High Current Density and High Brightness H- Sources for Accelerators

1
High Current Density and High Brightness H-
Sources for Accelerators

• Vadim Dudnikov
• Brookhaven Technology Group, Inc.

FNAL, December 2005
2
ACKNOWLEDGMENTS

• I am very grateful to the ISIS Team for
  choosing Charge Exchange Injection and Penning
  SPS for ISIS operation and for successful
  demonstration of its high performance in real
  accelerator operation.

3
Penning SPS in the ISIS RFQ
4
Abstract

• Operation Experience of Compact
  Surface-Plasma Sources (CSPS) under operation in
  different laboratories around the world, will be
  considered.
• Features of CSPS are small volume, small
  gaps between electrodes, high plasma density and
  high emission current density and high
  brightness, high pulsed gas efficiency and low
  electron current.
• In many versions of CSPS were reached very
  long operation time.
• Features of CSPS important for long time
  operation will be considered.

5
Contents

• Introduction.
• Historical remarks.
• Negative ion production in surface- plasma
  interaction.
• Cesium catalysis.
• Surface Plasma Sources- SPS.
• Charge-exchange cooling. Electron
  suppression.
• Beam extraction, formation, transportation.
• Space charge neutralization. Instability
  damping.
• SPS design. Gas pulser, cesium control,
  cooling.
• SPS life time. SPS in accelerators.
• Further development.
• Summary.
• Acknowledgment.

6
Horst Klein (20 ICFA Workshop summary).

• The ion sources, and especially the H-
  sources, are still somewhat a black magic.
  Therefore intense theoretical and experimental
  work has to be performed in different labs to
  achieve the new requirements. In Europe the
  Negative Ion Source network, supported by the
  European Union, with its 8 partners will help to
  reach the goal. But also such a meeting as we
  have had in Femilab is very helpful and
  intensifies the worldwide collaboration.
  Concerning the different types of ion sources, I
  think the most promising candidates for H- are
  the Penning ion source and the volume source
  (Large Volume SPS). The ECR source may be a hope
  for the future.
• Intuition and hand experience are important
  components for H- sources development.

7
H- beam brightness in different SPS ( R.Welton,
SNS).
Beam brightness and pulse current of operational
ion sources (points) and new facility
requirements (rectangles) Magnetron sources
1-DESY, 2-BNL, 3-ANL, 4-FNAL. Multicusp RF
sources 5-DESY, 6-SSC Penning sources 7-RAL
and 8-INR. Multicusp surface conversion sources
9-KEK and 10 LANL Multicusp filament sources
11-TRIUMF and 12-Jyvaskyla.
8
Ion Source requirements for new accelerators
projects ( from R. Scrivens review)
Ion Source parameters required for selected high
power project. 1rms, normalized, in mm mrad
9
HUASHUNG ZHANG, ION SOURCES, Springer,
1999. p.326

• Based on the achievements of positive ion
  sources, H- ion sources have
• been developed in two ways
• 1) Negative ions are extracted from the plasma of
  positive ion sources. Before the 1970's, the H-
  current was limited to less than 5 mA. This is
  because in a general high temperature plasma (Te
  gt 10 eV) the H- formation cross section(10-18
  cm2 ) is 3 to 4 orders less than the H-
  destruction cross sections (2 to 7x10-14 cm2).
• In 1962, Krohn 7 discovered that the yield of
  sputtered negative ions increased by one order
  while Cs ions impacted the metal target.
• Unfortunately, this result was not immediately
  used to develop a NIS up to 1970. An H- surface
  plasma source (SPS) was invented by introducing
  cesium into the hydrogen discharge plasma at
  1971.
• It quickly led to increasing the H- current to
  several Amperes. Also the cesium sputter NISs
  were rapidly developed.
• Since discovering, at the end of the 1970s, that
  the dissociative attachment cross section of
  highly vibrationally excited H2-molecules in a
  low-temperature plasma is higher by 104-105 than
  the groundstate8,9, high-intensity volume H-
  ion sources have been developed.
• At the end of the 1980's, H- volume ion sources
  combined with cesium has evolved with domination
  of surface- plasma generation of negative ions.

10
Adsorption of alkaline metals significantly
increases the secondary emission of negative ions

• In 1961, by Ahmet Ayukhanov (Tashkent
  Electronics Institute) was observed that the
  adsorption of alkaline metals significantly
  increases the secondary emission of negative
  ions. A little later the investigations of this
  effect were presented by Krohn (Argonne Nat.
  Lab.). However, even with the presence of cesium
  on the surface the intensity of beams of negative
  ions obtained by the secondary emission did not
  exceed the microampere level.
• These results became the basis of
  secondary-emission sputtering negative ion
  sources with a microampere level intensity for
  tandem accelerators.
• A. Ayukhanov, PhD. Thesis, Secondary emission of
  negative ions with bombardment by alkali positive
  ions. 1961.
• U. A. Arifov, and A. Kh. Ayukhanov, Izvestiya AN
  Uzbek. SSR, Ser.
• Fiz. Mat. Nauk. No. 6, 34 (1961).
• in book U. A. Arifov, Interactions of Atomic
  Particles with a Solid (Nauka, Moscow, 1986).
• V. E. Krohn, J. Appl. Phys. 33, 3523 (1962).

11
Budker Institute of Nuclear Physicswww.inp.nsk.su
12
History of Surface Plasma Sources
Development
(J.Peters, RSI, v.71, 2000)
Cesium Catalysis Enhancement of negative ion
production by admixture into discharge a
substance with a low ionization potential, such
as cesium.
13
Intensity of Negative Ion Beams 1971-discovery
of Cesium Catalysis.
14
H-/D- LV SPS for Tokomac Neutral Beam
Injectors 0A, 1 MeV, 1000s, 1 Billion
15
History of Charge Exchange Injection (Rees,
ISIS , ICFA Workshop)

• 1. 1951 Alvarez, LBL (H-)
• 1956 Moon, Birmingham Un. (H2)
• 2. 1962-66 Budker, Dimov, Dudnikov,
  Novosibirsk
• first achievements
  discovery of e-p instability.IPM
• 3. 1968-70 Ron Martin, ANL 50 MeV
  injection at ZGS
• 4. 1972 Jim Simpson, ANL 50-200 MeV,
  30 Hz booster
• 5. 1975-76 Ron Martin et al, ANL 6 1012
  ppp
• 6. 1977 Rauchas et al, ANL IPNS
  50-500 MeV, 30 Hz
• 7. 1978 Hojvat et al, FNAL 0.2-8
  GeV, 15 Hz booster
• 8. 1982 Barton et al, BNL 0.2-29
  GeV, AGS
• 9. 1984 First very high intensity
  rings PSR and ISIS
• 10. 1980,85,88 IHEP, KEK booster, DESY III (HERA)
• 11. 1985-90 EHF, AHF and KAON design studies.
  SSC
• 12. 1992 AGS 1.2 GeV booster injector
• 13. 1990's ESS, JHF and SNS 4-5 MW sources

16
INP Novosibirsk, 1965, bunched beam
Other INP PSR 1967 coasting beam instability
suppressed by increasing beam current fast
accumulation of secondary plasma is essential
for stabilization 1.8x1012 in 6 m
first observation of an e- driven instability?
coherent betatron oscillations beam loss with
bunched proton beam threshold 1-1.5x1010,
circumference 2.5 m, stabilized by feedback
(G. Budker, G. Dimov, V. Dudnikov, 1965). F.
Zimmermann
V. Dudnikov, PAC2001, PAC2005
17
Cs PATENTV. Dudnikov, The Method for Negative
Ion Production, SU Author Certificate,
C1.H013/04, No. 411542, Application filed at 10
March, 1972, granted 21 Sept,1973, published
Bul. No 2, 15 Jan.1974.
Enhancement of negative ion production by
admixture into discharge a substance with a low
ionization potential, such as cesium.
18
SPS was developed in cooperation of BDD, G.Dimov,
V.Dudnikov,and Yu.Belchenko
19
SPS for Accelerators was developed in cooperation
with G. Derevyankin
20
History of Volume Sources Development
(J.Peters, RSI, v.71, 2000)
Blue frame is separate Surface Plasma dominated
H- formation Development of Volume Sources is
finished by conversion into Large Volume SPS.
21
Marthe BacalFourth IAEA Technical Meeting on
Negative Ion Based Neutral Beam Injectors9 May
2005

• What ion source for volume production ??
• New ion sources were proposed for making use of
  the volume production mechanism. The magnetic
  multipole, used in 1976 in our first experiments
  (Nicolopoulou et al, J. Phys. 1977) was modified
  by the addition of a magnetic filter. This seemed
  to solve the problem of H- destruction by fast
  electrons, since they were eliminated from the
  extraction chamber.
• However, this solution was only partial, for two
  reasons
• the negative ions may not be formed in the
  extraction chamber, but in the driver, near the
  filaments
• the magnetic multipole is very efficient to
  dissociate molecules, but H atoms destroy H- and
  H2(v) !
• When cesium was introduced in the magnetically
  filtered multipole , it appeared as a suitable
  source for producing atoms and positive ions for
  surface production. Obviously, this device is not
  suitable for volume production !! It is really a
  good Large Volume Surface Plasma Source, not a
  Volume Source.

22
General Diagram of the Surface-Plasma Mechanism
for Production of Negative Ions in a Gas
Discharge
Surface plasma generation of H- on anode often is
a dominant process of H- formation in discharges
without Cs, as well with Cs
23
Schematic Diagrams of Surface Plasma Sources
(a) planotron (magnetron) flat cathode (b)
planotron geometrical focusing (cylindrical
and spherical) (c) Penning discharge SPS
(Dudnikov type SPS) (d) semiplanotron (e) hollow
cathode discharge SPS with independent
emitter (f) large volume SPS with filament
discharge and based emitter (g) large volume SPS
with anode negative ion production (h) large
volume SPS with RF plasma production and emitter
1- anode 6- hollow
cathode 2- cold cathode emitter 7-
filaments 3- extractor with 8-
multicusp magnetic magnetic system
wall 4- ion beam
9- RF coil 5- biased emitter 10-
magnetic filter
24
Probability of H- emission as function of work
function (cesium coverage)
25
Schematic of negative ion formation on the
surface Michail Kishinevsky, Sov. Phys. Tech.
Phys, 45 (1975)
26
Coefficient of negative ionization as function of
work function and particle speed
27
Enhancing surface ionization and beam formation
in volume-type H- ion sourcesR.F.Welton,
M.P.Stockli, M.Forrette, C.Williams, R.Keller,
R.W.Thomae, EPAC 2002, Paris.

• Cleary, once again Cs must reside on the surface
  for the vast majority of its lifetime in the
  source and therefore surface ionization must
  account for the observed enhancement of H- yield.
• In these cases, the term volume ion source is
  misleading since, most of the H- results from
  surface, rather than volume ionization processes.
  Therefore, ion source design, careful
  consideration should be granted the interior
  surfaces of the source.
• Correct classification of ion sources is
  important, because it should determine a
  direction of devices optimization to optimize a
  volume production, or surface-plasma production.
  Incorrect speculation of main mechanism of
  negative ion generation was reason of long time
  delay in improving of beam parameters.

28
First version of Planotron (Plain Magnetron) SPS,
INP, 1972, Beam current up to 230 mA, 1.5x10 mm2
, J1.5 A/cm2 with Cs
29
H- energy spectra from planotron

• The ion spectra from a planotron usually
  have two peaks separated by a valley. The
  location of the first peak coincides with the
  energy eUex imparted to the negative ions by the
  extraction voltage. The ion energy of the second
  peak is higher than that of the first peak by an
  amount close to eUd. The oscillograms in the
  upper part of illustrate the change in the
  spectra, as a result of increasing the discharge
  voltage Ud from 120 V ( l ) to 210 V (4) by
  reducing the cesium supply. The oscillograms
  (1-4) in the lower part of Figure illustrated
  how the spectra vary as a result of increasing
  the hydrogen supply to the discharge chamber

30
Cross sections of Planotron (Magnetron) SPS of
second generation 3.7 A/cm2 with Cs (0.75 A/cm2
without Cs)
31
H- current density from planotron with Cs
(3.7A/cm2) and without Cs (0.75 A/cm2), INP,
Novosibirsk, 1972
32
Schematic of semiplanotron SPS

• 1- emission aperture
• 2- anode
• 3- cathode
• 4- cathode insulator
• 5- discharge channal
• 6- extractor
• 7- magnet with magnetic insertions.

33
Beam Current vs an Arc Current for Different Slit
Geometry in the Semiplanotron

• Dependences of the ?- ion beam current on
  the discharge current have the N-shaped form with
  three sections linear growth at small discharge
  currents, saturation or a falling section at
  medium currents, and linear, but slow growth at
  the high currents.

34
Cross section through LANL version ofSPS WITH
Penning Discharge.
Beamlet images at pepper-pot scinti1lator
(noiseless discharge). Emission slit 0.5x10
mm2. Vertical Y Plane Horizontal X Plane
35
Schematic of ISIS version of Penning discharge
SPS
36
Cathode and Plasma Plate of ISIS Penning SPS
after long time operation
37
H- Energy Spectra from Penning SPS
38
Review of Scientifi Instruments, March 2002,
Volume 73, Issue 3, pp. 1157-1160 Investigation
of the mechanism of current density increase in
volume sources of hydrogen negative ions at
cesium adding

• V.P. Goretsky, A.V.Ryabtsev, I.A. Soloshenko,
  A.F. Tarasenko, A.I. Shchedrin
• Institute of Physics of National Academy
  of Sciences of Ukraine, 46 prospect Nauki, Kiev
  03650, Ukraine
• In the present article the influence of adding
  cesium into the volume and on the surface of an
  ion source on its emission characteristics is
  studied both theoretically and experimentally. It
  is shown that cesium in the volume at conditions
  of a real ion source brings in a significant
  contribution to kinetic processes, but weakly
  influences the current of H ions extracted from
  the source. It is shown both theoretically and
  experimentally that an observed increase of the
  current of H ions with cesium added is due to
  the conversion of fast particles at the anode
  surface.
• Thus, on the basis of experimental results and
  calculations it can be stated that cesium in a
  volume of the source under study can not lead to
  the increase of current H- ions. Observed growth
  of this current with cesium introduction is due
  to conversion of hydrogen atom at discharge anode
  surface, covered by cesium. In other words,
  cesium adding results in the transformation of
  the source of H- ions of volume type to the
  source of surface-plasma type.
• Yu.Belchenko,G.Dimov, V.Dudnikov, Nucl.Fusion,
  14, 113 (1974)

39
Operation of Dudnikov type Penning source with
LaB6 cathodesK.N. Leung, G.J. DeVries, K.W.
Ehlers, L.T. Jackson, J.W.Stearns, and M.D.
Williams (LBL)M.G. McHarg, D.P. Ball, and W.T.
Lewis (AFWL)P.W. Allison (LAML)

• The Dudnikov type Penning source has been
  operated successfully with low work function LaB6
  cathodes in a cesium-free discharge. It is found
  that the extracted H current density is
  comparable to that of the cesium-mode operation
  and H current density of 350 mA/cm2 have been
  obtained for an arc current of 55 A. Discharge
  current as high as 100 A has also been achieved
  for short pulse durations. The H yield is
  closely related to the source geometry and the
  applied magnetic field. Experimental results
  demonstrate that the majority of the H ions
  extracted are formed by volume processes in this
  type of source operation.
• Review of Scientific Instruments --
  February 1987 -- Volume 58, Issue 2, pp. 235-239

40
H- Detachment by Collisions with Various
Particles and Resonance Charge-Exchange Cooling
Resonance charge -exchange cooling
41
Cesium escaping from a pulsed discharge in SPS

• there is a strong suppression of the gas and
  cesium flow from the emission slit by the high
  density plasma of the discharge.

42
Gas trapping by discharge in CSPS

• qo-gas flux without discharge
• qp- gas flux with discharge
• Id- discharge current

43
H- Beam Intensity of SPS
Years
Beam intensity vs discharge current for first
version of semiplanotron 1976
Evolution of H- beam intensity in ISIS
44
Emittance, Brightness, Ion Temperature
d
y
Emission slit
l
Emittance
Normalized emittance
x
?x
Normalized brightness
?a
Half spreads of energy of the transverse motion
of ions
Reduced to the plasma emission slit
Characteristics of quality of the beam formation
45
Discharge Stability and Noise
n,1016 cm-3
noiseless
Diagram of discharge stability in coordinates of
magnetic field B and gas density n
no discharge
n
noisy
Bmin
B, kG
µ e?/m (?2 ?2)
µ
noiseless
The effective transverse electron mobility µ vs
effective scattering frequency ? and cyclotron
frequency ?
? / ?
46
Noise of discharge voltage
Dependence of discharge noise of magnetic field
47
Discharge Noise Suppression by Admixture of
Nitrogen
P.Allison, V. Smith, et. al. LANL
no N2
QN2 0.46 sccm
48
Design of SPS with Penning Discharge
49
Fast, compact gas valve, 0.1ms, 0.8 kHz

• 1 -current feedthrough
• 2- housing 3-clamping
• screw 4-coil 5- magnet
• core 6-shield 7-screw
• 8-copper insert 9-yoke
• 10-rubber washer-
• returning springs
• 11-ferromagnetic plate-
• armature 12-viton stop
• 13-viton seal 14-sealing
• ring 15-aperture
• 16-base 17-nut.

50
Photograph of a fast, compact gas valve
51
CSPS with Penning discharge
52
Discharge voltage
Noiseless operation
Discharge current
100 Hz
Extraction voltage
Tested for 300 hs of continuous operation with
H- Currentgt100mA
Extraction current
H- current after magnetic analyzer
53
Beam Formation and Diagnostics of SPS with
Penning Discharge
54
Emittance measurement, Direct Brightness
determination

• Ion beam
• Collector 1 with collimator for J.
• Collector 2 with collimator s1 for B
• Beamlet
• Deflector Horizontal
• Deflector Vertical
• Screen with collimator s2 for B detection
• Collector 3 for B detection.
• B I L2/s1 s2, s1s20.1x0.1 mm2, L250mm,
• I10-6 A.

55
Emittance diagramms
0.5X10 mm mm exn 90 0.06 p mm mrad eyn 90
0.2 p mm mrad Tx 16 eV, Ty2 eV

• H- beam current 80 mA, Energy 23 keV,

56
Beam instability with a secondary electron
emission
57
Beam instability with current density fluctuation
58
Beam current density distribution for different
currents/extraction voltages
59
Dependence of current and pick current density
for different extraction voltages on discharge
current
60
BINP version Penning DT SPS for UMD
1- cathode 2-anode 3-extractor 4- ground
ext. 5-magnet 6-insularors 7-cooler. 1 ms,
10 Hz, 1 A/cm2 Teff 1 eV
61
Design of Fermilab Magnetron with a Slit
Extraction
62
Fermilab Magnetron with a Slit Extraction
63
Simulation of H- Ion Beam Extraction from the
Slit Magnetron
Current density
2
Electrodes trajectories and equipotentials
5
J, A/cm2
Y, mm
0
Y, mm
2
250
Emittance plot
0
0
10
5
X, mm
Slit 2x10 mm I87 mA U21 kV neutral 95
X, mrad
-250
-2.5
2.5
X, mm
64
Discharge Parameters and Beam Intensity in
Fermilab Magnetron
200
time, mks
0
0
Beam current, mA
80
100
0
time, mks
65
Beam Intensity vs Discharge Current and
Extraction Voltage in Fermilab Magnetron
66
Extraction System of BNL Magnetron
67
H- Current vs Extraction Voltage for Magnetron
H- Current, mA
Extraction voltage, kV
68
Design of the first Version of Semiplanotron SPS
V. Dudnikov, INP, 1976
1- Cathode 5cm long 2- Anode -discharge
chamber 3- Magnetic insert 4- Magnetic poles
5-emission slit, d0.5 mm 6- Extractor 7-
cylindrical grove for plasma confinement 8-
plasma trap for discharge triggering. H- Beam up
to 0.9 A, 1 ms, 10 Hz, slit 0.7x45 mm2 0.22 A,
slit 1x10 mm2 .
69
NI Beam intensity as function of discharge
current in the Semiplanotron SPS
70
Design of a Semiplanotron SPS for accelerators
71
Semiplanotron SPS with a Slit Extraction
72
Beam Current vs an Arc Current for Different Slit
Geometry in the Semiplanotron

• Dependences of the ?- ion beam current on
  the discharge current have the N-shaped form with
  three sections linear growth at small discharge
  currents, saturation or a falling section at
  medium currents, and linear, but slow growth at
  the high currents.

73
Polarized Negative Ion Source with a Resonance
Ionizer
A.Belov,V. Dudnikov, et. al.
analyzer
extractor
solenoid
Plasma Source
Ionizer, SPS
D-, D,e, H-
74
DC SPS with a High Emission Current Density
75
Anode of DC SPS
76
Collector current Ic vs. discharge current Id
and extraction voltage Vex
Extraction aperture of D0.4 mm
Extraction aperture of D1 mm
77
Compact DC SPS with Hollow Cathode Discharges
1- cylindrical cathode body 2- channel for
cesium delivery 3- channel for working gas 4-
insulator (ring) 5- anode chamber 6- hollow
cathode channel 7- drifted plasma 8-
extraction aperture 9- spherical emitter 10-
magnetic pole 11- extractor 12- ion beam.
78
Assembly of the negative ion source in vacuum
chamber
1- gas tube 2- electric vacuum feedthroughs
3- high voltage flange 4- high voltage
insulator 5- high voltage feedthrough 6- base
flange 7- cooling rods 8- Cs catalyst
supply 9- cathode-emitter 10- cathode
insulator 11- gas discharge chamber anode 12-
magnet poles 13- suppression electrode 14-
extraction electrode 15- permanent magnet 16-
high voltage insulators 17- base plate-magnetic
yoke 18- ion beam 19- vacuum chamber 20-
high voltage insulator. .
Brookhaven Technology Group
79
Typical Assembling of CSPS on the Vacuum Flange
80
Emittance of DC SPS, 25 keV, 1.5 mA
81
(DuoSPS) Possible adaptation of NIE in the real
Duoplasmatron
82
SPS with Helicon Plasma Generation and Ion/Atom
Converter
Ion flux conversion to fast atoms in
converter. Laser diagnostics and control cesium
distribution. Cesium trapping by full ionization
with laser excitation in discharge chamber. Laser
beam attenuation for control cesium density
without discharge.
83
Helicon discharge plasma source for SPS

• A discharge in Hydrogen gas with helicon
  type antenna in longitudinal magnetic field was
  developed and tested as plasma generator for H-
  source with Jim Alessi at the BNL in 1993.
• A quartz cylinder 34 mm ID, helicon
  antenna, solenoid and flanges are shown left. Ion
  current density of 0.1 A/ cm2 was extracted with
  a discharge power of 0.4 kW, RF frequency of 40
  MHz. The same efficiency was produced before in
  RF ion source in the Budker Institute of Nuclear
  Physics (BINP), Novosibirsk, Russia in DC mode of
  operation with optimized resonance magnetic
  field.

84
Helicon Discharge Surface Plasma Source.
1- gas valve 2- discharge volume 3- discharge
vessel 4- helicon saddle like antenna 5-
magnetic coil 6- ion/atom converter 7- electron
flux 8- emission aperture (slit) 9- extraction
electrode 10-suppression /steering electrode
11- ion beam.
85
Antennas of RF plasma generator.
With replacing of the ordinary helix antenna
shown in (a) by saddle type (b) a plasma flux
density was increased up to 5 times from 140
mA/cm2 to 700 mA/cm2 with 14 MHz RF frequency
and power of 2.5 kW and magnetic field of B86
Gauss. The plasma flux to the wall was reduced
significantly. This big difference is determined
by plasma generation near the wall with ordinary
helix antenna and a much picked plasma generation
with the saddle type antenna.
a- ordinary helix antenna b- saddle type antenna.
86
FNAL SPS in preaccelerator, 0.75 MV, 0.1 A
87
ANL SPS in preaccelerator, 0.75 MeV, 80 mA
88
LEBT with Solenoidal Focusing ( BNL, LANL)
89
Semiplanatron SPS on the flange
Schematic of semiplanotron SPS (cross section
parallel to the magnetic field). 1-ion source
flange 2- insulator flange 3-vacuum insulator
4- gas discharge chamber-anode (st.st.) 5-
cathode (molybdenum) 6- anode insert 7-cathode
insulator (ceramic) 8-discharge channel 9-
emission slit 10- source holders 11- high
voltage insulators 12- magnetic yoke 13- base
plate 14- gas valve 15- cathode nuts 16 cesium
oven 17- ion beam 18- extractor 19- permanent
magnets (NdBFe, 10x25x50 mm3) 20- magnetic
inserts 21- gas tube 22-cathode cooling.
90
Semiplanatron SPS on the Flange
Schematic of semiplanotron SPS (cross section
perpendicular to the magnetic field). 1-ion
source flange 2- insulator flange 3-vacuum
insulator 4- gas discharge chamber-anode
(st.st.) 5- cathode (molybdenum) 6- anode
insert 7-cathode insulator (ceramic)
8-discharge channel 9- emission slit 10- source
holders 11- high voltage insulators 12-
magnetic yoke 13- base plate 14- gas valve 15-
cathode nuts 16 cesium oven 17- ion beam 18-
extractor 19- permanent magnets (NdBFe, 10x25x50
mm3) 20- magnetic inserts 21- gas tube
22-cathode cooling
91
Schematic of upgraded Compact Surface Plasma
Source.
Left-cross section along the magnetic field
right- cross section perpendicular to the
magnetic field 1-cooled anode 2- high
thermoconductive insulator AlN 3- discharge gap
4- cathode with channel for HCD 5-plasma plate
with emission aperture 6- cooled high voltage
flange 7- first extractor-electron collector 8-
permanent magnet with magnetic poles and yoke 9-
high voltage insulators 11- grounded extractor
12- suppresser of positive ions 13- ion beam
14- gas valve 15- cesium delivery system16
-cooling chanel17-magnetic yoke .
92
DC CSPS with HC Penning discharge Yu. Belchenko,
BINP

• The source uses a Penning discharge with a
  hydrogen and cesium feed through the hollows in
  the cathodes. Discharge voltage is about 6080 V,
  current 9 A, hydrogen pressure 45 Pa, magnetic
  field 0.050.1 T, and cesium seed ,1 mg/h.
  Negative ions are mainly produced on the cesiated
  anode surface due to secondary ion/atom emission.
  DC H- beam current up to 15 mA.

93
Sputtering yield

• Sputtering and flakes formation are main
  reason of failures. Operation below a sputtering
  threshold is good for a lifetime increase.

94
Average current and source lifetime in hours and
in A hr . Circle is anticipated parameters of BTG
phase II.
95
Summary 1

• The CSPS have high plasma density, high
  emission current density. They are very small,
  simple and effective have a high brightness in
  noiseless mode of operation, and high pulsed gas
  efficiency. The CSPS are very good for pulsed
  operation and continues operation during many
  months has been achieved. Negative ion formation,
  charge-exchange cooling of H- below 1 eV, high
  brightness beam extraction, formation,
  transportation, space charge neutralization,
  brightness preservation instability dumping are
  discussed. Practical aspects of SPS design,
  simulation and operation, a gas pulsing and
  cesium admixture control, lifetime enhancement of
  selected SPS are described and compared.

96
Summary 2

• Features of all discussed CSPS are small
  volume, small gaps between electrodes, high
  plasma density and high emission current density.
  These features have complicated the long time
  operation of CSPS with high beam parameters,
  because a sputtering rate, flakes formation,
  deposition of insulators surface and probability
  of short circuit of electrodes should be high.
  But in many versions of CSPS was reached a very
  long operation time.

97
Summary 3

• The operation time of ion source is limited
  by cathode erosion in plasma, deposition of
  conducting films to the insulators and flakes
  formation with a short circuit of a discharge gap
  between insulated electrodes. A typical current
  of DC discharge Id1-10 A is small enough for
  long time conducting by these short circuit. It
  was observed, than during operation of CSPS with
  a pulsed discharge with low impedance forming
  line a flake formation is significantly
  suppressed and short circuit, created by
  deposition could be recovered. Short circuit
  created by conductive film deposition to the
  insulator or flakes can curry a low DC current
  but can be evaporated by high pulsed current.
  Evaporated material form a dust accumulated in
  any pockets in gas discharge chamber without
  disturbing of discharge.