ESS-Bilbao Initiative Workshop. Front Ends for High Intensity

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Front Ends for High Intensity
Alan Letchford (ISIS-RAL)

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ESS-Bilbao Initiative Workshop. Front Ends for High Intensity

  1. 1. Front Ends For High Intensity Alan Letchford STFC RAL ISIS Injector Group ESS-Bilbao Initiative Workshop March 2009
  2. 2. Outline • Front Ends • Challenges • Ion sources • LEBTs • RFQs • MEBTs • Choppers • Funnels • Diagnostics •Outlook Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  3. 3. Front Ends The ‘Front End’ is not precisely defined. Rarely taken to mean anything above 10-20 MeV. Often refers to just the first 2-3 MeV. Ion Source Radio Frequency Linac Drift Tube H+ or H- Quadrupole (for example) Low Energy Medium Energy Beam Transport Beam Transport Funnel Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  4. 4. Front Ends Rather obviously, no linac can operate without a front end. Getting the front end right is important as it defines the available current for the machine. The front end defines the emittance for the whole linac. Beam artefacts generated here may propagate along the linac and lead to loss. Chopping and funnelling are challenging and essential in some scenarios. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  5. 5. Challenges H+ Ion Sources. For a long pulse neutron source with only a linac, an H+ ion source can be used. H+ sources can deliver >100mA at duty factors up to 100%. Eg CEA SILHI ECR source: H+ Intensity > 100 mA at 95 keV H+ fraction > 80 % Reliability > 95 % Emittance < 0.2 mm.mrad CW or pulsed mode Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  6. 6. H- Ion Sources. For a neutron source with synchrotron or compressor ring an H- ion source in required for charge exchange injection. H- source performance does not match that of H+ sources. Currents up to 60mA and duty factors approaching 10% have been demonstrated but not simultaneously for extended periods. High currents require caesium which can limit lifetimes. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  7. 7. H- Ion Sources. Eg SNS RF driven multicusp source Baseline LBNL source has been developed to >35mA at 4% duty factor. 40mA The Large Volume External Antenna Source has demonstrated >60mA but chamber heating an issue. 2 week production run RMS emittance ~0.2 mm mrad Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  8. 8. H- Ion Sources. Eg RAL FETS Penning Surface Production Source Development of the ISIS source has demonstrated feasibility of 60 40 both >60mA and 7% duty factor. 20 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Time (us) -20 On FETS power supplies will allow -40 both to be achieved Discharge Current (A) Beam Current (mA) -60 Extract Volts (kV) simultaneously. -80 1.2ms 35mA beam at 50Hz State of the art diagnostics and modelling will lead to reduced emittance. The Penning source can be changed in ~2 hours. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  9. 9. Low Energy Beam Transport. There are two approaches: solenoids or einzel lenses. Space charge effects are very high at these particle velocities. Einzel lenses are short whereas solenoidal LEBTs allow for space charge compensation through background gas ionisation. Both systems can introduce aberrations if the full aperture is used. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  10. 10. LEBT. Electrostatic solutions may be problematic when operated close to a caesiated ion source. Space charge compensation in negative hydrogen beams is less well understood than for positive beams. >90% compensation is expected but gas pressures can also lead to beam stripping. Compensation takes time leading to an initially mismatched part of the beam. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  11. 11. LEBT. Eg SNS electrostatic H- LEBT incorporating pre-chopper Beam experiences full space charge but design is very compact. HV sparking has limited performance of LEBT chopper. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  12. 12. LEBT. Eg SILHI 2 solenoid H+ LEBT Almost 100% compensation is possible. Higher gas pressures are required to achieve full compensation in solenoids. Large emittance growth can occur for some operating points Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  13. 13. Radio Frequency Quadrupole. The RFQ is the default accelerating structure from 10s of keV up to 2-5 MeV due to its strong focussing and efficient bunching. Although the beam dynamics is quite mature the diversity of manufacturing methods suggests an optimum way of engineering the structure has not yet been found. High surface fields can make RFQs prone to field emission issues. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  14. 14. RFQ. High transmission and low emittance growth for high intensity beams leads to relatively long structures. 4-rod and 4-vane types are both feasible although 4-vane is possibly easier to cool at high duty factor. 4-vane structures can be bolted, brazed, electron beam or laser welded. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  15. 15. RFQ. Eg ISIS RFQ >95% transmission for >30mA but low frequency and low duty factor. Approaching 5 years of almost faultless operation. Matching to DTL is not optimal. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  16. 16. RFQ. Eg J-PARC RFQ 30mA H- at 3%. Employs Pi mode stabilising loops. Cavity is inside an external vacuum tank. Experiencing sparking issues. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  17. 17. RFQ. Eg LEDA RFQ 100mA H+ up to CW 6.7MeV, 8m long. Output current dropped during pulsed operation requiring up to 110% electrode voltage to cure – trapped ions may be the cause. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  18. 18. Beam Chopper. For injection into a ring at high intensity, chopping the linac beam at the ring revolution frequency is essential for low loss acceleration. Ideally there should be no partially chopped bunches in the linac which requires extremely fast switching times. High voltage switching limits mean chopping has to be done at low energy in the Medium Energy Beam Transport. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  19. 19. Beam Chopper. Even for a H+ linac with no ring, chopping may still be necessary. Reducing average current without reducing bunch charge requires chopping. Alternative would be to reduce source output and retune whole linac for lower current. A chopper may be required to remove slow beam transients at the beginning and end of pulse or ramping current at switch on. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  20. 20. Medium Energy Beam Transport. Placing the chopper in the MEBT places constraints on the MEBT design. Large drifts necessary for the deflectors and beam dumps and a relatively parallel beam through the chopper results in quite low phase advance in the MEBT. Matching between the MEBT and RFQ and following structure – which have relative large phase advances – and controlling emittance growth can be challenging. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  21. 21. Beam Chopper. Eg CERN Linac4 Chopper Uses a meander type deflector mounted inside a quadrupole. UP to 30% emittance growth in MEBT seen in simulations. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  22. 22. Beam Chopper. Eg J-PARC Chopper Uses 2 RF deflectors in MEBT plus induction gap pre-chopper in LEBT. Low Q deflector cavities allow ~10ns rise times. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  23. 23. Beam Chopper. Eg RAL FETS Chopper Two stage chopping to achieve fast rise time and long flat-top. Discrete deflector plates and delay lines instead of meander. Sub 2ns rise and fall Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  24. 24. Funnel. Beam funnelling has been proposed as a solution to achieving higher currents than available from a single ion source (mainly applicable to H-) or to reduce space charge in the front end. A low energy for funnelling reduces the amount of duplicated equipment. A higher energy may be preferable to control dispersion effects. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  25. 25. Funnel. Eg Frankfurt 2 beam RFQ Novel concept of two convergent RFQs and RF deflector in a single cavity. Funnelling has been experimentally demonstrated. It isn’t clear if dispersion is controlled. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  26. 26. Funnel. Eg Los Alamos half funnel. A 5 MeV H- beam was successfully ‘funnelled’ with good transmission and emittance growth. Proof of principle that funnelling can be achieved. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  27. 27. Diagnostics. Even at 3 MeV the beam power in a high intensity front end can be significant: nearly 20 kW on RAL FETS for example. Non destructive diagnostics are an attractive proposition and can be applied throughout the linac. For H- beams laser photo detachment techniques allow for online profile and emittance measurement. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  28. 28. Diagnostics. Eg RAL FETS laser diagnostics. The RAL front end test stand will employ laser wire tomography for full 2D non destructive beam density measurement. -5 1*10 hPa -4 1*10 hPa Laser 2 1 electrostatic LEBT A laser stripping based beam axis emittance measurement 1*TP ion CC D source system is being ca 1*TP 2 einzel lenses 2*TP me ra R=40mm magnetic coils differentiell pumping developed. tank 1, 2 slit position of emittance scanner TP = Turbopump Faraday cup Scintillator Dumping system Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  29. 29. Outlook. High intensity front ends in operation or under development include (but not limited to): H- 30 mA 50 Hz 300 µs 0.6 MeV 202.5 MHz ISIS H- 60 mA 50 Hz 2 ms 3 MeV 324 MHz RAL FETS H- 30 mA 25 Hz 0.5 ms 3 MeV 324 MHz J-PARC H- 30 mA 60 Hz 1 ms 2.5 MeV 402.5 MHz SNS H- 20 mA 60 Hz 2 ms 3 MeV 350 MHz PEFP H+ 70 mA 4 Hz 36 µs 3 MeV 325 MHz FAIR H+/H- 20 mA 2.5 Hz 3 ms 2.5 MV 325 MHz HINS H- 40 mA 50 Hz 1.2 ms 3 MeV 352 MHz SPL D+ 125 mA CW CW 5 MeV 175 MHz IFMIF H+ 100 mA CW CW 6.7 MeV 402.5 MHz LEDA Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop
  30. 30. Discussion. • Use H- even for long pulse to enable laser wire diagnostics. • Design a dismantleable/repairable RFQ and have a spare. • Include fast chopper even if there is no ring. Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop

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