- Email: [email protected]
On the generation of dense plasma in ECR ion sources
Nuciear Instruments and Methods 198 (1982) 593-594 North-Holland Publishing Company
593
Letter to the Editor ON THE GENERATION
S.
OF DENSE PLASMA
IN ECR ION SOURCES
PESJk
Laboratory Received
of Physics, Institute of Nuclear Sciences ‘B. KidriF-Vi&a, 2 November
1981 and in revised form
11February
The generation of dense plasma in the low-pressure resonances for plasma production is proposed.
Yugoslavia
1982
microwave
Recently the electron cyclotron resonance (ECR) ion source has attracted a great deal of attention as a long lifetime source which provides intense beams of multicharged heavy ions [ 11. ECR sources are under construction or development in several cyclotron laboratories [2-51. The charge state of the produced heavy ions depends on the relative electron-ion velocity and on the product of the ion confinement time 7i and the hot electron density ne [6]. Starting from the fact that the plasma cutoff density is proportional to the square of the pump frequency w it is noted that by increasing w the electron density generated in the microwave discharge and, therefrom, the charge state of the produced ions can be increased. According to these extrapolations several high-frequency ECR ion sources are now under development [2,4,5]. The ion confinement time in the combined mirror-multipole magnetic field applied in the source however is proportional to the ion-ion 90’ scattering time [7]. Therefore the quality factor ne7i of an ECR ion source is nkarly unaffected by the increase of the electron density. Besides, since 7i depends logarithmically on the mirror ratio and the ion temperature must be kept low in this electron bombardment source, only slight improvements of the ion confinement can be achieved. It follows that in a practical way the main benefit from an increase of the electron density is a higher beam intensity of the extracted highly charged ions. The content of the present paper is concerned primarily with the generation of dense plasma in the low-pressure microwave discharge of an ECR ion source. Plasma production (and heating) by microwaves at the ECR frequency has been demonstrated to be an effective technique in a variety of plasma devices. Based on the collisionless wave-electron interaction, this method is characterized by high efficiency of coupling the microwave energy to the plasma and high rate of heating the bulk of electrons. In the present ECR ion sources a low-temperature dense plasma is generated in a low-pressure (p == 10m3 mbar) microwave discharge and subsequently heated at the ECR. The maximum
0167-5087/82/0000-0000/$02.75
P.O. Box 522, 11001 Beograd,
0 1982 North-Holland
discharge
of an ECR ion source
is discussed.
The use of wave
value of the electron density generated in the discharge depends mainly on the pump power and the transparence of the medium. Due to the curvature of the magnetic field lines, the wave propagation in the ECR source is usually oblique (0 = L (K, B) # 0, n/2). On entering the magnetized plasma an incident wave is split into two electromagnetic modes. One of these modes has a large perpendicular (with respect to the magnetic field lines) electric field component and at the ECR it can transfer an important part of the incident wave energy to the perpendicular electron motion. In the special case of rigorously parallel and perpendicular wave propagation this “resonant” mode becomes a right-hand circularly polarized wave and an extraordinary wave, respectively. It has the following cuttofs: x=lfy, where x = w&/w2 = n Jnec, y = w,,/w = B/B,, W&= e2n,/r0tne being the electron plasma frequency frequency, w,, = eB/m, the electron cyclotron and B the magnetic field induction. The second electromagnetic mode propagates up to the plasma cutoff,x = 1. It should be noted that propagating through an inhomogeneous plasma confined by a nonuniform magnetic field the electromagnetic modes may interact strongly. This can lead to a partial reflection or mode conversion and reduce substantially the absorbed wave energy. The ratio of the energy flow in the “resonant” mode to the energy flow in the “nonresonant” one can be made sufficiently large by an appropriate choice of the pump wave polarization and propagation conditions. The previous discussion of the wave propagation in magnetized cold plasma reveals a variety of microwave discharge conditions which can be used for the plasma production in an ECR ion source. The microwave discharge at low gas pressure may be maintained in a large frequency band expanding from large y-values up to the harmonics of the electron cyclotron frequency (y = n- ’ where n = 1,2,3,. . .). Moreover the electron density generated in the discharge ranges up to and beyond the plasma cutoff density. One has to note for instance that
S. PeSiC / Generation of dense plasma
594 in the microwave
discharge sustained by the wave field of the “resonant” mode at y> 1 an electron density exceeding the plasma cutoff density (an overdense plasma) can be generated. The resonances are of particular interest for the plasma production in microwave discharge. In general there are two types of resonances. One of them is related to some characteristic motion of the charged particles. In the present ECR sources this phenomenon (the cyclotron resonance) is used for the plasma generation (and heating). It should be pointed out that the electric field strength for microwave breakdown is significantly lowered when the ECR conditions (y = 1) are met. The second type of resonances is related to the plasma collective properties. Near these (wave) resonances the resonant electromagnetic mode slows down and it converts partially into a quasi-longitudinal plasma wave which can interact efficiently with the charged particles. Unlike the particle resonances, the charged particles may attain large velocities near the wave resonances only because of the resonant wave amplification. Whatever the resonance mechanism is employed however, the dominant process for transfering the microwave energy to the plasma is the collisionless wave-particle interaction. In one-dimensionally inhomogeneous plasma the wave resonance is excited in a density region in which the component of the dielectric tensor along the density gradient vanishes. This condition defines three frequency branches. Here we shall be interested only in the medium frequency branch determined by, w,=min(wr,,w,,>,
T=O;
W= r
depends upon the ratio of the width of the evanescent region to the average wavelength. In the frequency range of interest for the ECR ion sources the average wavelength is comparable to the plasma radius and an important part of the incident wave energy can be transmitted to the wave resonance. We propose the use of this phenomenon for the plasma generation in the plasma injector of the ECR ion source. In the frequency range ace > w > w LH high absorption efficiency (7 = SO-90%) in an overdense plasma (x 5 100) has been reported in numerous papers (see for example refs. 8,9). It has been established that the angle between the density gradient and the magnetic field depends on the gas pressure as well as on the species of the applied gas. Generally it increases as the gas pressure decreases. The threshold nature of the variation of the electron density with the magnetic field has been experimentally confirmed. The experimental results are in qualitative agreement with the theoretical predictions for the wave conversion in an inhomogeneous plasma. By using the microwave discharge based on the absorption associated with the wave conversion, the electron density produced in the plasma injector can be substantially increased without raising the pump frequency. In the ECR source VINIS [5] an overdense plasma will be produced in a microwave discharge of this type. Microwave radiation at the same frequency (f= 18 GHz) will be applied for the plasma poduction in the plasma injector and the electron heating in the interaction chamber. After the ignition of the microwave discharge at ECR conditions the magnetic field strength can be varied in the range y 5 1.7 to optimize the microwave energy absorption. Using the averaged equation of energy balance a ratio of the total number of electrons to the microwave power y = 2 X 10’ ‘W- ’ has been calculated for the discharge in Ne at p = 10P3 mbar.
References
where 5 is the angle between the density gradient and the magnetic field lines. The second expression can be written in the form, x=
y2-1 y2 cos2{ - 1
(2)
One concludes that for y cosl> 1 the wave resonance occurs in an overdense plasma (x > 1). The high frequency energy is efficiently absorbed by the electrons near this wave resonance. It should be noted however that usually the electromagnetic mode which exhibits a resonant behaviour near the wave resonance can reach this region only by tunneling through the wave evanescence region. The efficiency of wave tunneling
(I] R. Geller, IEEE Trans. Nucl. Sci. NS-23 (1976) 904. [2] Y. Jongen, C. Pirart and G. Ryckewaert, Workshop on ECR-ion sources and related topics, Darmstadt, GSI-81-l (1981) p. 1. [3] V. Bechtold, L. Friedrich and H. Schweickert, ibid., p.23. [4] L. Aldea, H. Beuscher, R.K. Bhandari, H.G. Mathews, J. Reich and P. Wucherer, ibid., p. 60. [S] S. PeSiC, D. CiriC, N. NeSkovic and B. Perovic, Ion source for the VinEa Cyclotron-VINIS, Beograd (1981). (61 R. Geller, IEEE Trans. Nucl. Sci. NS-26 (1979) 2120. [7] S. PeSiC, Fizika 13 suppl. 2 (1981) 118. [S] V.N. Budnikov, V.E. Golant and A.A. Obuchov, Phys. Lett. 31A (1970) 76. [9] B.V. Galaktionov, V.E. Golant, V.V. D’yachenko and O.N. Shcherbinin, Zh. T&h. Fiz. 40 (1970) 2322 (Sov. Phys.-Tech. Phys. 15 (1971) 18131.
593
Letter to the Editor ON THE GENERATION
S.
OF DENSE PLASMA
IN ECR ION SOURCES
PESJk
Laboratory Received
of Physics, Institute of Nuclear Sciences ‘B. KidriF-Vi&a, 2 November
1981 and in revised form
11February
The generation of dense plasma in the low-pressure resonances for plasma production is proposed.
Yugoslavia
1982
microwave
Recently the electron cyclotron resonance (ECR) ion source has attracted a great deal of attention as a long lifetime source which provides intense beams of multicharged heavy ions [ 11. ECR sources are under construction or development in several cyclotron laboratories [2-51. The charge state of the produced heavy ions depends on the relative electron-ion velocity and on the product of the ion confinement time 7i and the hot electron density ne [6]. Starting from the fact that the plasma cutoff density is proportional to the square of the pump frequency w it is noted that by increasing w the electron density generated in the microwave discharge and, therefrom, the charge state of the produced ions can be increased. According to these extrapolations several high-frequency ECR ion sources are now under development [2,4,5]. The ion confinement time in the combined mirror-multipole magnetic field applied in the source however is proportional to the ion-ion 90’ scattering time [7]. Therefore the quality factor ne7i of an ECR ion source is nkarly unaffected by the increase of the electron density. Besides, since 7i depends logarithmically on the mirror ratio and the ion temperature must be kept low in this electron bombardment source, only slight improvements of the ion confinement can be achieved. It follows that in a practical way the main benefit from an increase of the electron density is a higher beam intensity of the extracted highly charged ions. The content of the present paper is concerned primarily with the generation of dense plasma in the low-pressure microwave discharge of an ECR ion source. Plasma production (and heating) by microwaves at the ECR frequency has been demonstrated to be an effective technique in a variety of plasma devices. Based on the collisionless wave-electron interaction, this method is characterized by high efficiency of coupling the microwave energy to the plasma and high rate of heating the bulk of electrons. In the present ECR ion sources a low-temperature dense plasma is generated in a low-pressure (p == 10m3 mbar) microwave discharge and subsequently heated at the ECR. The maximum
0167-5087/82/0000-0000/$02.75
P.O. Box 522, 11001 Beograd,
0 1982 North-Holland
discharge
of an ECR ion source
is discussed.
The use of wave
value of the electron density generated in the discharge depends mainly on the pump power and the transparence of the medium. Due to the curvature of the magnetic field lines, the wave propagation in the ECR source is usually oblique (0 = L (K, B) # 0, n/2). On entering the magnetized plasma an incident wave is split into two electromagnetic modes. One of these modes has a large perpendicular (with respect to the magnetic field lines) electric field component and at the ECR it can transfer an important part of the incident wave energy to the perpendicular electron motion. In the special case of rigorously parallel and perpendicular wave propagation this “resonant” mode becomes a right-hand circularly polarized wave and an extraordinary wave, respectively. It has the following cuttofs: x=lfy, where x = w&/w2 = n Jnec, y = w,,/w = B/B,, W&= e2n,/r0tne being the electron plasma frequency frequency, w,, = eB/m, the electron cyclotron and B the magnetic field induction. The second electromagnetic mode propagates up to the plasma cutoff,x = 1. It should be noted that propagating through an inhomogeneous plasma confined by a nonuniform magnetic field the electromagnetic modes may interact strongly. This can lead to a partial reflection or mode conversion and reduce substantially the absorbed wave energy. The ratio of the energy flow in the “resonant” mode to the energy flow in the “nonresonant” one can be made sufficiently large by an appropriate choice of the pump wave polarization and propagation conditions. The previous discussion of the wave propagation in magnetized cold plasma reveals a variety of microwave discharge conditions which can be used for the plasma production in an ECR ion source. The microwave discharge at low gas pressure may be maintained in a large frequency band expanding from large y-values up to the harmonics of the electron cyclotron frequency (y = n- ’ where n = 1,2,3,. . .). Moreover the electron density generated in the discharge ranges up to and beyond the plasma cutoff density. One has to note for instance that
S. PeSiC / Generation of dense plasma
594 in the microwave
discharge sustained by the wave field of the “resonant” mode at y> 1 an electron density exceeding the plasma cutoff density (an overdense plasma) can be generated. The resonances are of particular interest for the plasma production in microwave discharge. In general there are two types of resonances. One of them is related to some characteristic motion of the charged particles. In the present ECR sources this phenomenon (the cyclotron resonance) is used for the plasma generation (and heating). It should be pointed out that the electric field strength for microwave breakdown is significantly lowered when the ECR conditions (y = 1) are met. The second type of resonances is related to the plasma collective properties. Near these (wave) resonances the resonant electromagnetic mode slows down and it converts partially into a quasi-longitudinal plasma wave which can interact efficiently with the charged particles. Unlike the particle resonances, the charged particles may attain large velocities near the wave resonances only because of the resonant wave amplification. Whatever the resonance mechanism is employed however, the dominant process for transfering the microwave energy to the plasma is the collisionless wave-particle interaction. In one-dimensionally inhomogeneous plasma the wave resonance is excited in a density region in which the component of the dielectric tensor along the density gradient vanishes. This condition defines three frequency branches. Here we shall be interested only in the medium frequency branch determined by, w,=min(wr,,w,,>,
T=O;
W= r
depends upon the ratio of the width of the evanescent region to the average wavelength. In the frequency range of interest for the ECR ion sources the average wavelength is comparable to the plasma radius and an important part of the incident wave energy can be transmitted to the wave resonance. We propose the use of this phenomenon for the plasma generation in the plasma injector of the ECR ion source. In the frequency range ace > w > w LH high absorption efficiency (7 = SO-90%) in an overdense plasma (x 5 100) has been reported in numerous papers (see for example refs. 8,9). It has been established that the angle between the density gradient and the magnetic field depends on the gas pressure as well as on the species of the applied gas. Generally it increases as the gas pressure decreases. The threshold nature of the variation of the electron density with the magnetic field has been experimentally confirmed. The experimental results are in qualitative agreement with the theoretical predictions for the wave conversion in an inhomogeneous plasma. By using the microwave discharge based on the absorption associated with the wave conversion, the electron density produced in the plasma injector can be substantially increased without raising the pump frequency. In the ECR source VINIS [5] an overdense plasma will be produced in a microwave discharge of this type. Microwave radiation at the same frequency (f= 18 GHz) will be applied for the plasma poduction in the plasma injector and the electron heating in the interaction chamber. After the ignition of the microwave discharge at ECR conditions the magnetic field strength can be varied in the range y 5 1.7 to optimize the microwave energy absorption. Using the averaged equation of energy balance a ratio of the total number of electrons to the microwave power y = 2 X 10’ ‘W- ’ has been calculated for the discharge in Ne at p = 10P3 mbar.
References
where 5 is the angle between the density gradient and the magnetic field lines. The second expression can be written in the form, x=
y2-1 y2 cos2{ - 1
(2)
One concludes that for y cosl> 1 the wave resonance occurs in an overdense plasma (x > 1). The high frequency energy is efficiently absorbed by the electrons near this wave resonance. It should be noted however that usually the electromagnetic mode which exhibits a resonant behaviour near the wave resonance can reach this region only by tunneling through the wave evanescence region. The efficiency of wave tunneling
(I] R. Geller, IEEE Trans. Nucl. Sci. NS-23 (1976) 904. [2] Y. Jongen, C. Pirart and G. Ryckewaert, Workshop on ECR-ion sources and related topics, Darmstadt, GSI-81-l (1981) p. 1. [3] V. Bechtold, L. Friedrich and H. Schweickert, ibid., p.23. [4] L. Aldea, H. Beuscher, R.K. Bhandari, H.G. Mathews, J. Reich and P. Wucherer, ibid., p. 60. [S] S. PeSiC, D. CiriC, N. NeSkovic and B. Perovic, Ion source for the VinEa Cyclotron-VINIS, Beograd (1981). (61 R. Geller, IEEE Trans. Nucl. Sci. NS-26 (1979) 2120. [7] S. PeSiC, Fizika 13 suppl. 2 (1981) 118. [S] V.N. Budnikov, V.E. Golant and A.A. Obuchov, Phys. Lett. 31A (1970) 76. [9] B.V. Galaktionov, V.E. Golant, V.V. D’yachenko and O.N. Shcherbinin, Zh. T&h. Fiz. 40 (1970) 2322 (Sov. Phys.-Tech. Phys. 15 (1971) 18131.