J. Plasma Fusion Res. SERIES, Vol.4 (2001) 301-304

Enhancement of X-ray Generation in a Microwave Discharge
GOLUBEV Serguey, ZORIN Vladimir, VODOPYANOV Alexander, GAUDIN Christeller, RAZIN Sergueyl
ROUILLE Clauder and BACAL Marthel*Institute for Applied Physics of Russian Academy of Sciences,
46 Ulyanov St., 603600 Nizhny Novgorod, Russia
tLaboratoire de Physique et Technologie
des Plasmas, U.M.R. 7648 du C.N.R.S.,
Ecole Polytechnique, 91 I 28 Palaiseau, France(Received: 5 December 2000 / Accepted: l8 August 2001)

Abstract
In the present paper we describe the observation of enhancement of X-ray generation in a
microwave plasrna at a sub-harmonic of the gyro-frequency, where the equality , =l r" is satisfied(here ar is the microwave frequency, rrrs is the gyro-frequency of electrons). Our experiments show, forthe first time, that the X-ray flux produced at sub-harmonic resonance is five times higher than that
produced at the cyclotron resonance.

Keywords:
Sub-harmonic resonance, gyro-frequency, plasma, X-ray, microwave discharge, magnetic trap, electron
cyclotron resonanceOne of the prospective X-ray plasma sources isat the midplane of the magnetic mirror, both during themicrowave discharge and in the afterglow of the piasma.

based on a microwave discharge in a magnetic trap at

low gas pressure, under electron cyclotron resonance

Ikegami et al. [2] also found a resonant increase of the
X-ray emission when the second and third harmonics ofthe electron cyclotron frequency associated with themagnetic field in the midplane of the magnetic mirror
reached the frequency of the heating microwave.
We describe here the observation of the resonant
increase of X-ray emission at a sub-harmonic of thegyro-frequency, namely at o = | o)s, when launching
microwaves in a compact X-ray source transverse to themagnetic field lines. It was suggested earlier that thenonlinearity of the equations of electron motion which(ECR), i.e. under conditions when there is a region in
the trap where the resonance on the fundamental
harmonic a= o)u is fulfilled (arg is the gyro-frequencyof electrons). In such a relatively small source a plasma
with density of the order 10r0-l0rr cm-3 containing<< hot >> electrons with energies of a few tens of keV isobtained. When these electrons impinge on a solidtarget, Iocated inside the plasma chamber, X-rays withconsiderably higher intensity are generated []. The useof permanent magnets to produce the mirror magnetic

field prevents the axial injection of microwaves,

determines resonant interaction on the sub-harmonic cantherefore transverse injection of microwaves was used.
Ikegami et al. [2] showed that when the microwave

be related to the non-homogeneity of the microwave
pumping [3] as well as with non-homogeneity of the dc

power is fed across the magnetic flux lines, the hot

magnetic field [4]. We experimentally investigate theelectrons are bunched in a closed-shell structure located*Corresponding author's e-mail: bacal@

02001 by The Japan Society of Plasma

lptp.polytechnique.fr301

Science and Nuclear Fusion Research

Golubev S. et al., Enhancement of X-ray Generation in a Microwave Discharge
6.Magnetron

1.Nal deteclor 2.Al loii 15 um 3-Vacuum
cnamoer

efficiency of X-ray production in the ECR discharge fordifferent resonance conditions at a = 0)H and a = I a".
The specific feature of our experiment was to keep afixed frequency of the microwave source (2.45GH2).

The transition from sub-harmonic to cyclotron
resonance was performed by varying the dc magnetic

5.Magnetic
coils

field in the trap.
The experimental setup is shown on Fig. 1. The

lo.Microwave//t-./
z.Lealolate S.Langmuirprobe IMicrowavedetector window

microwave source for producing the plasma is amagnetron from a microwave oven with a frequency ofll.WaveguideFig. 1 Schematic presentation of the experimental setup.

2.45GHz at a power of lkW, operated in a pulsed mode,with the pulse length lms. The linearly polarized
microwaves are transferred through a cylindricalwaveguide with a diameter of 9.3cm into the vacuum
chamber with characteristic dimensions 14x14xl4cm3.

f

The vacuum chamber was placed in a simple axisymmetric magnetic trap, created by two pulsed solenoids.
The curent pulse length was 13ms and the mirror ratio
was 3.5. Moveable targets made of different materials

ctj'6co.c$Lwere introduced in the midplane of the dischargechamber filled with argon. When working with a*magnetic field of l75OGauss 16 = I o4l near the target,
the magnetic field value of 875Gauss (a = roos) was notpresent in the chamber. A NaI scintillation detector(designed primarily for counting photons) was used tomeasure the X-ray spectrum, by measuring theamplitude of the 1Ops pulses, which is proportional tothe photon energy. The spectral resolution of the00.5 1.0 1.5 2.0 2.5 3.0
Norrnal ized rnagnetic f ield

detector is 287o, the working energy range is 15-Fig. 2 Integrated X-ray spectrum intensity versus

normalized magnetic tield BlB". Curve a corre-150keV. In this experiment the lowest measured photon

sponds to pumping at the fundamental harmonic,
with optimum pressure 4x10-5Torr, curve b - toenergy is determined by the Al window of 200pm
thickness. For measuring the X-ray dose a calibrated
dosimeter DRG-O5M was used (minimum energy of themeasured photon is l5keV). For the estimation of theplasma density in the discharge, we measured thecurrent with a Langmuir probe, located on the axis
outside the magnetic trap at a considerable distancefrom the magnetic mirror. In addition, one could

pumping at the half-harmonic, with optimum
oressure 2x10{Torr.

10-sTorr (curve a on Figure 2), while the maximum at
half-harmonic of the gyro-frequency at = I a, (i.e. BlB"= 2) was measured at a lower pressure, 2xl0-sTorr(curve b on Figure 2). Both maxima cannot be observed

estimate the plasma electron density by measuring theintensity of the microwave radiation transmitted throughthe plasma. The absorption coefficient of the microwave

at the same pressure. No other maximum was observedby increasing the magnitude of the magnetic field up toBlB" = (.J, irrespective the pressure.
It is important to note that the X-ray flux measured

signal by the plasma was measured with an antenna and

HF diode. located outside the chamber, behind thewhen pumping on the half-harmonic of the gyrofrequency (at lower pressure) exceeds by a factor five

microwave window.

During the herein described experiments thethe flux obtained when pumping on the fundamentalBremsstrahlung flux with various targets was studied atfixed microwave frequency (2.45GH2) versus the magnetic field, at different gas pressures. The maximum X-harmonic.

The pressure dependence of the discharge
characteristics for the half harmonic of the gyro-ray flux at the fundamental harmonic of the gyrofrequency o) = @n was measured for a pressure of 4x

frequency is illustrated in Figures 3 and 4. Typical time302

Golubev S. et al., Enhancement of X-ray Generation in a Microwave DischareezE o.o2zE o.o2co) 0.00
a -0.02(ucc -0.04
o -0.06co) 0.00
a -0.02(Ecc -0.04
o -0.06
c -0.08-0.08

Ec -0.05

z6o -0.10

a0.00.9)co)II -0.15

II(L -0.20TL0.0(Uc.9)

o(dz-0.1

(Uco)

-0.2
-0.3
-0.4-0.5

aE

z0 0.5 1 1.5 20-5
-10-15-20,250.0-0.1

-o.2
-0.3
-0.4
-0.5

Tirne, nF00.5

1Time, ms

Fig. 3 Oscilloscope traces of (from top to bottom):Fig. 4 Oscilloscope traces of (from top to bottom):transmitted microwaves, electron current to theprobe, signal of the X-ray detector. Discharge atpressure of 2x1 0-5Torr.

transmitted microwaves, electron current to theprobe, signal of the X-ray detector. Discharge atpressure of 5x10-5Torr.

dependences of the microwave detector signals, from theThe X-ray Bremsstrahlung spectrum recorded when

Langmuir probe and the X-ray detector for two different

using a light target such as aluminum (showing no
characteristic lines in the observed energy range) ispressures are displayed. The analysis of thesedependences shows the existence of two different

shown on Fig. 5. Note that the intensity spectrum decays
following an exponential function exp(-ElkT"), with ftI.
= 30 keV. When heavier targets are used, a substantial
contribution to the X-ray intensity is brought by thecharacteristic lines of the target material (lSkeV or
64keV, if a molybdenum or a tungsten target is used,discharge modes with different electron densities andtemperatures. At low gas pressure (2x10-5Torr, Fig. 3) adischarge with low plasma density is obtained. In thiscase there is no temporal variation of the transmitted

microwave signal (Fig. 3, upper oscillogram), whichmeans that the plasma density is much less than the cut-respectively).
The total accumulated X-ray dose was measured by
two methods: (1) by integrating the measured spectra

off value. The Bremsstrahlung recorded by the X-ray
detector can be seen from the lowest oscilloscope trace
in Figure 3. At higher pressures (5x10-5Ton, Fig. 4) at a
certain time the discharge mode is suddenly changing.
The signal from the probe placed on the longitudinal
axis of the magnetic system increases abruptly (by afactor ten relative to the signal before the change of thedischarge mode), while the signal recorded by themicrowave diode decreases, and the X-ray radiation
disappears. The further increase in the pressure leads tothe disappearance of the low-density discharge phase,
while the high-electron-density discharge phase starts
from the very beginning of the microwave pulse.

and (2) by using a dosimeter. Under optimum
conditions, the dose at 30cm from the radiation source
was determined to be 3.57mGy/s.

In conclusion, we underline our main experimental

results : (a) the first observation of an X-ray flux
produced under conditions of sub-harmonic resonance,and (b) the significantly higher flux of the lattercompared to the X-ray flux produced in the fundamental
cyclotron resonance regime under optimum plasma
conditions (gas pressure, see Fig. 2).The relatively highintensity of X-ray radiation in the sub-harmonic regime303

Gofubev S. et al., Enhancement of X-ray Generation in a Microwave Discharge

explanations and made the corresponding calculations.
Work is in progress to find the genuine cause of the subharmonic resonant enhancement of X-ray generation.
One should also realize that in the sub-harmonic
regime the magnitude of the mirror magnetic field isdouble the magnitude of the latter in the fundamental

oJ.iEP.

:ljr

C'eoTl<rrItq)

cg

harmonic regime. This fact points to considerablyimproved confinement properties of the magnetic trap

tt

for energetic electrons.
I10I

R
6_aAcknowledgement
We are grateful to Professors Hideo Ikegami.

TII

Evgeniy Suvorov and Michail Tokman for enlighteningdiscussions, and to INTAS (Project 97-0094) for its

Isupport.

0 10 20 30 40 50 60 70 80 90 100
Quantum energy, kev

References

Fig. 5 Spectrum of Brehmsstrahlung with Al target (4 =30keV) when pumping at the half-harmonic of thetll

M. Bacal, C. Gaudin, A. Bourdier, J. Bruneteau,
J.M. Buzzi, K.S. Golovanivsky, L. Hay and C.

gyro-frequency.

Rouille, Nature (London) 384,421 (1996).
[2] H. Ikegami, H. Ikesi, M. Hosokawa, S. Tanaka andK. Takayama, Phys. Rev. Lett. 19, 778 (1967).
t3l S. Corti, F. De Luca and G. Grosso, RF Preionization in Tokamak tore: Effect of Subharmonics of {lson the absorption of the Wave in Proceedings of theJoint Varenna-Grenoble International Symposium
on Heating in Toroidal Plasmas, 1978, p. 225-230.
t4l N.S. Ginzburg and M.D. Tokman, Sov. Phys. Tech.
Phys. 29,604 (1984).

of electron acceleration was recorded in characteristiclines of heavy material targets. This is convincing
evidence that under suitable plasma conditions a fractionof electrons are accelerated up to relatively highenergies.

The detailed comparative theoretical analysis of Xray production in these two regimes is a rather exciting
and interesting problem, but it lies outside the scope of
this paper. We have considered a number of theoretical304