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An ultra compact 10 GHz electron-cyclotron-resonance ion source (ECRIS) for multiply charged ions production
Nuclear Instruments
& *H__ -_
and Methods in Physics Research B 98 (1995) 525-527
Beam Interactions with Materials&Atoms
@ ELSEVIER
An ultra compact 10 GHz electron-cyclotron-resonance ion source (ECRIS) for multiply charged ions production M. Schlapp a,* ‘I, R. Trassl a, E. Salzborn a, R.W. McCullough H.B. Gilbody b
b, T.K. McLaughlin
b,
aInstitutfir Kernphysik, Strahlenzentrum, b Department
Universitiit Giessen, 35392 Giessen, Germany of Pure and Applied Physics, The Queen’s University of Belfast, Belfast, Northern Ireland, UK
Abstract There is considerable current interest in the use of beams of highly charged ions for a wide variety of applications. In particular, there is a need for sources with low power consumption for installation in high voltage terminals where typically only a few hundred Watts of electrical power is available. An ultra-compact (200 mm long and 120 mm outside diameter) 10 GHz electron-cyclotron-resonance (ECR) ion source has been constructed, making use of recent advances in permanent magnet and solid state microwave technology. The magnetic structure comprises only two ring magnets producing an axial magnetic field configuration with a mirror ratio of 1.6 and a Halbach-hexapole magnet for the radial field with a maximum field of 0.94 T inside the plasma chamber of 25 mm inner diameter. Preliminary tests of this source have shown efficient ECR plasma heating with microwave power levels in the range l-50 W and total extracted ion currents of the order of 1 mA. First results from a prototype source using different microwave injection methods will be presented.
1. Introduction For many years, our group has investigated charge exchange and ionization processes in ion-ion collisions. Employing a crossed-beams technique, one ion beam is produced by a 5 GHz ECR ion source with energies up to 20 keV . q. The ion source producing the second ion beam is installed on a high voltage terminal (up to 400 kV), where only 1 kW of electrical power is available. Up to now, there are no experimental investigations of reactions between two different species of highly charged ions. One reason may be that nobody could afford so far two powerful ECR ion sources most suited for providing intense beams of multiply charged ions for a crossed-beams experiment. Therefore, theoretical calculations [ 11 for quasi-resonant electron capture in isoelectronic systems like:
04++
F5++
OS++
F4+,
F5++ Ne6+-+ F6++ Nesf, could not be verified so far experimentally. Our first attempt in this field was the construction of a 2.45 GHz ECRIS, where the necessary resonance magnetic field strength of 87.5 mT can easily be obtained with
’ Corresponding author. ’ This work contains parts of the thesis of M. Schlapp, Giessen D26.
permanent magnets [2]. This ion source is able to produce very high currents of singly charged ions, e.g. 6.5 n~4 of He+ at an extraction voltage of 20 kV. However this low microwave frequency was not high enough for the plasma electrons to gain sufficient energy to produce substantial yields of multiply-charged ions. Operation at higher microwave frequencies allows smaller dimensions of the plasma chamber reducing both the power consumption by the plasma and the amount of permanent magnet material which results in lower weight and lower costs [3].
2. Source description
The smallest diameter of plasma chamber in which microwaves with a frequency of 10 GHz can still efficiently propagate was determined. Fig. 1 shows the attenuation of the microwaves in a circular copper waveguide as a function of the diameter for both TE,, and TM,, modes. According to this calculation the inner diameter of the plasma chamber was chosen to be 25 mm with a wall thickness of 2.5 mm. Fig. 2 shows the mechanical setup of the 10 GHz ECR ion source. The complete assembly is mounted on a NW 150 PN flange and has a length of 200 mm at a weight of approximately 15 kg. The plasma chamber is water-cooled to protect the permanent magnets from demagnetization at high temperatures and terminates with a DN 35 CF flange to which different waveguide couplings were connected.
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00180-8
5. PRODUCTION/METHODS/APPLICATIONS
M. Schlapp el al. /N~rcl. Instr. and Meth. in Phys. Rex L?98 (1995) 525-527
526
~__ i_ _ A TM,, - mode 0 TE ,, - mode
-. 16
,16
20
22
24
26
26
._A 30
32
id [mm] Fig. 1. Attenuation copper-waveguide.
of 10 GHz microwave
radiation
in a circular-
The whole body of the ion source is at extraction potential (up to 20 kV) and the ions are accelerated to ground potential. The optimized extraction geometry consisting of a spherical extraction electrode and a conical puller electrode was designed on the basis of [gun computer program simulations [4]. In order to simplify the mechanical assembly of the ion source, the distance between the extraction aperture and the puller electrode was fixed following optimization during test runs. The ion source is only pumped through the extraction aperture and during tests a residual gas pressure of 5 X lo-’ mbar inside the plasma chamber was reached.
The magnetic structure was designed based on computer simulations calculated with the Permag [S] and Pandira [6] codes. The magnetic field for the radial confinement of hot plasma electrons was produced by a hexapole magnet made from NdFeB permanent magnet material with a remanence of 1.12 T and a coercivity of 1920 kA/m. It has a length of 75 mm and consists of 24 trapezoidal segments where the angle of magnetization varies by 45” from one segment to the next. A detailed description of this hexapole geometry is given elsewhere [7]. With this hexapole magnet a magnetic field strength of 0.94 T is obtained at the plasma chamber inner wall. This value corresponds to a ratio of Bmax/BECR = 2.6 at the resonance magnetic field strength of 0.36 T corresponding to a cyclotron frequency of 10 GHz. The axial magnetic mirror field is provided by two outer NdFeB rings magnetized radially with respect to the magnet chamber and in opposite directions and with four axially-magnetized inner rings positioned over the hexapole magnet producing an axial mirror ratio B,,,/B,,, = 1.85. Fig. 3 shows the excellent agreement between our measurements of the axial magnetic field produced by the complete structure compared with the results of the computer simulations. The mirror field can be varied by adjusting the position of the four inner ring magnets to optimize the desired charge state of the extracted ions. In contrast to ECR ion sources, which use solenoid coils and where the axial magnetic field drops to zero in the extraction region, the magnetic field in this source becomes negative on either side of the
ring magnet (axially magnetized)
insulation (max. 20 kV)
NW 150 PN \
__
ring magnet (radially magnetized)
/
gas inlet
-DN
------.
35 CF
plasma chamber
water cooling \ puller electrode
n
Ll
’ Fig. 2. Schematic diagram of the 10 GHz ECRIS.
5cm
’
M. Schlapp et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 525-527
527
proved the most effective with the other coupling methods resulting in excessive thermal heating by the plasma.
0.6
3. First results g
0.2
cs
0.0
The first charge state distributions of oxygen, neon and argon ions extracted from the 10 GHz ECR ion source are shown in Fig. 4. Ion currents were extracted at 12 kV with a gas pressure of 1 X 10P6 mbar in the extraction region. Beams were focussed by an Einzel lens, analyzed using a 45” bending magnet and measured in a 20 mm diameter Faraday-cup approximately 1.5 m from the extraction aperture.
-0.2
-0.6
0
paZtion
Fig. 3. Axial structure.
25
20
5
magnetic
[En]
field in the midplane
of the magnetic
4. Conclusion mirror
peaks.
It would
appear
from
ion trajectory
simula-
tions that such a magnetic field in the extraction region improves the ion beam quality. The position of the extraction electrode was chosen to be at the maximum of the magnetic field. First tests of the 10 GHz ECR ion source showed that the best results were obtained with the injection of approximately 50 W of microwave power. This allows the use of a compact solid state microwave system with low electrical power consumption. The efficiency of microwave coupling to the plasma was tested using a helical antenna, a coaxial line and a rectangular to circular waveguide transition. The rectangular to circular waveguide transition
A compact all-permanent 10 GHz ECR ion source with low power requirements has been constructed and successfully operated to produce useful currents of multiply charged ions. It should be possible to increase the charge state distributions and extracted ion currents by further reduction of the source dimensions enabling higher magnetic fields and a microwave frequency up to 14 GHz to be used.
Acknowledgements The authors gratefully acknowledge the support of the Queen’s University of Belfast and the British Council.
o,,,,, /,,,::l,
References Ill R.K. Janev and D.S. Belie, J. Phys. B. 15 (1982) 3479.
La M. Liehr, R. Trassl, M. Schlapp and E. Salzborn, Rev. Sci. Instr. 63(4) (1992) 2541.
131 P. Sortais et al., Proc. 11th Workshop on Electron Cyclotron
Ii
lo-Z0
1
2
3
Charge
4
5
6
7
State
Fig. 4. Charge state distribution of various elements obtained from the 10 GHz ECRIS with an extraction voltage of (I,, = 12 kV.
Resonance Ion Sources, KVI Report 996, Groningen (1993) p. 97. [41 R. Becker, Nucl. Instr. and Meth. A 298 (1990) 13. [51 Z.Q. Xie and T.A. Antaya. The PERMAG code - for the calculation of the air core 3D multipole field produced by oriented permanent magnets NSCL, Michigan State University, Sept. 87. [61A.M. Winslow, J. Comput. Phys. 2 (19.57) 149. 171K. Halbach, Nucl. Instr. and Meth. 169 (1980) 1.
5. PRGDUCTION/METHODS/APPLICATIONS
& *H__ -_
and Methods in Physics Research B 98 (1995) 525-527
Beam Interactions with Materials&Atoms
@ ELSEVIER
An ultra compact 10 GHz electron-cyclotron-resonance ion source (ECRIS) for multiply charged ions production M. Schlapp a,* ‘I, R. Trassl a, E. Salzborn a, R.W. McCullough H.B. Gilbody b
b, T.K. McLaughlin
b,
aInstitutfir Kernphysik, Strahlenzentrum, b Department
Universitiit Giessen, 35392 Giessen, Germany of Pure and Applied Physics, The Queen’s University of Belfast, Belfast, Northern Ireland, UK
Abstract There is considerable current interest in the use of beams of highly charged ions for a wide variety of applications. In particular, there is a need for sources with low power consumption for installation in high voltage terminals where typically only a few hundred Watts of electrical power is available. An ultra-compact (200 mm long and 120 mm outside diameter) 10 GHz electron-cyclotron-resonance (ECR) ion source has been constructed, making use of recent advances in permanent magnet and solid state microwave technology. The magnetic structure comprises only two ring magnets producing an axial magnetic field configuration with a mirror ratio of 1.6 and a Halbach-hexapole magnet for the radial field with a maximum field of 0.94 T inside the plasma chamber of 25 mm inner diameter. Preliminary tests of this source have shown efficient ECR plasma heating with microwave power levels in the range l-50 W and total extracted ion currents of the order of 1 mA. First results from a prototype source using different microwave injection methods will be presented.
1. Introduction For many years, our group has investigated charge exchange and ionization processes in ion-ion collisions. Employing a crossed-beams technique, one ion beam is produced by a 5 GHz ECR ion source with energies up to 20 keV . q. The ion source producing the second ion beam is installed on a high voltage terminal (up to 400 kV), where only 1 kW of electrical power is available. Up to now, there are no experimental investigations of reactions between two different species of highly charged ions. One reason may be that nobody could afford so far two powerful ECR ion sources most suited for providing intense beams of multiply charged ions for a crossed-beams experiment. Therefore, theoretical calculations [ 11 for quasi-resonant electron capture in isoelectronic systems like:
04++
F5++
OS++
F4+,
F5++ Ne6+-+ F6++ Nesf, could not be verified so far experimentally. Our first attempt in this field was the construction of a 2.45 GHz ECRIS, where the necessary resonance magnetic field strength of 87.5 mT can easily be obtained with
’ Corresponding author. ’ This work contains parts of the thesis of M. Schlapp, Giessen D26.
permanent magnets [2]. This ion source is able to produce very high currents of singly charged ions, e.g. 6.5 n~4 of He+ at an extraction voltage of 20 kV. However this low microwave frequency was not high enough for the plasma electrons to gain sufficient energy to produce substantial yields of multiply-charged ions. Operation at higher microwave frequencies allows smaller dimensions of the plasma chamber reducing both the power consumption by the plasma and the amount of permanent magnet material which results in lower weight and lower costs [3].
2. Source description
The smallest diameter of plasma chamber in which microwaves with a frequency of 10 GHz can still efficiently propagate was determined. Fig. 1 shows the attenuation of the microwaves in a circular copper waveguide as a function of the diameter for both TE,, and TM,, modes. According to this calculation the inner diameter of the plasma chamber was chosen to be 25 mm with a wall thickness of 2.5 mm. Fig. 2 shows the mechanical setup of the 10 GHz ECR ion source. The complete assembly is mounted on a NW 150 PN flange and has a length of 200 mm at a weight of approximately 15 kg. The plasma chamber is water-cooled to protect the permanent magnets from demagnetization at high temperatures and terminates with a DN 35 CF flange to which different waveguide couplings were connected.
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00180-8
5. PRODUCTION/METHODS/APPLICATIONS
M. Schlapp el al. /N~rcl. Instr. and Meth. in Phys. Rex L?98 (1995) 525-527
526
~__ i_ _ A TM,, - mode 0 TE ,, - mode
-. 16
,16
20
22
24
26
26
._A 30
32
id [mm] Fig. 1. Attenuation copper-waveguide.
of 10 GHz microwave
radiation
in a circular-
The whole body of the ion source is at extraction potential (up to 20 kV) and the ions are accelerated to ground potential. The optimized extraction geometry consisting of a spherical extraction electrode and a conical puller electrode was designed on the basis of [gun computer program simulations [4]. In order to simplify the mechanical assembly of the ion source, the distance between the extraction aperture and the puller electrode was fixed following optimization during test runs. The ion source is only pumped through the extraction aperture and during tests a residual gas pressure of 5 X lo-’ mbar inside the plasma chamber was reached.
The magnetic structure was designed based on computer simulations calculated with the Permag [S] and Pandira [6] codes. The magnetic field for the radial confinement of hot plasma electrons was produced by a hexapole magnet made from NdFeB permanent magnet material with a remanence of 1.12 T and a coercivity of 1920 kA/m. It has a length of 75 mm and consists of 24 trapezoidal segments where the angle of magnetization varies by 45” from one segment to the next. A detailed description of this hexapole geometry is given elsewhere [7]. With this hexapole magnet a magnetic field strength of 0.94 T is obtained at the plasma chamber inner wall. This value corresponds to a ratio of Bmax/BECR = 2.6 at the resonance magnetic field strength of 0.36 T corresponding to a cyclotron frequency of 10 GHz. The axial magnetic mirror field is provided by two outer NdFeB rings magnetized radially with respect to the magnet chamber and in opposite directions and with four axially-magnetized inner rings positioned over the hexapole magnet producing an axial mirror ratio B,,,/B,,, = 1.85. Fig. 3 shows the excellent agreement between our measurements of the axial magnetic field produced by the complete structure compared with the results of the computer simulations. The mirror field can be varied by adjusting the position of the four inner ring magnets to optimize the desired charge state of the extracted ions. In contrast to ECR ion sources, which use solenoid coils and where the axial magnetic field drops to zero in the extraction region, the magnetic field in this source becomes negative on either side of the
ring magnet (axially magnetized)
insulation (max. 20 kV)
NW 150 PN \
__
ring magnet (radially magnetized)
/
gas inlet
-DN
------.
35 CF
plasma chamber
water cooling \ puller electrode
n
Ll
’ Fig. 2. Schematic diagram of the 10 GHz ECRIS.
5cm
’
M. Schlapp et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 525-527
527
proved the most effective with the other coupling methods resulting in excessive thermal heating by the plasma.
0.6
3. First results g
0.2
cs
0.0
The first charge state distributions of oxygen, neon and argon ions extracted from the 10 GHz ECR ion source are shown in Fig. 4. Ion currents were extracted at 12 kV with a gas pressure of 1 X 10P6 mbar in the extraction region. Beams were focussed by an Einzel lens, analyzed using a 45” bending magnet and measured in a 20 mm diameter Faraday-cup approximately 1.5 m from the extraction aperture.
-0.2
-0.6
0
paZtion
Fig. 3. Axial structure.
25
20
5
magnetic
[En]
field in the midplane
of the magnetic
4. Conclusion mirror
peaks.
It would
appear
from
ion trajectory
simula-
tions that such a magnetic field in the extraction region improves the ion beam quality. The position of the extraction electrode was chosen to be at the maximum of the magnetic field. First tests of the 10 GHz ECR ion source showed that the best results were obtained with the injection of approximately 50 W of microwave power. This allows the use of a compact solid state microwave system with low electrical power consumption. The efficiency of microwave coupling to the plasma was tested using a helical antenna, a coaxial line and a rectangular to circular waveguide transition. The rectangular to circular waveguide transition
A compact all-permanent 10 GHz ECR ion source with low power requirements has been constructed and successfully operated to produce useful currents of multiply charged ions. It should be possible to increase the charge state distributions and extracted ion currents by further reduction of the source dimensions enabling higher magnetic fields and a microwave frequency up to 14 GHz to be used.
Acknowledgements The authors gratefully acknowledge the support of the Queen’s University of Belfast and the British Council.
o,,,,, /,,,::l,
References Ill R.K. Janev and D.S. Belie, J. Phys. B. 15 (1982) 3479.
La M. Liehr, R. Trassl, M. Schlapp and E. Salzborn, Rev. Sci. Instr. 63(4) (1992) 2541.
131 P. Sortais et al., Proc. 11th Workshop on Electron Cyclotron
Ii
lo-Z0
1
2
3
Charge
4
5
6
7
State
Fig. 4. Charge state distribution of various elements obtained from the 10 GHz ECRIS with an extraction voltage of (I,, = 12 kV.
Resonance Ion Sources, KVI Report 996, Groningen (1993) p. 97. [41 R. Becker, Nucl. Instr. and Meth. A 298 (1990) 13. [51 Z.Q. Xie and T.A. Antaya. The PERMAG code - for the calculation of the air core 3D multipole field produced by oriented permanent magnets NSCL, Michigan State University, Sept. 87. [61A.M. Winslow, J. Comput. Phys. 2 (19.57) 149. 171K. Halbach, Nucl. Instr. and Meth. 169 (1980) 1.
5. PRGDUCTION/METHODS/APPLICATIONS