Ion Sources for ISIS and Beyond

1. Ion Sources for ISIS and Beyond John Thomason Reg Sidlow, Mark Whitehead, Dan Faircloth

2. The ISIS Ion Source In normal operation the ISIS ion source is represented to most staff simply by the display screens in the MCR, and only has any impact when its output drops below 35 mA

3. Those who venture further afield may catch a glimpse of one of our expert operatives in action in the ICR

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 0.6  10 mm  600 mA/cm 2  • 200-250  s, 50 Hz (  1% duty factor) • 26 days’ average continuous running •  20 ml/min H 2 ,  3 g/month Cs • normalised emittance  0.17  mm mrad (665 keV, 35 mA, rms) Noise is typically present during the first 200  s of the arc current pulse, and so beam is only extracted after this noise has been damped

5. Emittance Plots These emittance plots are 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

6. The ISIS Ion Source Assembly source components ion source assembly magnet assembly This is the assembly which 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

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 (2  5.5 kHz transformers) • To be replaced in April 2004 by a 665 keV 202.5 MHz RFQ The ion source is floated at -665 kV on the EHT platform and the H  ions accelerate towards ground potential through the acceleration column

8. Gas Discharge H + , H 2 + and H 3 + ions formed in the plasma will eventually reach the edges of the plasma potential and be 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 the number of further ionisations they can precipitate

9. H 0 Potential Well The H 0 potential well used in the diagrams of pages 10 and 12 can be approximated by constructing integrals for a second electron at position r within the charge cloud of the first electron: the result is a screened Coulomb potential

10. Interaction at the Cathode Surface Even if the H 0 gets close to the cathode surface, electrons from the Fermi level should not be able to overcome the potential barrier presented by the work function and be donated to form H  . This is still the case if Cs is added to reduce the work function from 4.5 eV to 1.5 eV

11. Image Potentials Luckily physics comes to our aid (for once!) and because of the image potential of the incoming particle in the infinite conducting plane of the cathode the energy level of the whole H 0 potential well is dropped as it approaches the surface

12. H - Formation At Cathode Surface Now H 0 s coming within about 5 a 0 of the cathode can pick up an electron to form H 

13. Extraction • Thermal H at  700 K • H  from cathode <  160 eV H  formed at the cathode may have up to 160 eV of KE after acceleration down and then back up the discharge potential. If these H  s were extracted they would lead to unwanted halo effects. Ribs on the underside of the aperture plate stop fast H  being extracted

14. Resonant Charge Exchange The fast H  s then undergo resonant charge exchange with slow thermal H 0 s in the aperture region. Before H - H After H H -

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 the world at present can produce all the outputs required for next generation accelerator projects such as new spallation neutron sources and neutrino factories. Typical required parameters based on the specifications of the ESS project will be produced by intensive development of the ISIS source

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 designed to 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

17. • Emittance scanners modified to allow   75 mm scanning compared with  28 mm previously Diagnostic equipment includes emittance scanners identical to those used on the RFQ test facility

18. Extraction at Higher Potential • Field strength for 25 kV extraction can be achieved using existing power supply, sector magnet coil and yoke mv 2 /r = Bev ½mv 2 = eV I  V 3/2  V  B 2  I  B 3 17 kV  25 kV 35 mA  62 mA For a non- space-charge limited source it should be possible to increase output current by increasing the extract potential. A new extract power supply has been designed for this purpose, with the existing sector magnet design being able to meet the increased demand for field strength • 0-25 kV, 0-3 ms, <50 Hz, <2 A

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

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

21. The standard ISIS geometry has inadequate termination of the field at the exit of the sector 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 17 keV normalised  Hrms = 0.29  mm mrad  Vrms = 0.20  mm mrad 17 keV normalised  Hrms= 0.05  mm mrad  Vrms= 0.22  mm mrad

22. Because the standard ISIS extraction electrode is not correctly terminated the electric field across it falls away rapidly at the open ends of the slit. This results in severe aberrations in the focus of the sector magnet. Closing the slit ends, and other refinements, should cure this problem 17 keV normalised  Hrms= 0.05  mm mrad  Vrms= 0.06  mm mrad 17 keV normalised  Hrms= 0.07  mm mrad  Vrms= 0.26  mm mrad 17 keV normalised  Hrms= 0.05  mm mrad  Vrms= 0.22  mm mrad

23. Thermal Modelling ALGOR An FEA model of the source components has been produced using ALGOR software, which allows the thermal behaviour of the ISIS source to be investigated for standard parameters. This model will be used to study increased pulse widths and possible new cooling regimes

24. HTCs for air and 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

25. 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 600 520 440 360 280 200

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

27. The Top Loading Ion Source ion source assembly magnet assembly 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

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 beam has been proposed by George Doucas of Oxford University, and optimised by summer student Iris Yiu

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

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