of multiply break-downs appeared to be the most interesting one (Fig. 14). In

this case the arc lengthens at movement along the interelectrode gap, its extinction takes place with

further ignition in a narrow interelectrode gap (Fig. 15). This mode was investigated in details in the

diagnostic chamber.

а) hydrogen, initial pressure 40 MPa б) nitrogen, initial pressure 20 MPa Fig. 14. Oscillograms of the discharges in the chamber of power electric discharge launcher.

Fig. 15. Arc movement in the interelectrode gap in the mode of multiply break-downs

For the discharge in nitrogen, in a view of low arc mobility it was possible to estimate the discharge channel temperature directly in the chamber of the electric discharge generator. For hydrogen it was dome for the diagnostic chamber at lower values of the discharge current amplitude and pressure.

The temperature of the discharge channel in nitrogen was estimated on the basis of oscillograms. Sharp voltage fluctuation at the pressure oscillogram can be explained as movement of the cathode spots of the first type (Fig. 14b). The thermal cathode spot of the second type with common molten bath takes place near discharge current maximum. Thus the voltage at the discharge gap decreases sharply.

The arc temperature was estimated on its conductivity. The most complete researches of the

discharge were carried out in the diagnostic chamber at current amplitude up to 500 kA. The origin of

heterogeneities in a kind of current channels can be seeing in photos of the discharge in the diagnostic

chamber already to the 12-th µs. Conductivity of the channel was determined on the 28-th µs, when the

discharge has not lost the compact form yet. At that moment 5 arc channels with average current

density in each channel of 3.0·10

A/cm

are observed that is typical for arc discharges with a

thermal cathode spot. The arc temperature is T ~18·10

К, which corresponds to temperature

measurements in a transitive zone between the arc and surrounding gas on half- width of D-lines of

natrium absorption К 5890, 5896 Ǻ, observable on a background of a continuous spectrum of

radiation of the arch – (11-14) ·10

K.

More rough estimates of the discharge channel temperature in hydrogen on its conductivity for the moment of the current first maximum gives magnitude (25 - 30)·10

К. Inaccuracy in temperature determination is connected with inaccuracy of determination of geometrical dimensions and uncertainty of metal vapor concentration in the arc. The temperature of the discharge channel for the discharge in nitrogen is about 50·10

К. In the latter case it was supposed that the arc burns near to a place of initiation in metal vapors of electrodes.

The recent researches have shown that the steady condition of the discharge channel in ~200 µs is connected with occurrence of powerful electrode jets. Dependence of field intensity in the discharge channel on material of electrodes indicates about jet influence.

Two types of jets, connected with formation of spots of various types, depending on time of electrode worming-up, current density, dimensions and material of an electrode, etc., are observed.

The jets of the first type are connected with formation of fast moving electrode spots. Two types of jets can exist simultaneously. For the first time it was possible to observe the anode and cathode jets through the expanding semitransparent discharge channel (Fig. 16).

The jets of the second type flows out from the general molten bath of the electrode, which is formed after its worming-up (Fig. 17, 80 µs).

4 µs 8 µs 16 µs

20 µs 24 µs 28 µs

Fig. 16. Formation of cathode and anode jets at expansion of semitransparent discharge channel in hydrogen (1 - cath- ode, 2 - anode).

Overall picture of the discharge channel evolution in hydrogen is shown in Fig. 17.

0 мкс 24 мкс 80 мкс 128 мкс

176 мкс 264 мкс, 120 кА 304 мкс 320 мкс, 70 кА

А К

Fig. 17. Shadow photos of the discharge channel evolution in hydrogen with incomplete filtration of self radiation (tungsten electrodes with diameter 6 mm, interelectrode distance 10 mm, initial hydrogen pressure 1 MPa J

- 125 kA)

Parameters of the erosion plasma near the electrode end were determined for the second type jet at 70 µs after the ignition of discharge by brightness temperature T and pressure P measurements [8-9].

The assumption about equality of magnetic and gas-kinetic pressures at the jet base was used for determination of P. The observed jet compression was the basis for this assumption. Magnetic pressure is equal to:

P[MPa]=1.6·10

J r

2

2

[A

/cm

], P=177 MPa at J=3.16·10

А and r=0.3 cm.

The magnitude of metal vapor concentration n and average ion discharge m were determined from the system of equations:

( )















⋅

=

= +

−

−3 3/2 21

3/2

eV cm 10 6 A

)kT m + n(1

= P

n m

T kTln A m

i 1/2

there i – average magnitude of ionization potential for tungsten in the point m +1/2.

The system at T=59·10

К and P=177 MPa for tungsten plasma has the solution: m =3.1, n=5.3·10

cm

.

For magnitudes determined at cathode and anode T, m and n, the magnitude of conductivity σ is ~400 (Ω·cm)

. That corresponds to field strength of E~2.5·10

V/cm at current density of j~10

A/cm

.

Total voltage drop at the zones near electrodes is ~1 kV. Then the total length of zone l, at which this voltage drop take place, is l=U/E=1000/2500=0.4 cm, at total length of the discharge gap of 1.7 cm.

Zones, at which the greatest amount of energy is released, are placed near cathode and anode

and are about 0.2 cm each. The analogue zones were observed in [12, 13] near anode and cathode at

roentgenograms.

One of the reasons of high near electrode voltage drops can be interaction of jets with the surface of the opposite electrode or between each other.

Another one reason of possible high voltage drop in near electrode areas can be non- coincidence of current and jet rate.

Additional field density E, stipulated by rate u across the arc magnetic field B will be, as it is known, E = u × ^{B, where}

r B J

π µ

= 2

Estimations of jet rate give the magnitude of about 10

m/s. If J~10

A, r=3⋅10

m, that corresponds to the experimental data, then E~10

V/cm.

The surface layer ejection from the whole surface of the cathode end face, detected for the first time [14], in addition to erosion in a kind of vapor and drops, is not early known erosion mechanism in similar condition. (Fig. 17 – two last frames).

At dJ/dt=6·10

A/s [15] are investigated the discharges in air at atmospheric pressure and in helium at pressure up to 15 MPa. The maximal amplitude of the discharge current is ≈600kA.

The measurements of pressure along the discharge axis on different diameters have allowed to estimate distribution of current density on the channel radius. Comparison of pressure on the axis, averaged on two diameters 0.2 and 0.4 sm, is carried out for the atmospheric discharge in air.

Distribution of current density on radius is uniform at discharge current amplitude of 144 kA. At increase of the discharge current amplitude up to 290 kA the current density on the smaller radius is higher. In this case the average pressure at diameter of 0.2 sm is 171 MPa. Based upon this pressure magnitude and considering that plasma density in the central part does not exceed 10

cm

it was determined that the temperature in the center is not lower than 10

К.

A clear, uniformly melted, bulging zone with a diameter of 0.4 sm can be seen on the hemispherical cathode at increase of the discharge current up to 500 kA. Occurrence of bulging zone at higher currents agrees with the assumption of increase of current density in the central parts of the discharge. After the discharge current maximum there is sharp increase of rate of the discharge channel expansion caused by reduction of the magnetic pressure magnitude.

If the average density of particles in the discharge channel before the beginning of the stage of magnetic confinement corresponds to the average density of the discharge channel in a stage of thermal wave, then by estimations for the discharge in air the density in the channel is in some times below atmospheric. Therefore the probable temperature of the discharge channel at discharge current amplitude of 500 kA can reach 5·10

K.

The average pulse pressure of 436 MPa is registered for the discharge in helium at initial pressure in the chamber of 10 MPa, discharge current amplitude of 570 kA and diameter of 0.4 sm.

Theoretical calculations show that the temperature on the channel axis is ~10

K [16].

Heating of gas in the discharge chamber is carried out due to shock wave energy. Thus

coincidence of experimental and calculated curves of pulse pressures on the axis and on the wall of

the discharge chamber (Fig. 18) [15, 16] is obtained.

а) б)

Fig. 18. Computational modeling of gas heating by shock waves in fast discharge in helium at initial pressure 15 MPa (Р

– pressure at the discharge axis, Р

– pressure at the wall, а – discharge chamber radius 2 cm, б - 1 sm)[16].

Power shock waves, moving away from the discharge channel, are formed at fast energy input (rate of current rise ≥10

A/s) at generating of the super high pressure discharge. Thus the energy emitted by the channel is absorbed by the front of the shock wave. The calculations show [16] that it is possible to increase the discharge channel temperature, focusing the energy of reflected shock waves along the discharge axis, passing the additional current pulse at the moment of focusing.

Conclusion

As it has been shown, for powerful pulse discharges in dense gas, the mechanism of energy transfer from the arc to the surrounding gas and parameters of the discharge channel are determined first of all by rate of energy input, geometry of the discharge chamber, a kind and initial pressure of the plasma forming gas. Depending on a combination of these parameters there can be a turbulent heat transfer from the arc to gas, Joule heating of gas at movement of the arc along the electrodes, gas heating by shock waves. Thus the electrode erosion jets play a crucial role. Their occurrence is connected with high density of energy supplied to the electrodes. Erosion jets are responsible for high voltage drops near the electrodes. They also create an intensive vortex gas movement in a volume, improving the heat exchange between the arc and the surrounding gas.

The work has been performed under partial support of Russian Fond of Phyndamental Researches (Grant № 02-02-16770 and № 00-15-96604).

References

[1] R.V.Mitin Stationary and pulse arcs of high and superhigh pressure and metods of their diagnos- tics / in collect. Properties of Low temperature Plasma and Methods of their Diagnostic edited by M.F.Zhykov, Novosibirsk, SO «Nauka», 1977, p. 105-138.

[2] B.P.Giterman, D.I.Zenkov, A.I.Pavlovski, N.N.Petrov, E.N.Smirnov, G.M.Spirov. Investigation of powerful quasistationary discharge at megaampere currents. // JTF, v. 52, issue 10, №10, 1982, p.1983-1986.

[3] Ph.G.Rutberg, A.A. Bogomaz, A.V. Budin, V.A. Kolikov, A.G.Kuprin Heating of gas with high initial density by powerful electric arc // Izvestya RAS Energetica, 1998, № 1, p. 100-106.

[4] A.A. Bogomaz, V.S.Borodin, B.P.Levchenko, Ph.G.Rutbrg. Investigation of high current dis- charge in dense plasma generators. // JTF, v. 47, issue 1, 1977, p.121-133.

[5] E.A.Asisov, A.A. Bogomaz, B.P.Levchenko, Ph.G.Rutberg, V.A.Yagnov. High current dis-

charge in nitrogen at power supply from the inductive storage. // JTF, v. XLIX, 1979, p.441-

443.

[6] I.A.Glebov, Ph.G. Rutberg. Powerful Plasma Generators, Moscow, Energoatomizdat, 1985, 153p.

[7] A.A. Bogomaz, A.V. Budin, V.A. Kolikov, М.E. Pinchuk, A.A. Pozubenkov, Ph.G. Rutberg.

Investigation of influence of surrounding arc gas on the magnitude of voltage drops near elec- trodes. / Proceeding of XVIII International Conf. - Action of intensive energy flows on the substance (1-6 March 2003) – «Physics of the Extreme State of Substance – 2003», Cher- nogolovka, 2003, p. 162-163.

[8] A.A. Bogomaz, A.V. Budin, V.A. Kolikov, М.E. Pinchuk, A.A. Pozubenkov, Ph.G. Rutberg.

Influence of cathode and anode jets on the properties of a high-current electric arc. // Techni- cal Physics, vol. 47, № 1, 2002, p. 26-33.

[9] Ph.G. Rutberg, A.A. Bogomaz, A.V. Budin, V.A. Kolikov, М.E. Pinchuk, A.A. Pozubenkov.

Investigation of anode and cathode jets influence on electric arc properties with current up to 500 kA. // IEEE Transaction on Plasma Science (IEEE Trans. Plasma Sci.), vol. 31, № 2, april,2003, p. 201-206.

[10] A.A. Bogomaz, A.V. Budin, S.V.Zacharenkov, V.A.Kolikov, A.I.Kulishevich, I.P.Makarevich, Ph.G.Rutberg, A.Ph. Savvateev. Application of pulse plasma generators for hupervelosity ac- celeration of bodies. // Izvestia RAS Energetica, 1998, № 1, p. 64-79.

[11] A.V.Budin, V.A.Kolikov, B.P.Levchenko, V.V.Leont’ev, I.P.Makarevich, Ph.G.Rutberg, N.A.Shirokov. Factor of transformation of electric power of the arc into the internal energy of the working gas and their shares in the power balance of the electric discharge light-gas launcher. // JTF, v. 64, issue 9, 1994, p. 198-199.

[12] H. Chuagui, M. Favre, R. Savedra, E.S. Wyndham, P. Choi, C. Dumitrescu Zoita, L. Soto, R.

Aliaga Rossel, and I.H. Mitchell. Observation of the plasma dynamics of a vacuum spark from its soft x-ray emission. // Phys. Plasmas, vol. 4, № 10, 1997, pp. 3696-3702.

[13] H. Chuagui, M. Favre, R. Savedra, E.S. Wyndham, L. Soto, P. Choi, and C.D. Zoita. Observa- tion of vacuum spark dynamics from its x-ray emission. // IEEE Trans. Plasma Sci., vol. 26,

№ 4, aug., 1998, p. 1162-1167.

[14] A.A. Bogomaz, A.V. Budin, V.A. Kolikov, М.E. Pinchuk, A.A. Pozubenkov, Ph.G. Rutberg.

Features of the electrode erosion for discharge-current amplitudes above 10

A. // Doklady Physics, vol.48, № 1, 2003, p. 1-4.

[15] D.A.Andreev, A.A.Bogomaz, Ph.G.Rutberg, A.M.Shakirov. High current discharge of а Z- pinch type in dense media. // JTF, v. 62, issue 6, 1992, p.74-82.

[16] K.V.Dubovenko Interaction of shock waves with plasma of the high current discharge in a

chamber of high pressure // JTF, v. 62, issue 6, 1992, p.83-93.