Fussmann, G.

Waldmann, O.

English

MPG.PuRe

Spontaneous generation of suprathermal electron beams in a stationary magnetic arc discharge

Max Planck Society eDoc Server

Spontaneous generation of suprathermal electron beams in a stationarymagnetic arc dischargeO. Waldmann1, G. Fussmann1,21 Max-Planck-Institut für Plasmaphysik, TI Greifswald, EURATOM Association, Germany2 Humboldt-Universität zu Berlin, Institut für Physik, GermanyIntroductionAlbeit the plasma generator PSI-2 has been designed and constructed to run under completelystationary conditions fluctuations of a number of parameters cannot be suppressed even in longterm operation. Most evident are low frequency (a few Hz) intensity variations of the plasmacolumn which can be observed with the naked eye. These do occur in particular during theconditioning phase after venting of the machine; most likely they are caused by sudden gasrelease from the walls.In this paper we concentrate on another fluctuation phenomenon which seems to be presentunder all plasma conditions and, is still found after many hours of operation: short spikes in thevoltage between the heated cathode and the grounded anode. The variations are up to about afactor of two and are followed by a similar change of the floating voltage of the neutralizer plateat the end of the device. From the time difference it can be inferred that suprathermal electronsare produced during each spike in the relatively short anode-cathode region which can pass thewhole plasma region without suffering collisions. Interestingly, each spike event seems to havea small precursor.Plasma Generator PSI-20.5 m6cathode (LaB  )regiondischargestagedifferentialplasma flow BplasmadiaphragmpumpingMo anodeplatetarget  chambervalveneutralizergas inletFigure 1: Plasma Generator PSI-2PSI-2 is a linear plasma device with a stationary arcdischarge. A steady discharge current in the range 50 toabout 500 A can be chosen. For stabilization a resistorof 1 Ohm is in series with the plasma (about 0.1 to 1Ohm). The plasma is produced in the discharge regionwhich consists of a heated, cylindrical, hollow LaB6-cathode and a cylindrically-shaped Mo-anode. The ge-ometry of the discharge region is the same as in [1].Guided by an axial magnetic field the plasma streams through a differential pumping systemand a so called target chamber and is terminated at the neutralizer plate (l ≈ 2.66 m) (cf.Fig. 1). Parameters for this work are: IAC = 100 . . .125 A, B = 0.1 T, ne = 1018 . . .1019 m−3,33rd EPS Conference on Plasma Phys. Rome, 19 - 23 June 2006 ECA Vol.30I, P-1.049  (2006)Te = 2 . . .15 eV, Ti = 0.5 . . .0.7 Te, pneutral = 0.04 . . .0.6 Pa, pAC = 4 . . .7 Pa.Experimental ResultsVoltage TracesIn Fig. 2 a typical temporal course for a spike event in a hydrogen plasma is shown. Suchevents happen irregularly with a similar signature but are less pronounced in argon discharges.The anode-cathode voltage (UAC) is suddenly enhanced by a factor of two (from -70 V to-140 V) within time intervals of about 1 µs. The same signature but delayed by 0.5 µs is foundin the floating voltage of the neutralizer plate (Unp) which covers the whole plasma cross sectionat the end of the discharge. In the case of argon a time delay of 1 µs was measured.-140-120-100-80-60-40-30 -25 -20 -15 -10 -5  0  5  10  15  20Voltage [V]Time [µs]UACUnpFigure 2: Voltage traces in hydrogenWith the measured time of flight τ andthe length l = 2.66 m the velocity of thesuprathermal electrons can be calculated.This value can be compared with the theoret-ical velocity of an electron accelerated withthe corresponding voltage.vmeas =lτ= 5.5±0.2 ·106 ms(1)vtheo =√2eme√UAC = 7.3 ·106ms(2)The measured velocity is in fair agreement with the theoretical value in this approximation. Forargon we get vmeas = 2.7±0.1 ·106 ms ,vtheo = 3.1 ·106 ms . There is no other reasonable process inthe plasma (e.g. waves) that could transfer the information of the spike at such a high velocity.While varying the trigger voltage one can see that there is up to one event per minute fora voltage of UAC ≤ −164V. There is a saturation of about 3500 events per minute at UAC ≤−100V. Estimating the capacitance of the neutralizer plate as Cnp = 8ε0R (a thin disk a RadiusR = 8.5 cm), we get Q = CU = 1.7 ·10−9Coulomb for one event.As power supplies we use EA-PS 9072-120 from EA company with a guaranteed currentcontrol time of less than 1 ms. If the plasma resistance changes for some reason the deviceadjusts the voltage in order to keep the current constant. According to our observations theactual control time constant is, however, much lower than stated and certainly in the range of 1µs or less. The voltages and currents were measured with a digital signal analyzer (TektronixTDS 754C, 500 MHz, 2 GS/s) and cross-checked with a Tektronix DSA 602A (1 GHz, 2 GS/s).We performed non-averaged as well as averaged measurements in order to approve the signal-to-noise ratio.33rd EPS 2006; O.Waldmann et al. : Spontaneous generation of suprathermal electron beams in a stationary magne... 2 of 4Floating Potential of a Probe-1.4-1.2-1-0.8-0.6-0.4-0.2 0 0.2-60 -40 -20  0  20  40  60  80  100  120Voltage [V]Time [µs]UAC + 25.6 VUnp + 13.2 VUP + 1.5 VFigure 3: Voltage traces in argonAdditionally we put a double probe in thetarget chamber and measured the floating po-tential of both tips alternately. The spikescould also be found in the floating potential ofthe probe. In Fig. 3 the probe was just outsidethe visible plasma column. The same coursecan be found also inside the plasma but thesignal to noise ratio is worse. In Fig. 3 theaverage respective voltages were subtractedfrom the curves in order to compare the height of the three voltage spikes. For a biased probethe qualitative behaviour was about the same for both probe tips (biased and non-biased).Increment of Electron Density-180-150-120-90-60-30-1 -0.5  0  0.5  1  1.5  2  2.5  3Voltage [V]Time [µs]+0 sccm+125 sccm+175 sccmFigure 4: Voltage traces in case of additionalgas puffing. UAC (left set) and Unp (right set ofcurves).We also used a valve in the target cham-ber (cf. Fig. 1) to inject additional workinggas. This results in an increase of the electrondensity in this area without increasing it in thedischarge region. As to be seen in Fig. 4 thefast electrons are now slowed down in the tar-get chamber (τ is 470 ns, 640 ns, and 730 ns,respectively). Estimating the mean free pathlengths of the thermal (λ ∼ T 2e /ne) and thesuprathermal electrons (λ ∼ U2AC/ne) for aplasma density of ne ≈ 1 ·1019 m−3 these arefound to be 0.1 m and 200 m, respectively. Incase of the increased density also the motion of the suprathermal electrons becomes affected bycollisions. The peaks in Unp are consequently decreased and broadened as expected.TheoryA heated cathode emits electrons as long as the electric field at its surface is positive. Forvanishing field conditions severe space charge limitation occur and the extracted current densityis determined by the Child-Langmuir law [2]j = 49ε0√emeU3/2ACL2, (3)33rd EPS 2006; O.Waldmann et al. : Spontaneous generation of suprathermal electron beams in a stationary magne... 3 of 4where L is the characteristic length over which the potential drops from 0 to −UAC. Underplasma conditions this coincides with the electrostatic sheath length, i.e. a few times the Debyelength (λD =√ε0kBTee2ne) and we get the relation:j ∼U3/2AC ne/Te. (4)We are not yet in the position to identify the proper reason for the occurrence of the voltagespikes but density fluctuations in front of the heated cathode could be causal. In fact, a suddendecrease of the electron density would cause a reduction of the extracted current and thus reducethe density further because there are now less ionization events in the neighboring dischargeregion. There is hence an instability tending to quench the discharge. For this reason a controlsystem is needed which rises the voltage to avoid current quenching. Until now we have nosatisfying explanation for the occurrence of the precursor event.Conclusions and SummaryWe observe short voltage spikes in the anode-cathode voltage in a linear stationary plasmadevice PSI-2. The voltage of the spikes is twice as high as the mean discharge voltage. Thecurrent of the discharge is controlled. Each event seems to have a smaller precursor about 5 µsahead.The spikes can also be found in the floating voltage of the neutralizer plate at the end of thedischarge. It is inferred that suprathermal electrons are produced during the short phases withenhanced voltage. The corresponding energy of the electrons in the pulse is of order 100-150 eVand thus much larger than the electron temperature which is measured to be Te ≤ 10 eV. Suchsuprathermal electrons can pass nearly collisionless the whole plasma length of 2.6 m under theprevailing low density conditions.Density fluctuations that lead to variations of the electrostatic sheath length in front of thecathode are identified as a possible mechanism to produce the spikes.Apart from being an interesting physical phenomenon by itself the effect could have alsopractical aspects since the distortion of the electron Maxwellian distribution (bump in tail) canhave consequences for the interpretation of various measurements and moreover may lead toexcitation of plasma waves.References[1] D. Naujoks, G. Fussmann and H. Meyer, Contrib. to Plasma Physics 38, 127–133 (1998)[2] C.D. Child, Phys. Rev. (Series I) 32, 492–511 (1911)33rd EPS 2006; O.Waldmann et al. : Spontaneous generation of suprathermal electron beams in a stationary magne... 4 of 4

Spontaneous generation of suprathermal electron beams in a stationary magnetic arc discharge

Abstract

Similar works

Full text

Available Versions

MPG.PuRe

Max Planck Society eDoc Server

MPG.PuRe