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A high-current low-emittance dc ECR proton source
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A309 (1991) 37-42 North-Holland
Section A
A high-current low-emittance dc ECR proton source Terence Taylor and John S.C . Wills
Chalk River Laboratories, AECL Research, Chalk Ricer, Ontario, KOJ IJO, Canada
Received 18 June 1991
A simple high-current low-emittance do ECR proton source has been developed for a cw RFQ accelerator injector . Proton fractions of up to 90% have been obtained at beam current densities as high as 350 mA/cmz. The normalized rms emittance of a 25 mA beam from a single 4.0 mm diameter extraction aperture was 0.07 Tr mm mrad corresponding to a brightness of 5 A/(Tr mm mrad)Z. Similar results were obtained with multiple aperture extraction systems.
1. Introduction
2. Design
The resonant absorption of microwaves by electrons orbiting in a magnetic field is a convenient means of exciting a cold plasma suitable for the production of a high-quality ion beam . Electron cyclotron resonance (ECR) ion sources have many advantages over their arc-discharge competitors : the mass spectrum, as well as the charge state distribution, of the beam from an ECR ion source is modified relatively easily by adjusting either the plasma generator design or the operating parameters ; an ECR plasma generator is more dependable than an arc-discharge plasma generator with a short-lived cathode; the ionization efficiency of an ECR ion source is very high and, consequently, the gas load on the vacuum system is very low; in principle, all of the power supplies for an ECR ion source can be at ground potential, avoiding the isolation transformers associated with most arc-discharge ion sources and simplifying the control system . The advantages of ECR plasma generators in the production of multiply-charged ions have been recognized for many years [1]. However, the designers of high-current ion sources have hardly begun to exploit microwave and, especially, ECR plasma generators . Sakudo et al . [2] have generated a few hundred mA of several different ions with microwave ion sources designed for ion implantation . An ECR ion source developed by Torii et al . [31 has produced up to 200 mA of oxygen ions for the fabrication of buried SiO 2 layers . Nevertheless, high-current accelerator injectors are still predominantly arc-discharge ion sources. The present article describes a very simple high-current low-emittance de ECR proton source developed at Chalk River Laboratories as an injector for a ew RFQ accelerator .
Several factors must be considered in developing a design for a high-current low-emittance ECR ion source . Firstly, the ions are, of necessity, created in a substantial magnetic field . The divergence of an ion beam extracted from an axially symmetric magnetic field can be deduced from [41
Elsevier Science Publishers B.V .
1 qBr
2 p where q is the charge of the ions, B is the magnetic induction at the extraction aperture, r is the radius of the aperture and p is the momentum of the extracted ions . The contribution of the thermal velocities of the ions is neglected . The magnetic induction that satisfies the ECR condition is given by
where m and e are, respectively, the mass and the charge of the electron and to is the drive frequency. Assuming that the magnetic induction is constant, Bc can be substituted for B in eq . (1). The ion beam divergence then becomes l gmcer
Thus, the quality of the beam from an ECR ion source may be limited by the frequency of the microwave generator. (For example, a 35 keV proton beam extracted from a 30 GHz ECR ion source through a 5 mm diameter aperture would have an unacceptably high divergence of at least 50 mrad . On the other
38
T. Taylor, J.S.C. Wills / A dc ECR proton source
hand, at a microwave frequency of 2.45 GHz, the influence of the magnetic field would be so small that the divergence would be determined by the optical aberations .) Secondly, although a low microwave frequency is essential for a low-divergence ion beam, low-frequency microwaves cannot propagate through a high-density plasma . To be precise, microwaves of frequency m are exponentially attenuated above a critical density given by eo MW (The critical density for 2.45 GHz microwaves, for example, is only 7 X 10" ) cm -3 , far lower than the density required in a high-current ion source .) As a consequence, low-frequency microwaves can be coupled to a high-density plasma at the surface only . In other words, in an ECR ion source with a plasma density greater than nc, the resonance condition must be satisfied by the magnetic induction at the point where the microwaves are introduced to the plasma . Finally, the magnetic induction must be essentially constant throughout the plasma generator so that the plasma will be confined radially . If the magnetic field declines significantly anywhere between the point where the plasma is generated and the point where the ions are extracted, the plasma will diffuse along the diverging magnetic field lines, proportionately reducing the density at the extraction aperture . KLASMA ELECTRODE -,
An ion source that roughly conforms to the above considerations was assembled largely from existing components for a preliminary demonstration . It is depicted schematically in fig. 1. The WR284 drive line incorporates a 1000 W cw microwave power supply based on a 2.45 GHz magnetron, a dual directional coupler for power monitoring and a motorized three stub tuner for impedance matching . Active tuning elements are essential because the dielectric properties of the plasma vary with the rf power, the gas feed rate and the magnetic-field configuration. The microwave power is introduced to the plasma chamber through a two-layer window. The first layer, a 10 mm thick quartz plate terminating the waveguide, creates a vacuum seal . The second layer, a 4.0 mm thick plate of boron nitride adjacent to the plasma, conducts away the heat generated by electrons backstreaming from the extraction column . The water cooled oxygen-free high-conductivity (OFHC) copper plasma chamber is 90 mm in diameter and 100 mm long . An axial magnetic induction of up to 100 mT is generated by two solenoids. The solenoids can be translated axially to vary the distribution of the magnetic induction in the plasma chamber or to gain access to the plasma chamber and the extraction column . The extraction column is a multi-aperture triode . The diameter of the apertures in the plasma electrode and the deceleration electrode is 4.0 mm while the apertures in the acceleration electrode are 3 .5 mm in DECEL ELECTRODE
ACCEL ELECTRODE
..wOV.OApQO~
9.
-~
_
~, !HN
Y
RF WINDOW
PLASMA CHAMBER
SOLENOID
Fig. 1 . Schematic of high-current low-emittance do ECR proton source .
T. Taylor, J S. C. Wills / A de ECR proton source
39
diameter . The apertures are shaped to maximize brightness [5]. A close-packed hexagonal array of up to seven apertures can be accommodated, however, the measurements reported here were made either with a single aperture or with three collinear apertures to facilitate diagnosis. The centre-to-centre spacing of the apertures is 6 .5 mm . The acceleration gap is 6.0 mm while the deceleration gap is 2.0 mm . The electrodes were fabricated from OFHC copper faced with TZM molybdenum alloy. The facing improves resistance to sparking initiated by backstreaming electrons. 3. Performance Prior to the installation of the extraction column, the plasma generator was studied with a double cylindrical Langmuir probe [6]. The saturation ion current density and the electron temperature were measured as a function of the peak magnetic induction, the axial position of the solenoids, the microwave power and the hydrogen feed rate . The probe could be translated both axially and radially via a slot in the plasma electrode. The maximum ion current density exceeded 300 mA/em z. The electron temperature was typically 20 eV . The density declined rapidly with increasing radial displacement . At half of the radius of the chamber, the density was invariably less than half of the maximum. On the other hand, the density varied relatively little with axial position . Following the Langmuir probe studies, the extraction column was installed for the measurement of beam currents, mass spectra and emittances . The current density of an ion beam extracted from three apertures and collected in a Faraday cup is shown in fig. 2 as a function of the magnetic induction at the microwave window . The microwave power was 400 W and the hydrogen mass flow rate was 3.5 seem (5 .3 l.Lg/s) . The three stub tuner was adjusted for matched impedance at each value of the magnetic induction. At the maximum beam current density, the magnetic induction adjacent to the microwave window was 87 .5 mT, satisfying the ECR condition for 2.45 GHz microwaves . The linear background is probably attributable to the excitation of left-hand circularly polarized electromagnetic waves that travel parallel to the magnetic field [7]. (The critical plasma density for these L-waves propagating in a modest magnetic field is considerably higher than the critical plasma density for the incident microwaves .) Measurements similar to those shown in fig. 2 were made for various solenoid positions with either one or both of the solenoids energized. The optimum magnetic induction at the microwave window invariably satisfied the ECR condition. Keeping in mind that the
Fig. 2. Beam current density vs axial magnetic induction at the microwave window with matched impedance. two solenoids were energized by a single power supply and that the solenoids were fastened together, the magnetic field configuration that generated the highest ion current density is shown in fig. 3 . The magnetic induction is reasonably uniform over the length of the plasma chamber. Thus, the plasma is confined radially so that the density is essentially constant along the axis . The beam current density is plotted as a function of the microwave power at a fixed hydrogen mass flow rate of 1 .0 seem (1 .5 lkg/s) in fig. 4. Fig. 5 shows the
0 -50
-25
0
25
50
75
100
125
Axial Displacement (mm)
150
Fig. 3. Magnetic induction on the axis of the solenoids as a function of axial displacement from the microwave window. The dashed vertical lines define the axial extent of the plasma chamber.
40
T. Taylor, J.S .C. Wills / A dc ECR proton source 1000 H
H,'(HS')
100 H2; ( Nie~)
H2
Q) 10
có
N `
N '
Q) C).,
1 00
Fig. 4. Beam current density vs microwave power with BN lining (solid) and without (open) . beam current density as a function of the flow rate at a fixed power of 1000 W. The ion current density increased continuously with increasing microwave power and with decreasing hydrogen mass flow rate . The ion source was unstable at hydrogen mass flow rates of less than 1 .0 sccm (1 .5 Fig/s) . It is worth noting that the current density was almost unchanged when the plasma electrode was lined with boron nitride. Although the measurements displayed here were generated with a single extraction aperture, similar results were obtained with three collinear apertures, except that the hydrogen mass flow rate was, of course, tripled. The
50
100
150
200
Displacement (mm)
250
Fig. 6. Typical mass spectrum for three collinear extraction apertures. Contributions of species in parentheses are relatively insignificant . intensity varied by only 12% from beamlet to beamlet when three apertures were used . During these measurements, the magnetic induction adjacent to the microwave window was only about 80 mT, significantly lower than the 87 .5 mT that corresponds to the electron cyclotron resonance. Current densities as high as 350 mA/cmz could be obtained by operating on the ECR resonance at a microwave power of 1000 W. However, the ion source usually performed more stably off resonance, and, in any case, the beam current density was substantial even below the resonance. 10
~08 0
00
Fig. 5. Beam current density vs hydrogen mass flow rate with BN lining (solid) and without(open) .
200
400
600
800
1000
Microwave Power (W)
1200
Fig. 7. Species fractions vs microwave power with BN lining (solid) and without (open).
T. Taylor, J S. C. Wills / A de ECR proton source
41
60 ~
10
X 08
o
0
Hz+ H3
40
U cÓ06
204 U
X -20
02
-40
00
-60 -40
Flow Rate (seem)
Fig. 8. Species fractions vs hydrogen mass flow rate with BN lining (solid) and without (open) .
A typical mass spectrum for three collinear extrac-
tion apertures is shown in fig . 6. The measurements
were made with a system described elsewhere [8]. The species fractions calculated from similar spectra are plotted against the microwave power and the hydrogen mass flow rate in figs . 7 and 8. The proton fraction, like
the beam current density, increased continuously with
increasing microwave power and with decreasing hydrogen mass flow rate . However,
the addition of a
boron nitride plasma electrode liner changed the mass
-20
0
x (mm)
20
40
Fig. 9. Typical phase space diagram for three collinear extrac tion apertures. Contours are at 2% (dashed) and 5% (solid) of maximum intensity.
waist . In this case, the beam current was 25 mA per aperture so that the brightness was 5 A/( ,rr mm mrad)Z.
The emittance of the beam from a closely packed
array of N extraction apertures can be inferred from the one- and three-aperture measurements because the
radius of the waist is determined almost entirely by the number of apertures and the aperture separation . Assuming that the centre-to-centre spacing of the apertures is fixed at 6.5 mm and that the rms divergence is
spectra
dramatically . Indeed, the maximum proton fraction increased from 55% to 90%. Similar results were obtained with liners of quartz or alumina, The proton fraction
enhancement observed here
was foreseen by Chan et al . [9] using a comprehensive plasma generator model that simultaneously solved a set of particle balance equations based on 11 atomic and molecular reactions . The model predicted that the
proton fraction would increase significantly if the hydrogen atom recombination coefficient of the wall material were reduced. Quartz has a much smaller hydrogen atom recombination coefficient than copper [10]. The emittance was measured for a single extraction
aperture and for a linear array of three extraction
apertures with a two slit emittance measuring system
[8]. A typical phase-space plot for three collinear aper-
tures is shown in fig. 9. The rms divergence of the beamlets from each of three apertures as well as the rms divergence of the entire beam are plotted as a function of the extraction voltage in fig. 10 . The minimum normalized rms emittance for a single aperture
was 0.07 rr mm mrad corresponding to an rms divergence of 15 mrad and an rms radius of 0.5 mm at the
Extraction Voltage (kV) Fig. 10 . Rms divergence vs extraction voltage for each of three beamlets and for entire beam .
42
T. Taylor, J.S.C. Wills / A dc ECR proton source
a constant 15 mrad, the normalized rms emittance is given by E ( = 0.2 vfN ,rr
mm mrad) .
4. Discussion A simple ECR plasma generator has proved to be a practical device for generating dc proton beams with high current densities and low emittances . Hydrogen ion beams of up to 130 mA have been extracted from three apertures having a total area of only 0.38 cm 2. Proton fractions as high as 90% have been achieved by lining the plasma electrode with suitable materials. Because the rms divergence is only 15 mrad, exceptionally bright multi-beamlet proton beams can be obtained . For example, a hexagonal array of seven 4 mm diameter apertures would yield a 280 mA proton beam with a normalized rms emittance of just 0 .5 rr mm mrad . The success of the present ECR proton source has encouraged the development of an improved design . A dc waveguide break has been developed so that the microwave power supply can be at ground potential. An automatic impedance matching network will replace the three stub tuner. A single-layer aluminum nitride window, that dissipates the energy of the backstreaming electrons as well as providing a vacuum seal, is being evaluated. The solenoids will be isolated from the plasma chamber so that the associated dc power supplies can be at ground potential . The extraction system has been completely redesigned to facilitate installation and maintenance and to ensure that the electrostatic field enhancement in the extraction gap is minimized. The improved design is expected to operate more reliably at the extraction voltages required for a high-current cw RFQ injector.
A cw RFQ has recently accelerated more than 75 mA of 50 keV protons from the original ion source to an energy of 600 keV [111 .
Acknowledgements The authors are indebted to E.C . Douglas and D.G . Hewitt for invaluable contributions to the design and the fabrication of the ion source, to G.F. Morin for indispensable assistance in the maintenance of the ion source test stand and to M.S . de Jong and N.A . Ebrahim for helpful comments on the manuscript .
References [1] R. Geller, Annu . Rev. Nucl . Part . Sci. 40 (1990) 15 . [21 N. Sakudo, Nucl . Instr . and Meth . B21 (1987) 168. [31 Y Torii, M. Shimada, 1. Watanabe, J. Hipple, C Hayden and G. Dionne, Rev. Sci. Instr. 61 (1990) 253 [41 W. Kraus-Vogt, H. Beuscher, H.L . Ragedoorn, J. Reich and P. Wucherer, Nucl . Instr . and Meth . A268 (1988) 5 . [51 W.S . Cooper, K.H . Berkner and R.V. Pyle, Nucl . Fusion 12 (1972) 263. [6] E.O . Johnson and L. Malter, Phys . Rev . 80 (1950) 58 . [71 T .H. Stix, The Theory of Plasma Waves (McGraw-Hill, New York, 1962) . [81 T. Taylor, M.S de Jong and W.L . Michel, 1988 Linear Accelerator Conf . Proc ., Continuous Electron Beam Accelerator Facility Rept . 89-001, June 1989, pp . 100-102. [91 C .F . Chan, C.F . Burrell and W.S . Cooper, J . Appl . Phys . 54 (1983) 6119 . [10] B.J . Wood and H. Wise, J. Phys . Chem . 65 (1961) 1976 . [111 G.M . Arbique, T. Taylor, M.H . Thrasher and J .S .C . Wills, Proc . 1991 IEEE Particle Accelerator Conference, to be published .
Nuclear Instruments and Methods in Physics Research A309 (1991) 37-42 North-Holland
Section A
A high-current low-emittance dc ECR proton source Terence Taylor and John S.C . Wills
Chalk River Laboratories, AECL Research, Chalk Ricer, Ontario, KOJ IJO, Canada
Received 18 June 1991
A simple high-current low-emittance do ECR proton source has been developed for a cw RFQ accelerator injector . Proton fractions of up to 90% have been obtained at beam current densities as high as 350 mA/cmz. The normalized rms emittance of a 25 mA beam from a single 4.0 mm diameter extraction aperture was 0.07 Tr mm mrad corresponding to a brightness of 5 A/(Tr mm mrad)Z. Similar results were obtained with multiple aperture extraction systems.
1. Introduction
2. Design
The resonant absorption of microwaves by electrons orbiting in a magnetic field is a convenient means of exciting a cold plasma suitable for the production of a high-quality ion beam . Electron cyclotron resonance (ECR) ion sources have many advantages over their arc-discharge competitors : the mass spectrum, as well as the charge state distribution, of the beam from an ECR ion source is modified relatively easily by adjusting either the plasma generator design or the operating parameters ; an ECR plasma generator is more dependable than an arc-discharge plasma generator with a short-lived cathode; the ionization efficiency of an ECR ion source is very high and, consequently, the gas load on the vacuum system is very low; in principle, all of the power supplies for an ECR ion source can be at ground potential, avoiding the isolation transformers associated with most arc-discharge ion sources and simplifying the control system . The advantages of ECR plasma generators in the production of multiply-charged ions have been recognized for many years [1]. However, the designers of high-current ion sources have hardly begun to exploit microwave and, especially, ECR plasma generators . Sakudo et al . [2] have generated a few hundred mA of several different ions with microwave ion sources designed for ion implantation . An ECR ion source developed by Torii et al . [31 has produced up to 200 mA of oxygen ions for the fabrication of buried SiO 2 layers . Nevertheless, high-current accelerator injectors are still predominantly arc-discharge ion sources. The present article describes a very simple high-current low-emittance de ECR proton source developed at Chalk River Laboratories as an injector for a ew RFQ accelerator .
Several factors must be considered in developing a design for a high-current low-emittance ECR ion source . Firstly, the ions are, of necessity, created in a substantial magnetic field . The divergence of an ion beam extracted from an axially symmetric magnetic field can be deduced from [41
Elsevier Science Publishers B.V .
1 qBr
2 p where q is the charge of the ions, B is the magnetic induction at the extraction aperture, r is the radius of the aperture and p is the momentum of the extracted ions . The contribution of the thermal velocities of the ions is neglected . The magnetic induction that satisfies the ECR condition is given by
where m and e are, respectively, the mass and the charge of the electron and to is the drive frequency. Assuming that the magnetic induction is constant, Bc can be substituted for B in eq . (1). The ion beam divergence then becomes l gmcer
Thus, the quality of the beam from an ECR ion source may be limited by the frequency of the microwave generator. (For example, a 35 keV proton beam extracted from a 30 GHz ECR ion source through a 5 mm diameter aperture would have an unacceptably high divergence of at least 50 mrad . On the other
38
T. Taylor, J.S.C. Wills / A dc ECR proton source
hand, at a microwave frequency of 2.45 GHz, the influence of the magnetic field would be so small that the divergence would be determined by the optical aberations .) Secondly, although a low microwave frequency is essential for a low-divergence ion beam, low-frequency microwaves cannot propagate through a high-density plasma . To be precise, microwaves of frequency m are exponentially attenuated above a critical density given by eo MW (The critical density for 2.45 GHz microwaves, for example, is only 7 X 10" ) cm -3 , far lower than the density required in a high-current ion source .) As a consequence, low-frequency microwaves can be coupled to a high-density plasma at the surface only . In other words, in an ECR ion source with a plasma density greater than nc, the resonance condition must be satisfied by the magnetic induction at the point where the microwaves are introduced to the plasma . Finally, the magnetic induction must be essentially constant throughout the plasma generator so that the plasma will be confined radially . If the magnetic field declines significantly anywhere between the point where the plasma is generated and the point where the ions are extracted, the plasma will diffuse along the diverging magnetic field lines, proportionately reducing the density at the extraction aperture . KLASMA ELECTRODE -,
An ion source that roughly conforms to the above considerations was assembled largely from existing components for a preliminary demonstration . It is depicted schematically in fig. 1. The WR284 drive line incorporates a 1000 W cw microwave power supply based on a 2.45 GHz magnetron, a dual directional coupler for power monitoring and a motorized three stub tuner for impedance matching . Active tuning elements are essential because the dielectric properties of the plasma vary with the rf power, the gas feed rate and the magnetic-field configuration. The microwave power is introduced to the plasma chamber through a two-layer window. The first layer, a 10 mm thick quartz plate terminating the waveguide, creates a vacuum seal . The second layer, a 4.0 mm thick plate of boron nitride adjacent to the plasma, conducts away the heat generated by electrons backstreaming from the extraction column . The water cooled oxygen-free high-conductivity (OFHC) copper plasma chamber is 90 mm in diameter and 100 mm long . An axial magnetic induction of up to 100 mT is generated by two solenoids. The solenoids can be translated axially to vary the distribution of the magnetic induction in the plasma chamber or to gain access to the plasma chamber and the extraction column . The extraction column is a multi-aperture triode . The diameter of the apertures in the plasma electrode and the deceleration electrode is 4.0 mm while the apertures in the acceleration electrode are 3 .5 mm in DECEL ELECTRODE
ACCEL ELECTRODE
..wOV.OApQO~
9.
-~
_
~, !HN
Y
RF WINDOW
PLASMA CHAMBER
SOLENOID
Fig. 1 . Schematic of high-current low-emittance do ECR proton source .
T. Taylor, J S. C. Wills / A de ECR proton source
39
diameter . The apertures are shaped to maximize brightness [5]. A close-packed hexagonal array of up to seven apertures can be accommodated, however, the measurements reported here were made either with a single aperture or with three collinear apertures to facilitate diagnosis. The centre-to-centre spacing of the apertures is 6 .5 mm . The acceleration gap is 6.0 mm while the deceleration gap is 2.0 mm . The electrodes were fabricated from OFHC copper faced with TZM molybdenum alloy. The facing improves resistance to sparking initiated by backstreaming electrons. 3. Performance Prior to the installation of the extraction column, the plasma generator was studied with a double cylindrical Langmuir probe [6]. The saturation ion current density and the electron temperature were measured as a function of the peak magnetic induction, the axial position of the solenoids, the microwave power and the hydrogen feed rate . The probe could be translated both axially and radially via a slot in the plasma electrode. The maximum ion current density exceeded 300 mA/em z. The electron temperature was typically 20 eV . The density declined rapidly with increasing radial displacement . At half of the radius of the chamber, the density was invariably less than half of the maximum. On the other hand, the density varied relatively little with axial position . Following the Langmuir probe studies, the extraction column was installed for the measurement of beam currents, mass spectra and emittances . The current density of an ion beam extracted from three apertures and collected in a Faraday cup is shown in fig. 2 as a function of the magnetic induction at the microwave window . The microwave power was 400 W and the hydrogen mass flow rate was 3.5 seem (5 .3 l.Lg/s) . The three stub tuner was adjusted for matched impedance at each value of the magnetic induction. At the maximum beam current density, the magnetic induction adjacent to the microwave window was 87 .5 mT, satisfying the ECR condition for 2.45 GHz microwaves . The linear background is probably attributable to the excitation of left-hand circularly polarized electromagnetic waves that travel parallel to the magnetic field [7]. (The critical plasma density for these L-waves propagating in a modest magnetic field is considerably higher than the critical plasma density for the incident microwaves .) Measurements similar to those shown in fig. 2 were made for various solenoid positions with either one or both of the solenoids energized. The optimum magnetic induction at the microwave window invariably satisfied the ECR condition. Keeping in mind that the
Fig. 2. Beam current density vs axial magnetic induction at the microwave window with matched impedance. two solenoids were energized by a single power supply and that the solenoids were fastened together, the magnetic field configuration that generated the highest ion current density is shown in fig. 3 . The magnetic induction is reasonably uniform over the length of the plasma chamber. Thus, the plasma is confined radially so that the density is essentially constant along the axis . The beam current density is plotted as a function of the microwave power at a fixed hydrogen mass flow rate of 1 .0 seem (1 .5 lkg/s) in fig. 4. Fig. 5 shows the
0 -50
-25
0
25
50
75
100
125
Axial Displacement (mm)
150
Fig. 3. Magnetic induction on the axis of the solenoids as a function of axial displacement from the microwave window. The dashed vertical lines define the axial extent of the plasma chamber.
40
T. Taylor, J.S .C. Wills / A dc ECR proton source 1000 H
H,'(HS')
100 H2; ( Nie~)
H2
Q) 10
có
N `
N '
Q) C).,
1 00
Fig. 4. Beam current density vs microwave power with BN lining (solid) and without (open) . beam current density as a function of the flow rate at a fixed power of 1000 W. The ion current density increased continuously with increasing microwave power and with decreasing hydrogen mass flow rate . The ion source was unstable at hydrogen mass flow rates of less than 1 .0 sccm (1 .5 Fig/s) . It is worth noting that the current density was almost unchanged when the plasma electrode was lined with boron nitride. Although the measurements displayed here were generated with a single extraction aperture, similar results were obtained with three collinear apertures, except that the hydrogen mass flow rate was, of course, tripled. The
50
100
150
200
Displacement (mm)
250
Fig. 6. Typical mass spectrum for three collinear extraction apertures. Contributions of species in parentheses are relatively insignificant . intensity varied by only 12% from beamlet to beamlet when three apertures were used . During these measurements, the magnetic induction adjacent to the microwave window was only about 80 mT, significantly lower than the 87 .5 mT that corresponds to the electron cyclotron resonance. Current densities as high as 350 mA/cmz could be obtained by operating on the ECR resonance at a microwave power of 1000 W. However, the ion source usually performed more stably off resonance, and, in any case, the beam current density was substantial even below the resonance. 10
~08 0
00
Fig. 5. Beam current density vs hydrogen mass flow rate with BN lining (solid) and without(open) .
200
400
600
800
1000
Microwave Power (W)
1200
Fig. 7. Species fractions vs microwave power with BN lining (solid) and without (open).
T. Taylor, J S. C. Wills / A de ECR proton source
41
60 ~
10
X 08
o
0
Hz+ H3
40
U cÓ06
204 U
X -20
02
-40
00
-60 -40
Flow Rate (seem)
Fig. 8. Species fractions vs hydrogen mass flow rate with BN lining (solid) and without (open) .
A typical mass spectrum for three collinear extrac-
tion apertures is shown in fig . 6. The measurements
were made with a system described elsewhere [8]. The species fractions calculated from similar spectra are plotted against the microwave power and the hydrogen mass flow rate in figs . 7 and 8. The proton fraction, like
the beam current density, increased continuously with
increasing microwave power and with decreasing hydrogen mass flow rate . However,
the addition of a
boron nitride plasma electrode liner changed the mass
-20
0
x (mm)
20
40
Fig. 9. Typical phase space diagram for three collinear extrac tion apertures. Contours are at 2% (dashed) and 5% (solid) of maximum intensity.
waist . In this case, the beam current was 25 mA per aperture so that the brightness was 5 A/( ,rr mm mrad)Z.
The emittance of the beam from a closely packed
array of N extraction apertures can be inferred from the one- and three-aperture measurements because the
radius of the waist is determined almost entirely by the number of apertures and the aperture separation . Assuming that the centre-to-centre spacing of the apertures is fixed at 6.5 mm and that the rms divergence is
spectra
dramatically . Indeed, the maximum proton fraction increased from 55% to 90%. Similar results were obtained with liners of quartz or alumina, The proton fraction
enhancement observed here
was foreseen by Chan et al . [9] using a comprehensive plasma generator model that simultaneously solved a set of particle balance equations based on 11 atomic and molecular reactions . The model predicted that the
proton fraction would increase significantly if the hydrogen atom recombination coefficient of the wall material were reduced. Quartz has a much smaller hydrogen atom recombination coefficient than copper [10]. The emittance was measured for a single extraction
aperture and for a linear array of three extraction
apertures with a two slit emittance measuring system
[8]. A typical phase-space plot for three collinear aper-
tures is shown in fig. 9. The rms divergence of the beamlets from each of three apertures as well as the rms divergence of the entire beam are plotted as a function of the extraction voltage in fig. 10 . The minimum normalized rms emittance for a single aperture
was 0.07 rr mm mrad corresponding to an rms divergence of 15 mrad and an rms radius of 0.5 mm at the
Extraction Voltage (kV) Fig. 10 . Rms divergence vs extraction voltage for each of three beamlets and for entire beam .
42
T. Taylor, J.S.C. Wills / A dc ECR proton source
a constant 15 mrad, the normalized rms emittance is given by E ( = 0.2 vfN ,rr
mm mrad) .
4. Discussion A simple ECR plasma generator has proved to be a practical device for generating dc proton beams with high current densities and low emittances . Hydrogen ion beams of up to 130 mA have been extracted from three apertures having a total area of only 0.38 cm 2. Proton fractions as high as 90% have been achieved by lining the plasma electrode with suitable materials. Because the rms divergence is only 15 mrad, exceptionally bright multi-beamlet proton beams can be obtained . For example, a hexagonal array of seven 4 mm diameter apertures would yield a 280 mA proton beam with a normalized rms emittance of just 0 .5 rr mm mrad . The success of the present ECR proton source has encouraged the development of an improved design . A dc waveguide break has been developed so that the microwave power supply can be at ground potential. An automatic impedance matching network will replace the three stub tuner. A single-layer aluminum nitride window, that dissipates the energy of the backstreaming electrons as well as providing a vacuum seal, is being evaluated. The solenoids will be isolated from the plasma chamber so that the associated dc power supplies can be at ground potential . The extraction system has been completely redesigned to facilitate installation and maintenance and to ensure that the electrostatic field enhancement in the extraction gap is minimized. The improved design is expected to operate more reliably at the extraction voltages required for a high-current cw RFQ injector.
A cw RFQ has recently accelerated more than 75 mA of 50 keV protons from the original ion source to an energy of 600 keV [111 .
Acknowledgements The authors are indebted to E.C . Douglas and D.G . Hewitt for invaluable contributions to the design and the fabrication of the ion source, to G.F. Morin for indispensable assistance in the maintenance of the ion source test stand and to M.S . de Jong and N.A . Ebrahim for helpful comments on the manuscript .
References [1] R. Geller, Annu . Rev. Nucl . Part . Sci. 40 (1990) 15 . [21 N. Sakudo, Nucl . Instr . and Meth . B21 (1987) 168. [31 Y Torii, M. Shimada, 1. Watanabe, J. Hipple, C Hayden and G. Dionne, Rev. Sci. Instr. 61 (1990) 253 [41 W. Kraus-Vogt, H. Beuscher, H.L . Ragedoorn, J. Reich and P. Wucherer, Nucl . Instr . and Meth . A268 (1988) 5 . [51 W.S . Cooper, K.H . Berkner and R.V. Pyle, Nucl . Fusion 12 (1972) 263. [6] E.O . Johnson and L. Malter, Phys . Rev . 80 (1950) 58 . [71 T .H. Stix, The Theory of Plasma Waves (McGraw-Hill, New York, 1962) . [81 T. Taylor, M.S de Jong and W.L . Michel, 1988 Linear Accelerator Conf . Proc ., Continuous Electron Beam Accelerator Facility Rept . 89-001, June 1989, pp . 100-102. [91 C .F . Chan, C.F . Burrell and W.S . Cooper, J . Appl . Phys . 54 (1983) 6119 . [10] B.J . Wood and H. Wise, J. Phys . Chem . 65 (1961) 1976 . [111 G.M . Arbique, T. Taylor, M.H . Thrasher and J .S .C . Wills, Proc . 1991 IEEE Particle Accelerator Conference, to be published .