Observation of directed bremsstrahlung from a hollow cathode plasma

Nuclear Instruments North-Holland

and Methods

OBSERVATION

in Physics

OF DIRECTED

Christoph SCHULTHEISS

Research

BREMSSTRAHLUNG

FROM A HOLLOW

CATHODE

PLASMA

and Frank HOFFMANN

Kemforschungsrentrum

Karlsruhe GmbH, Institut ftir Neutronenphysik

Received

1989 and in revised form 17 April 1990

7 November

187

B51 (1990) 187-191

und Reaktortechnik,

Postfach 3640, D-7500 Karlsruhe I, FRG

Directed bremsstrahlung emitted from a pinched hydrogen plasma source (T, = 9 keV) is observed in pulsed power driven pseudo spark discharges. The source is located on the discharge axis at the exit of the hollow cathode; the volume of the source is below 1 before the discharge; the central mm3. The emission half angles are Z”, 4O and 14”, depending on gas pressure and preionization axis of the emission cone is identical to the discharge axis. The 9 keV X-ray source is embedded in a pinch having a temperature of 1.5 keV. The onset of the pinch radiation follows the Bennett relation. The X-ray emissions obey the Kramer law of a radiating fully ionized plasma.

Since

one

decade

a fast

low

pressure

gas

discharge

-Anode

[l-3] is under investigation. Fig. 1 shows the setup of the discharge device, called pseudo spark chamber. The lower part is a hollow cathode, the upper part is a 10 cm long stack of free floating electrodes separated by insulators with a common discharge channel. The diameter of the channel is 4 mm. The chamber contains a low pressure gas-filling of typical 30 Pa. Either noble or molecular gases are used. If a high voltage pulse is applied between the anode and the cathode, a fast gas discharge occurs (10 to 100 ns). The breakdown voltage roughly obeys the Paschen law Ua = Ua( pd). In this case the geometric factor d is about the diameter of the common channel. It is not determined as usually by the gap-distance. Two modes of operation are used: the self-breakmode, where a dc-voltage is applied via a load resistor. Secondly the pulsed mode, where a water-filled pulsed power line (POLLUX, 3 0, 300 kV, 100 ns) is used to apply very high voltages to the chamber and to feed very large powers (5 GW) into the discharge. The discharge time defined by the time interval with an impedance below 10 D lies between 40 and 80 ns. There are two main features of our discharges: the intense EM-radiation, which is the subject of this paper and the formation of a magnetically self-pinched electron beam and an ion beam. The self-pinched electron beam is observed, if the current exceeds some 10 A. Depending on the total current the diameter of the electron beam at the anode exit (not shown in fig. 1) is between 0.1 and 0.5 mm. The maximum current density is estimated to be lo6 A/cm’. In this paper we present our measurements with the 3 !2 pulsed power line in detail, of which some results were already reported shortly [4]. called

pseudo

spark

0168-583X/90/$03.50

0 1990 - Elsevier Science Publishers

-Electrode -Insulator

a -I 2 a -E 2 a -:: 2

Fig. 1. Pseudo spark chamber; the lower section is the hollow cathode, the upper section acts as an accelerator. In addition emission cones of the directed bremsstrahlung are drawn in.

First of all a preionization system was used to match the discharge to the 100 ns long pulse from the driver. The preionization is generated by a shock wave [5]

B.V. (North-Holland)

C. Schuitheiss, F. Hoffmann / Bremsstrahiung

CrystalSpectrometer

I I I Voltage- * ‘Current---Scintillator-

1

Signal

I

t

I1

11

Fig. 2. Voltage and current signal of the discharge tion with the X-ray emission (5.6 keV spectrometer

in correlasignal).

running through the gas volume of the hollow cathode. We obtained the best results with igniting 60 ps before the main pulse in the area of the hollow cathode a 20 J shock wave. Then the charge density of the hydrogen plasma has decayed so far (lo6 cmP3) that plasma screening effects do no longer disturb the hollow cathode processes and plasma inhomogeneities are still avoided

Fl.

_from a hollow cathode plasma

The electric signals of the main discharge are shown in fig. 2. About 60 ns after the pulsed voltage is applied, the discharge starts. Just at the turn on the current, bremsstrahlung arises due to the focused electron beam hitting the anode. Depending on the pressure of the hydrogen gas-filling during or after the bremsstrahlung emission the plasma starts to radiate energetic X-rays (see fig. 2). The radiation lasts about 50 ns and normally ends at the current maximum. Fig. 3 demonstrates the shape of the emitted high energetic X-rays (filter: 2 cm lucite) photographed 30 cm away from the cathode in axial position. In the following it can be shown that the stepwise decrease of the intensity with radius at the photograph is not due to peripheral apertures like windows or the discharge channel, but is caused by the anisotropic emission of the X-rays from the source. The shadowgraph of an X-ray lead mask with the same arrangement is shown in fig. 4. The distance between the cathode and the mask is 25.3 cm. Using geometrical optical rules from the observed magnification in fig. 4 one concludes that the position of the source is the exit of the cathode within an error of 1 to 2 mm in axial direction. The X-ray source is pointlike since the projection of the radially far distant structure fields of the lead mask (faintly visible in fig. 4) have the same magnification as the one near the axis and show no optical distortion. Therefore the X-rays originate from one source. Furthermore, penumbra measurements in fig. 4 determine the diameter of the source to

Fig. 3. Shape of the directed bremsstrahlung, filtered with 20 mm lucite and photographed with a X-ray amplifier foil.

Fig. 4. Shadowgraph of a X-ray test mask (20 mm lucite filter); the umbra and penumbra determine position and size of the source.

C. Schultheiss, F. Hoffmann / Bremsstrahlung from a hollow cathode plasma

be below 1 mm in radial and axial direction. From large-angle shadowing effects (not shown here) the axis of the emission cones is identical to the discharge axis. The photographs in fig. 3 and 4 were taken for a 70 Pa discharge in hydrogen gas. We observe in various discharges three emission half angles for the emission cones, namely 2 O, 4 O, 14 o and without preionization [4] only 14O at 30 Pa. In a 70 Pa discharge we obtain mainly radiation within a 2O half angle. With decreasing pressures, an additional 4O contribution arises. The illumination of the 40-photographs sometimes suggests a ring-shaped emission. Besides the pseudo spark chamber arrangement fig. 1 shows the position of the source and the emission cones. In addition fig. 1 shows clearly that a simple shadowing effect of the discharge channel is not responsible for the anisotropic emission of the X-rays, because in this case the source with the smallest emission angle would have to be in the middle of the chamber and the other sources in positions between the middle of the chamber and the cathode. The energy spectrum of the plasma radiation was determined with an intensity-calibrated Bragg- and Laue-spectrometer positioned on the discharge axis (discharge pressure of 30 Pa H2). Depending on the energy range we used LiF, ADP and RAP single crystals. The diffracted radiation was detected time resolved with a NE 104 scintillator in combination with a NP 55 photomultiplier. Fig. 5 shows the result of the measurement for the maximum emission power. The spectra taken at later times are qualitatively identical within the statistical errors. The same statement applies also to measurements off the axis. The energy spectrum of fig. 5 corresponds to freefree-like transitions. Using the Kramer equation [7] (see below) for a radiating fully ionized plasma one finds two electron temperatures namely 1.5 keV and 9 keV. The photographs in fig. 3 and 4 relate to the pointlike T, = 9 keV source, since the 2 cm lucite window absorbs the T, = 1.5 keV radiation totally and makes this source invisible.

Energy

E[kcV]

Fig. 5. Energy distribution of the radiating plasma; the 9 keV radiation is directed.

189

The origin of the 1.5 keV source may be explained in the following way: From electrical measurements we know that the late phase of the voltage signal is purely inductive. The calculated diameter of the current channel decreases from initially 4 mm (diameter of the discharge channel) to about 0.5 mm. We assume therefore that in the late phase of the pseudo spark discharge a z-pinch is formed with a compression factor of about 50. Optical measurements carried out in hydrogen (HBline broadening) at 3 GW discharges indicate a fully ionized plasma in the main discharge [8]. Therefore we expect an electron density of n, = 5 x 10” cmm3 in the z-pinch. The sources radiate at total currents between 4 and 10 kA (see fig. 2); for these currents the Bennett equation I* = 2(4a/p,,)N kT, fits well to the 1.5 keV source (the line density N is defined by 2n/n,r dr). Late in the discharge the electron temperatures remain constant although the total current increases up to 60 kA. Thus radiation cooling may cause a nonadiabatic compression. The total energy radiated above 2 keV is 1.5 mJ for the isotropic 1.5 keV source and 0.03 mJ for the (directed) pointlike 9 keV source. In the case of the 9 keV source the solid angle-integration was restricted to the downstream emission cones only. The emission energy of 0.03 mJ relates to the 14O half-angle cone at a discharge-pressure < 0.4 mb. For lower emission angles (higher pressure) the emitted energy is below 0.03 mJ. The measured emitted energies of the two sources can be compared with the prediction of the Kramer equation [7] for a bremsstrahlung radiating fully ionized plasma. The spectral intensity of radiation per volume is d Jff -=CneniZz[-+7]“*gexp[--&I,

dv

where C = 5.44 x lo-52 Wm3K’/*/sr is a constant, the nuclear charge number and g the Gaunt-factor. The integration over all frequencies gives: Jff

=

c*cT,j

“*neniZ2,

Z

with

C * = 1 .I3 X 1O-41 W m3/K’/‘sr. The total radiated energy then is E = lJff V dt, where V is the volume of the plasma. The parameters in the Kramer equation for both sources are experimentally already known except the plasma density of the pointlike 9 keV source. The volumes used in the Kramer equation we obtained from the penumbra and the impedance measurements: The emission volume is l/2 mm3 for the pointlike 9 keV source at the exit of the cathode. The compressed discharge column for the 1.5 keV source has the estimated volume of 50 mm3. Within the statistical errors (from shot to shot) the parameter set of the 1.5 keV source fulfills the Kramer equation. For the pointlike 9 keV source we get agree-

C. Schultheiss, F. Hoffmann / Bremsstrahlung from a hollow cathode plasma

190 Comparison

1

T

/

I

,

,

*

Experiment

I

k ,

- Theory

*

t

i

/

a

1

z

P t Ei

lo-

Pressure

p[rbar]

Fig. 6. Comparison of the pressure variation measurement with the prediction of the Kramer law of a radiating plasma.

density of about 6 X 1017 cm-3, the same as in the 1.5 keV source. With the aid of the Kramer equation we can conclude that the plasma density of both sources is equal and that the high temperature of the 9 keV source does not result mainly from adiabatic compression as it does in the case of the 1.5 keV source. The formation of a z-pinch in the late phase of a pseudo spark is confirmed by varying the pressure between 10 and 90 Pa Ha, using again the Kramer equation. The impedance measurements give information about the pinch volume I/ and the electron density n, as a function of pressure. Introducing them into the Kramer formula the total radiated energy and the temperature of the pinch plasma (which contributes only weakly and is therefore only measured at 30 Pa) we obtain good agreement for the slopes of the graphs in fig. 6. Within a factor 2 the absolute values of theory and experiment agree also. The generation of the pointlike very hot 9 keV source may be a consequence of the radial flow of energetic electrons out of the hollow cathode [9] and of an energy deposition process at the exit of the hollow cathode. Concerning the energy deposition process some observations are already available: visible light streak measurements show that the discharge starts pointlike at the exit of the hoIIow cathode [lO,ll]. Next, an energy deficit of electrons hitting the anode is observable which point to a considerable energy loss of electrons on their way from the hollow cathode to the anode. In a 35 kV discharge the bremsstrahlung edge is at 24 keV [12]. Since the energy-loss is time and current independent, the deposition process may be governed by space charge trapping. However, the processes which lead to the directed emission of X-rays cannot be interpreted. A trivial ment with an electron

shadowing of the optical path either by peripheral apertures like window i.e. or by the axial channel of the pseudo spark chamber can be excluded. The error in the measurement of the axial position is only 1 to 2 mm. This would require axial errors in the order of centimeters. Dipole radiation or synchrotron- and FEL-effects can be excluded, because of the smalI radiation angles. One would need strong electron currents with relativistic energies. Such currents were not observed. Regular laser processes can be excluded, since we use hydrogen as the working gas and the energy spectrum corresponds to bremsstrahlung. Induced bremsstrahlung [13] might be suspected for our phenomenon. The gain rate is v(w$‘/w2)A, where Y is the electron-ion collision frequency, wr, and w the plasma and radiation frequency and A a factor which is positive only for anisotropic electron distributions. Estimates for our case require a A-factor > lo*. This is too high. So the origin of the anisotropic 9 keV component remains unsettled at the moment. Finally it should be mentioned that the observation of highly anisotropic emission of soft X-rays from a fast z-pinch is not new. In a 1 kJ plasma focus experiment the emission of 1 keV X-rays with an half angle of 3O from the plasma has been reported [14,15]. These investigations help to state that in highly transient plasmas with rotational symmetry a~sotropi~ emission of EMradiation can occur. Conclusion and outlook: First detailed measurements revealed directed bremsstrahlung from a hydrogen plasma in pseudo spark discharges have been presented. The main goal was to measure the X-ray energy distribution in hydrogen. Therefore the discharge power was chosen to be relatively low. The next steps which have to be taken is to look for still smaller emission angles by varying power and pressure and to check the influence of gases with Z > 1 on the process. We would like to express our gratitude to A. Citron, for supporting the experiments from the beginning and for critical reading of the manuscript. W. Schmidt we have to thank for many helpful discussions.

References [l] J. Christiansen

and Chr. Schultheiss, Z. Phys. A 290 (1979) 35. [2] W. Bauer, A. Brandelik, A. Citron, K. Mittag, A. Rogner, W. Schimassek and Chr. Schuhheiss, Proc. 6th IEEE Pulsed Power Conf., Arlington (1987) p. 240. [3] W. Batter, H. Ehrler, F. Hoffmann, K. Mittag and N. Niessen, Proc. 7th Int. Conf. on High Power Particle Beams, Karlsruhe, vol. 1 (1988) p. 233.

C. Schultheiss, F. Hoffmann / Bremsstrahlung from a hollow cathode plasma [4] F. Hoffmann. G. Jung, A. Rogner, C. Schultheiss, A. Kitamura and A. Citron, ibid. vol. II (1988) p. 1216. [5] H. Guthardt and T. Morita, J. Appl. Phys. 36 (1965) 2577. [6] J.I. Levatter et al., Proc. IEEE Pulsed Power Conf., San Diego (1983) p. 755. f7) T.F. Stratton, X-Ray Spectroscopy, in: Plasma Diagnostic Techniques, eds. R.H. Huddelstone and S.L. Leonard (Academic Press, New York, 1965). [8] W. Bauer, A. Brandelik, A. Citron, E. Ehrler, E. Halter, G. Melchior, K. Mittag, A. Rogner and C. Schultheiss, Laser and Particle Beams, vol. 5, part 4 (1987) p. 585. [9] K. Mittag, KfK-report 4612, Kemfor~hungs~ent~m Karlsruhe (1989); K. Mittag, Nucl. Instr. and Meth. A292 (1990) 465. [lo] W. Bauer, A. Brandelik, H. Ehrler, K. Mittag, A. Rogner,

[II]

1121

[13] [14] (151

191

W. Schimassek and C. Schultheiss, 6th IEEE Pulsed Power Conf., Arlington, VA (1987) p. 240. P. Choi, H. Chuaqui, J. Lunney, R. Reichle, A.J. Davies and K. Mittag, IEEE Trans. Plasma Science 17 (1989) 770. A. Kitamura and C. Schultheiss, ~rn~be~cht 14.04 OlP58F Kemfo~~h~gszent~m Karlsruhe (1988); W. Niessen, K. Kitamura and C. Schultheiss, to be published in Nucl. Instr. and Meth. W.B. Thompson, Laser and Particle Beams 6 (1988) 513. G. Herziger, H. Krompholz, L. Michel and K. Schiinbach, Phys. Lett. A64 (1978) 390. W. Neff, R. Noll, F. Riihl, R. Lebert, C.R. Haas, B. Weikl and G. Herziger, Nucl. Instr. and Meth. A285 (1989) 253.