10th December2003

ISIS LectureSeries

1

Ion Sources for ISIS and Beyond

John Thomason

Reg Sidlow, Mark Whitehead, Dan Faircloth

10th December2003

ISIS LectureSeries

2

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

10th December2003

ISIS LectureSeries

3

Those who venture further afield may catch a glimpse of one of ourexpert operatives in action in the ICR

10th December2003

ISIS LectureSeries

4

• 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

10th December2003

ISIS LectureSeries

5

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

10th December2003

ISIS LectureSeries

6

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

10th December2003

ISIS LectureSeries

7

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

10th December2003

ISIS LectureSeries

8

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

10th December2003

ISIS LectureSeries

9

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

10th December2003

ISIS LectureSeries

10

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

10th December2003

ISIS LectureSeries

11

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

10th December2003

ISIS LectureSeries

12

H- Formation At Cathode Surface

Now H0s comingwithin about 5 a0 ofthe cathode canpick up an electronto form H

10th December2003

ISIS LectureSeries

13

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

10th December2003

ISIS LectureSeries

14

Resonant Charge Exchange

Before

H-

H

After

H

H-

The fast Hs then undergo resonant charge exchange with slow thermal H0sin the aperture region.

10th December2003

ISIS LectureSeries

15

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

10th December2003

ISIS LectureSeries

16

• 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

10th December2003

ISIS LectureSeries

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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

10th December2003

ISIS LectureSeries

18

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

10th December2003

ISIS LectureSeries

19

Two papers based on finite element analysis (FEA) by Dan Fairclothwere presented at ICIS’03, Dubna, Russia, September 2003

10th December2003

ISIS LectureSeries

20

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

10th December2003

ISIS LectureSeries

21

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

10th December2003

ISIS LectureSeries

22

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

10th December2003

ISIS LectureSeries

23

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

10th December2003

ISIS LectureSeries

24

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

10th December2003

ISIS LectureSeries

25

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

10th December2003

ISIS LectureSeries

26

• 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

10th December2003

ISIS LectureSeries

27

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

10th December2003

ISIS LectureSeries

28

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

10th December2003

ISIS LectureSeries

29

• 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

10th December2003

ISIS LectureSeries

30

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