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An ECR-RFQ ion beam facility for materials research with highly charged slow ions
478
Nuclear Instruments
AN ECR-RFQ SLOW IONS
ION BEAM FACILITY FOR MATERIALS
and Methods in Physics Research B50 (1990) 478-480 North-Holland
RESEARCH
WITH HIGHLY CHARGED
D. HOFMANN ‘) H. BAUMANN ‘), C. FREUDENBERGER”, R. HERRMANN ‘jr A. SCHEMPP H. SCHMIDT-B&KING ‘), G. ZSCHORNACK 2), K. BETHGE I), H. KLEIN ‘), C. LYNEIS 3, and R. STOCK I)
‘),
Ii Universitiit Frankfurt am Main, Fachbereich Physik, Griifstrasse 39, D-6000 Frankfurt am Main, FRG ‘) Technische Universitiit Dresden, Sektion Physik, Mommsenstrasse 13, Dresden, DDR-8027, GDR ‘) Lawrence Berkeley Laboratory I Cyclotron Road Berkeley, CA 94720, USA
An ion beam facility is described which can produce intense beams of very highly charged ions with variable energy in the keV and MeV regime. This facility consists of an electron cyclotron resonance ion source (ECR) and a radiofrequency quadrupole accelerating structure (RFQ). It provides a relatively cheap and compact device for material research with highly charged ions at surfaces as well as for deep-lying solid layers.
Highly
charged
for studying
slow ions are very promising
the properties
of the surfaces
tools
of solids and
layers [l]. Due to their huge potential energy - a result of their empty electronic shells - such highly charged ions induce a high rate of electronic transitions in the target atoms. These X-ray and Augerelectron transitions yield information on the atomic structure in the solid state. E.g., a completely stripped Xes4+ ion stores about 200 keV potential energy in its empty shells, thus being able to give rise to the emission of more than lo4 electrons due to autoionisation processes, partially emitted from inner-shell transitions. For comparison, a Xe ’ + ion of 200 keV kinetic energy will ionize mainly the outermost target-atom shells and only a few characteristic Auger electrons or photons are emitted. Although within the last few years several new facilities for highly charged ion beams based on ECR (electron cyclotron resonance) and EBIS ion sources went successfully into operation [2], all these devices yield beams of either very low kinetic energy (Ekin < 20q keV, where q is the ion charge state) or high kinetic energy (E, > 5 MeV/u). In the first case the ion penetration depth in a solid is less than 1 pm, whereas in the second case the ion penetrates hundreds of pm. Thus in both cases the ion does not efficiently probe the interesting near-surface regime up to about 10 pm. Furthermore, the most interesting ion velocity regime for studying the basic atomic excitation processes of highly charged ions with matter is 0.01 < v,/v, < 0.3 (where up and v, are the ion and electron velocity, respectively). Within this velocity range the predominance of electronic excitation processes varies from transitions resulting mainly from quasimolecular level near-surface
0168-583X/90/$03.50 (North-Holland)
0 Elsetier Science Publishers B.V.
degeneration (“static” transitions) to dynamically induced transitions (e.g. direct ionisation to continuum states). Thus the whole complex aspect of the interaction of “static” and “dynamic” electronic transitions can be studied experimentally in this regime. To investigate these basic electronic excitation processes for highly charged ions, an ion beam facility is needed which provides also beam energies of several 100 keV/u. Based on recent progress in research on the ECR ion source [2] and radiofrequency quadrupole accelerators (RFQ) [3], we have proposed an ECR-RFQ ion beam facility at the Frankfurt University which yields highintensity beams of highly charged ions in the above mentioned velocity regime. The facility will consist (1) of a 14.6 GHz ECR ion source equivalent to the Berkeley AECR source [4], (2) of an energy-variable RFQ accelerating structure [3,5], and (3) of a beam bunching and pulsing device which allows a wide variation of the pulse structure of the beam. In fig. 1 the layout of the new facility is shown. The highly charged ion beam is created in a two-stage ECR ion source [4], where a frequency of 14.6 GHz is fed into both stages. The magnetic field for plasma trapping is provided by a NdFeB sextupole magnet and three solenoidal coils. The NdFeB magnets are mounted inside the vacuum vessel sealed in a thin stainless steel cladding. The mechanical construction of the vacuum system, the sextupole magnet and its cooling system is designed in such a way that nearly all plasma regions in the source can be optically viewed through a movable tube with an optical mirror at the end. The photons are recorded outside the source with standard photon spectrometers. The whole inner vacuum system with the plasma stages and the sextupole is insulated against
D. Hofmann et al. / An ECR ion beam facility for materials research
479
Table 3 Final ion beam energy 6-
SL
ST
1
P
Ion
5-
14N 20Ne 40Ar ‘29Xe 0
Fig. 1. Layout of the proposed ECR-RFQ facility: ECR = electron cyclotron resonance ion source, RFQ = high-frequency quadrupole accelerator, B = buncher, EL = electrostatic einzel lens, F = Faraday cup, G = high-vacuum gauge tube, L = lens, P = turbo-molecular pump, QL = quadrupole lens, SC = scattering chamber, SL = slits, ST = beam pulsing, V = valve, and RF = high-frequency resonator.
so that the ions can be accelerated
ground
energies
E,,
ing magnet.
= 509
kV before
entering
The charge-state-analyzed
up to kinetic
the 90 o analyzbeam
is chopped
Table 1 RFQ parameters Ion energy Injection energy Minimum specific charge Energy resolution Phase Accepted emittance Electrode voltage Minimum aperture Maximum current Rf frequency Length of resonator Rf power
Table 2 Expected
beam currents
lOC-200 keV/u 2.5-5 keV/u 0.20 +2& +20° 0.7 71 mmrad 70 kV 3.2 mm 25 mA 80-115 MHz 1.5 m 60 kW
209 pi
Energy [keV]
Charge
state
ECR + RFQ
ECR
4 7 7 9
lolo-
290 350 350 450
lololo- 600 lo- 850 lo- 750 10-1000 10-1500 10-1000 10-1500 10-1750
12 17 15 20 30 20 30 35
2800
1400-
2ooo- 4ooo
4000-
8000
12900-25
800
20 900-41800
by the steerer ST into bunches of less than 1 ns width and about 8 ns distance and postaccelerated by the RFQ structure [3,5]. This structure (1.5 m long) can accelerate ions with q/M > 0.15 (M is the ion mass) up to energies between 100 and 200 keV/u. The energyvariable structure is described in more detail in ref. [5]. Its most important parameters are given in table 1. A small radiofrequency structure on the exit side of the RFQ allows an energy variation of & 10%. A second 90” analyzing magnet with an entrance and exit slit system can provide ion beams with an energy resolution better than produced by a RFQ. If the highest charge states coming from the ECR are not injected into the RFQ, the first analyzing magnet
[4] in uA
4
N
Ne
Ar
5 6 7 8 9 10 12 14 16 17 20 25 30 35
> 50 >60 >l
> 50 >40 > 20 z 10 >l > 0.1
> > > >
Xe
Bi
60 60 80 70
> 20 >2 > 0.05 >O.OOl
>4 24 >4 >4 >4 >4 >2 > 0.01
0.1
0.2
. . ..‘.I..“““i 0.3 0.4
ionic configuration
>2 >3 > 1 > 0.01
0.5 q/M,
Fig. 2. Mean penetration depths of highly charged ions achieved with ECR ion beam facilities in dependence of the ionic charge state q (ion beam energy - q). The depth scale is estimated for Ar ions in Si. Region 1: ECR ion sources in Berkeley, Grenoble, Groningen, Jtilich, Karlsruhe, etc.; region 2: ECR facility at Argonne National Laboratory (ANL), Chicago, USA; region 3: Van de Graaff accelerators, etc.; Region 4: ECR-RFQ facility Frankfurt. VII. ACCELERATOR
DEVELOPMENT
480
D. Hofmann et al. / An ECR ion beam facility for materials research
0.1
0.2 0.3 0.4 ionicconfiguration
0.5
0.6
q/b
Fig. 3. Ratios of ion velocity (I+,) to electron velocity (ui, here i is taken for the Ar K-shell electron) as a function of the ion beam charge state per mass unit, produced by different ECR facilities.
will bend the highest charge states into a separate beam line for atomic physics experiments, where in particular hydrogen-like ions can be used for interesting ion-atom collision experiments. In table 2 the expected ion beam intensities and in table 3 the final beam energies are given as a function of the charge state q. In fig. 2 the penetration into a solid is plotted as a function of the ionic charge state in units of the ion’s nuclear charge for several existing ion beam facilities. For low-charge-state ions (q = 1) numerous traditional ion beam facilities exist where any penetration depth can be obtained. For highly charged ions and a few pm penetration, however, only the facility proposed here will provide the energies needed. Only the Atlas facility [6] and the EBIS facility at KSU [7] will yield energies for the highest charge states comparable to the accelerator device proposed here. The Atlas ECR, however, will be used almost exclusively for injecting the beam into the Atlas nuclear physics accelerator. In fig. 3 the ion velocity in units of the inner-shell target-electron velocity is shown as a function of the reduced ionic charge q/Z, for the different ECR facilities. It is evident from this figure that the proposed facility covers the region from “static quasimolecular” transitions to dynamically induced transitions here as well, thus enabling us to study the interplay of both transition regimes. The facility proposed here opens a new velocity
regime for highly charged ion beams, enabling a new research activity in basic atomic-collision physics and material analysis. The whole facility is rather small in size and also financially affordable for smaller laboratories. Furthermore, additional accelerating structures can easily be added to obtain an even wider energy range, thus allowing to study also deeper surface layers up to a depth of several tens of pm.
Ret erences [l] J. Andme, invited talk at Summer School on Atomic Phvsics of Highly Charged Ions, Cargese, Corsica, France, l-988, ed. R. Marrus (Plenum Press, New York, 1989). 121See e.g. Proc. lnt. Conf. on the Physics of Multiply Charged Ions, Groningen, The Netherlands, 1986, Nucl. lnstr. and Meth. B23 (1987) and Grenoble, France, 1988, J. Phys. Tome 50 (1989) Cl. [31 A. Schempp, Nucl. lnstr. and Meth. B40/41 (1989) 937. 141 C.M. Lyneis, J. Phys. Tome 50 (1989) Cl 698. 151A. Schempp et al., these Proceedings, (1st European Conference on Accelerators in Applied Research and Technology, Frankfurt am Main, FRG, 1989) Nucl. lnstr. and Meth. B50 (1990) 460. PI R. Pardo et al., 7th Workshop on ECR Ion Sources, Jiilich, 22-23 May 1986, Jiil-Conf-57 (1987) 223. 171 C.L. Cocke, P. Richard and J.S. Eck, Nucl. lnstr. and Meth. BlO/ll (1985) 838.
Nuclear Instruments
AN ECR-RFQ SLOW IONS
ION BEAM FACILITY FOR MATERIALS
and Methods in Physics Research B50 (1990) 478-480 North-Holland
RESEARCH
WITH HIGHLY CHARGED
D. HOFMANN ‘) H. BAUMANN ‘), C. FREUDENBERGER”, R. HERRMANN ‘jr A. SCHEMPP H. SCHMIDT-B&KING ‘), G. ZSCHORNACK 2), K. BETHGE I), H. KLEIN ‘), C. LYNEIS 3, and R. STOCK I)
‘),
Ii Universitiit Frankfurt am Main, Fachbereich Physik, Griifstrasse 39, D-6000 Frankfurt am Main, FRG ‘) Technische Universitiit Dresden, Sektion Physik, Mommsenstrasse 13, Dresden, DDR-8027, GDR ‘) Lawrence Berkeley Laboratory I Cyclotron Road Berkeley, CA 94720, USA
An ion beam facility is described which can produce intense beams of very highly charged ions with variable energy in the keV and MeV regime. This facility consists of an electron cyclotron resonance ion source (ECR) and a radiofrequency quadrupole accelerating structure (RFQ). It provides a relatively cheap and compact device for material research with highly charged ions at surfaces as well as for deep-lying solid layers.
Highly
charged
for studying
slow ions are very promising
the properties
of the surfaces
tools
of solids and
layers [l]. Due to their huge potential energy - a result of their empty electronic shells - such highly charged ions induce a high rate of electronic transitions in the target atoms. These X-ray and Augerelectron transitions yield information on the atomic structure in the solid state. E.g., a completely stripped Xes4+ ion stores about 200 keV potential energy in its empty shells, thus being able to give rise to the emission of more than lo4 electrons due to autoionisation processes, partially emitted from inner-shell transitions. For comparison, a Xe ’ + ion of 200 keV kinetic energy will ionize mainly the outermost target-atom shells and only a few characteristic Auger electrons or photons are emitted. Although within the last few years several new facilities for highly charged ion beams based on ECR (electron cyclotron resonance) and EBIS ion sources went successfully into operation [2], all these devices yield beams of either very low kinetic energy (Ekin < 20q keV, where q is the ion charge state) or high kinetic energy (E, > 5 MeV/u). In the first case the ion penetration depth in a solid is less than 1 pm, whereas in the second case the ion penetrates hundreds of pm. Thus in both cases the ion does not efficiently probe the interesting near-surface regime up to about 10 pm. Furthermore, the most interesting ion velocity regime for studying the basic atomic excitation processes of highly charged ions with matter is 0.01 < v,/v, < 0.3 (where up and v, are the ion and electron velocity, respectively). Within this velocity range the predominance of electronic excitation processes varies from transitions resulting mainly from quasimolecular level near-surface
0168-583X/90/$03.50 (North-Holland)
0 Elsetier Science Publishers B.V.
degeneration (“static” transitions) to dynamically induced transitions (e.g. direct ionisation to continuum states). Thus the whole complex aspect of the interaction of “static” and “dynamic” electronic transitions can be studied experimentally in this regime. To investigate these basic electronic excitation processes for highly charged ions, an ion beam facility is needed which provides also beam energies of several 100 keV/u. Based on recent progress in research on the ECR ion source [2] and radiofrequency quadrupole accelerators (RFQ) [3], we have proposed an ECR-RFQ ion beam facility at the Frankfurt University which yields highintensity beams of highly charged ions in the above mentioned velocity regime. The facility will consist (1) of a 14.6 GHz ECR ion source equivalent to the Berkeley AECR source [4], (2) of an energy-variable RFQ accelerating structure [3,5], and (3) of a beam bunching and pulsing device which allows a wide variation of the pulse structure of the beam. In fig. 1 the layout of the new facility is shown. The highly charged ion beam is created in a two-stage ECR ion source [4], where a frequency of 14.6 GHz is fed into both stages. The magnetic field for plasma trapping is provided by a NdFeB sextupole magnet and three solenoidal coils. The NdFeB magnets are mounted inside the vacuum vessel sealed in a thin stainless steel cladding. The mechanical construction of the vacuum system, the sextupole magnet and its cooling system is designed in such a way that nearly all plasma regions in the source can be optically viewed through a movable tube with an optical mirror at the end. The photons are recorded outside the source with standard photon spectrometers. The whole inner vacuum system with the plasma stages and the sextupole is insulated against
D. Hofmann et al. / An ECR ion beam facility for materials research
479
Table 3 Final ion beam energy 6-
SL
ST
1
P
Ion
5-
14N 20Ne 40Ar ‘29Xe 0
Fig. 1. Layout of the proposed ECR-RFQ facility: ECR = electron cyclotron resonance ion source, RFQ = high-frequency quadrupole accelerator, B = buncher, EL = electrostatic einzel lens, F = Faraday cup, G = high-vacuum gauge tube, L = lens, P = turbo-molecular pump, QL = quadrupole lens, SC = scattering chamber, SL = slits, ST = beam pulsing, V = valve, and RF = high-frequency resonator.
so that the ions can be accelerated
ground
energies
E,,
ing magnet.
= 509
kV before
entering
The charge-state-analyzed
up to kinetic
the 90 o analyzbeam
is chopped
Table 1 RFQ parameters Ion energy Injection energy Minimum specific charge Energy resolution Phase Accepted emittance Electrode voltage Minimum aperture Maximum current Rf frequency Length of resonator Rf power
Table 2 Expected
beam currents
lOC-200 keV/u 2.5-5 keV/u 0.20 +2& +20° 0.7 71 mmrad 70 kV 3.2 mm 25 mA 80-115 MHz 1.5 m 60 kW
209 pi
Energy [keV]
Charge
state
ECR + RFQ
ECR
4 7 7 9
lolo-
290 350 350 450
lololo- 600 lo- 850 lo- 750 10-1000 10-1500 10-1000 10-1500 10-1750
12 17 15 20 30 20 30 35
2800
1400-
2ooo- 4ooo
4000-
8000
12900-25
800
20 900-41800
by the steerer ST into bunches of less than 1 ns width and about 8 ns distance and postaccelerated by the RFQ structure [3,5]. This structure (1.5 m long) can accelerate ions with q/M > 0.15 (M is the ion mass) up to energies between 100 and 200 keV/u. The energyvariable structure is described in more detail in ref. [5]. Its most important parameters are given in table 1. A small radiofrequency structure on the exit side of the RFQ allows an energy variation of & 10%. A second 90” analyzing magnet with an entrance and exit slit system can provide ion beams with an energy resolution better than produced by a RFQ. If the highest charge states coming from the ECR are not injected into the RFQ, the first analyzing magnet
[4] in uA
4
N
Ne
Ar
5 6 7 8 9 10 12 14 16 17 20 25 30 35
> 50 >60 >l
> 50 >40 > 20 z 10 >l > 0.1
> > > >
Xe
Bi
60 60 80 70
> 20 >2 > 0.05 >O.OOl
>4 24 >4 >4 >4 >4 >2 > 0.01
0.1
0.2
. . ..‘.I..“““i 0.3 0.4
ionic configuration
>2 >3 > 1 > 0.01
0.5 q/M,
Fig. 2. Mean penetration depths of highly charged ions achieved with ECR ion beam facilities in dependence of the ionic charge state q (ion beam energy - q). The depth scale is estimated for Ar ions in Si. Region 1: ECR ion sources in Berkeley, Grenoble, Groningen, Jtilich, Karlsruhe, etc.; region 2: ECR facility at Argonne National Laboratory (ANL), Chicago, USA; region 3: Van de Graaff accelerators, etc.; Region 4: ECR-RFQ facility Frankfurt. VII. ACCELERATOR
DEVELOPMENT
480
D. Hofmann et al. / An ECR ion beam facility for materials research
0.1
0.2 0.3 0.4 ionicconfiguration
0.5
0.6
q/b
Fig. 3. Ratios of ion velocity (I+,) to electron velocity (ui, here i is taken for the Ar K-shell electron) as a function of the ion beam charge state per mass unit, produced by different ECR facilities.
will bend the highest charge states into a separate beam line for atomic physics experiments, where in particular hydrogen-like ions can be used for interesting ion-atom collision experiments. In table 2 the expected ion beam intensities and in table 3 the final beam energies are given as a function of the charge state q. In fig. 2 the penetration into a solid is plotted as a function of the ionic charge state in units of the ion’s nuclear charge for several existing ion beam facilities. For low-charge-state ions (q = 1) numerous traditional ion beam facilities exist where any penetration depth can be obtained. For highly charged ions and a few pm penetration, however, only the facility proposed here will provide the energies needed. Only the Atlas facility [6] and the EBIS facility at KSU [7] will yield energies for the highest charge states comparable to the accelerator device proposed here. The Atlas ECR, however, will be used almost exclusively for injecting the beam into the Atlas nuclear physics accelerator. In fig. 3 the ion velocity in units of the inner-shell target-electron velocity is shown as a function of the reduced ionic charge q/Z, for the different ECR facilities. It is evident from this figure that the proposed facility covers the region from “static quasimolecular” transitions to dynamically induced transitions here as well, thus enabling us to study the interplay of both transition regimes. The facility proposed here opens a new velocity
regime for highly charged ion beams, enabling a new research activity in basic atomic-collision physics and material analysis. The whole facility is rather small in size and also financially affordable for smaller laboratories. Furthermore, additional accelerating structures can easily be added to obtain an even wider energy range, thus allowing to study also deeper surface layers up to a depth of several tens of pm.
Ret erences [l] J. Andme, invited talk at Summer School on Atomic Phvsics of Highly Charged Ions, Cargese, Corsica, France, l-988, ed. R. Marrus (Plenum Press, New York, 1989). 121See e.g. Proc. lnt. Conf. on the Physics of Multiply Charged Ions, Groningen, The Netherlands, 1986, Nucl. lnstr. and Meth. B23 (1987) and Grenoble, France, 1988, J. Phys. Tome 50 (1989) Cl. [31 A. Schempp, Nucl. lnstr. and Meth. B40/41 (1989) 937. 141 C.M. Lyneis, J. Phys. Tome 50 (1989) Cl 698. 151A. Schempp et al., these Proceedings, (1st European Conference on Accelerators in Applied Research and Technology, Frankfurt am Main, FRG, 1989) Nucl. lnstr. and Meth. B50 (1990) 460. PI R. Pardo et al., 7th Workshop on ECR Ion Sources, Jiilich, 22-23 May 1986, Jiil-Conf-57 (1987) 223. 171 C.L. Cocke, P. Richard and J.S. Eck, Nucl. lnstr. and Meth. BlO/ll (1985) 838.