Ion Sources for ISIS and Beyond
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Transcript of Ion Sources for ISIS and Beyond
10th December2003
ISIS LectureSeries
1
Ion Sources for ISIS and Beyond
John Thomason
Reg Sidlow, Mark Whitehead, Dan Faircloth
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The ISIS Ion Source
In normal operation the ISISion source is represented to
most staff simply by the display screens in the MCR,
and only has any impact whenits output drops below 35 mA
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Those who venture further afield may catch a glimpse of one of ourexpert operatives in action in the ICR
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• The ion source used to produce H- ions on the ISIS spallation neutron source is a Penning surface plasma (SPS) source
• 35 mA through 0.610 mm 600 mA/cm2
• 200-250 s, 50 Hz ( 1% duty factor)
• 26 days’ average continuous running
• 20 ml/min H2, 3 g/month Cs
• normalised emittance 0.17 mm mrad (665 keV, 35 mA, rms)
Noise istypicallypresentduring thefirst 200 sof the arccurrentpulse, andso beamis onlyextractedafter thisnoise has beendamped
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Emittance Plots
These emittance plotsare taken from the RFQ
test facility in R8.Time resolved
animations show the emittance in the
established beam to be very stable in both planes
during the entire beam pulse
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The ISIS Ion Source Assembly
source components
ion sourceassembly
magnetassembly
This is the assemblywhich is actually replaced
during an ‘ion source change’, which takes
about 3 hours
This includes a 90 sector magnet which separates
electrons out of the H beam and shapes
the beam to be approximately circular
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The ISIS EHT Platform and RFQ
• Cockcroft Walton voltage multiplier 665 keV
• 10 M / 10 nF RC smoother
• 70 V PSU 6 kV 66 kV (25.5 kHz transformers)
• To be replaced in April 2004 by a 665 keV 202.5 MHz RFQ
The ionsource isfloated at-665 kV onthe EHTplatformand theH ionsacceleratetowardsgroundpotentialthroughtheaccelerationcolumn
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Gas Discharge
H+, H2+ and H3
+ ions formed in the plasma will eventually reach the
edges of the plasma potential andbe accelerated towards the
cathode surfaces. They rapidly pick up a single electron and then the
neutral species continue towards the cathode surfaces with the kinetic
energy already gained
The Penning B field causes any electrons attracted towards the anode to spiral around,
increasing their path length and hence thenumber of further ionisations they can precipitate
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H0 Potential Well
mass infinite of nucleus with
hydrogen of orbit Bohr first of radiusm102917.5
4a 11
20
0 2eem
0ar
-23
0
10001s e
a1
H For
1
22100
22100
re density Charge
rdensityy Probabilit
re
-V(r)2
rdrer1
e4 2r
0
2100
r
2100 rdre 0a
-2r2
ere
The H0
potential wellused in the diagrams ofpages 10 and 12can be approximatedbyconstructingintegrals fora secondelectron atposition rwithin thecharge cloudof the firstelectron:the result isa screenedCoulombpotential
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Interaction at the Cathode Surface
Even if the H0 getsclose to thecathode surface,electrons from theFermi level shouldnot be able toovercome thepotential barrierpresented by thework function andbe donated to formH. This is still thecase if Cs is addedto reduce the workfunction from4.5 eV to 1.5 eV
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Image Potentials
)(a z
(eV) E
4z27.22
- ΔE0
00a4e
22.27
Luckily physics comesto our aid (for once!)and because of the image potential of the incoming particle in the infiniteconducting plane of the cathode the energy levelof the whole H0 potentialwell is dropped as it approaches the surface
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H- Formation At Cathode Surface
Now H0s comingwithin about 5 a0 ofthe cathode canpick up an electronto form H
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Extraction
• Thermal H at 700 K
eV1.0kT23
KE
• H from cathode< 160 eV
H formed at the cathodemay have upto 160 eV ofKE afteraccelerationdown and thenback up the dischargepotential. If these Hs wereextracted theywould lead tounwanted haloeffects. Ribson theunderside ofthe apertureplate stop fastH being extracted
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Resonant Charge Exchange
Before
H-
H
After
H
H-
The fast Hs then undergo resonant charge exchange with slow thermal H0sin the aperture region.
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Development Goals
65 mA
1.0 ms, 50 Hz (short pulse)2.5 ms, 50/3 Hz (long pulse)
(possibly interleaved?)
normalised rms emittance <0.3 mm mrad at RFQ
matching point
maximised lifetime
No ion source in theworld at present can
produce all the outputsrequired for next
generation acceleratorprojects such as new
spallation neutron sources and neutrino
factories. Typical requiredparameters based
on the specifications of the ESS projectwill be produced by
intensive developmentof the ISIS source
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• HV platforms modified to accommodate new extract regulator, and new 3-phase isolation transformer installed
The Ion Source Development Rig
The ISDR has been designedto allow ion sources to be run at-35 kV, with duplicates of all the
equipment on the ISIS EHT platform,but allowing for higher powers and
longer pulse widths
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• Emittance scanners modified to allow 75 mm scanning compared with 28 mm previously
Diagnostic equipment includesemittance scanners identicalto those used on the RFQ test
facility
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Extraction at Higher Potential
• Field strength for 25 kV extraction can be achieved using existing power supply, sector magnet coil and yoke
mv2/r = Bev
½mv2 = eV
I V3/2
V B2
I B3
17 kV 25 kV
35 mA 62 mA
• 0-25 kV, 0-3 ms, <50 Hz, <2 A
For a non-space-chargelimited sourceit should bepossible toincrease output current by increasing theextractpotential. Anew extract power supplyhas beendesigned forthis purpose,with theexisting sectormagnet designbeing able tomeet theincreaseddemand forfield strength
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Two papers based on finite element analysis (FEA) by Dan Fairclothwere presented at ICIS’03, Dubna, Russia, September 2003
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Electromagnetic Modelling
MAFIA
An FEA model of the magnet flange and cold box has been produced using MAFIA software, which allows particle trajectory tracking through the extract and sector
magnet, with the intention of optimising the beam optics in these regions
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17 keV normalisedHrms= 0.29 mm mradVrms= 0.20 mm mrad
17 keV normalisedHrms= 0.05 mm mradVrms= 0.22 mm mrad
The standard ISIS geometry has inadequate termination of the field at the exit of thesector magnet, leading to particles continuing to be bent after leaving the magnet
By including a ‘maximag’ magnet steel insert in the face of the cold box, and adjusting the length of the pole tip, an on-axis, parallel beam can be produced
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17 keV normalisedHrms= 0.05 mm mradVrms= 0.06 mm mrad
17 keV normalisedHrms= 0.07 mm mradVrms= 0.26 mm mrad
17 keV normalisedHrms= 0.05 mm mradVrms= 0.22 mm mrad
Because thestandard ISISextractionelectrodeis notcorrectlyterminatedthe electricfield across it falls awayrapidly at theopen ends ofthe slit. Thisresults insevereaberrations inthe focus ofthe sectormagnet. Closing theslit ends, andotherrefinements,should cure this problem
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Thermal Modelling
ALGOR
An FEA model of the sourcecomponents has been produced
using ALGOR software,which allows the thermal
behaviour of the ISISsource to be investigated for standard parameters.This model will be used
to study increased pulse widths and possible new cooling
regimes
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0
50
100
150
200
250
300
350
0 500 1000 1500 2000 2500 3000
Time (seconds)
Moly
Copper
Steel
0
50
100
150
200
250
300
350
0 1000 2000 3000 4000
Time (Seconds)
1.00E-05bcdefgh
HTCs for airand water cooling channels have been studied in a CFD model by Oxford University to determine how to apply them correctly in the ALGOR model, and the range over which they are valid
Every effort has been made to measure real values of all the heat transfer coefficients (HTCs) in the source using the heat up and cool down of a thermal
test piece, rather leaving them as free parameters in the model
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600520440360280200
from model electrode
thermocouple surface maximum
typical ISIS
thermocouple
anode 456°C 496°C 535°C 400-600°C
cathode 501°C 585°C 631°C 440-530°C
source
body
416°C 441°C 441°C 390-460°C
Steady state and transient solutions for standard ISIS parameters have provided the first reliable values for the actual temperatures of electrode surfaces and the
temperature rises during the on-period of the pulse
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• 4X high H- current slit-extraction operation suggests discharge power may be decreased, and df extended to reach 5% df (50 Hz,1 ms) at 100 mA
• 4X operated at 250 mA H- for 1-2 days at LANL in 1987, df = 0.5% with no effort made to probe df limit
• 4X operated up to 2.3% df with circular apertures while extracting H- beam, up to 6% df in discharge-only mode
Los Alamos Scaled Penning Sources(Courtesy of Joe Sherman)
It appears that the most effective way to offset additional heating for longer pulse widths will be to scale up the sizes of source components. This approach was used at LANL during the 1980’s, but was not extended to an operational source
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The Top Loading Ion Source
ion sourceassembly
magnetassembly
The source and magnet assembly for the ISDR have been redesigned
in order to more easily accommodate larger source
components and more aggressive cooling strategies
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Energy Analysis
• A retarding potential energy analyser may be suitable for measuring H- energy distributions derived from the ion source
• SIMION modelling has determined the suitability of this technique for ΔE<1eV
A new energy analyser, which should be capable of resolving individual H energies in a 35 keV beamhas been proposed by George Doucas of Oxford University, and optimised by summer student Iris Yiu
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• Improved design now being manufactured
• Subject of Oxford University MPhys project, Jan-Mar 2004
Work on the energy analyser will be continued by Jenny Morrison, starting 15/1/04
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Future Work
• Extend thermal model to increased duty cycles with improved cooling and scaling of components
• Install new extraction electrodes and cold box optics and increase extraction potential to 25 kV
• Implement changes suggested by modelling and increase duty factors – deal with consequences of increased gas flow
• Lifetime testing of improved source
• Test effects of Penning field decoupled from sector magnet field
• Host HP-NIS annual meeting at The Cosener’s House, 1&2 April 2004