Warm and Cold Ion Linac: Comparison and Optimization

1. Warm and Cold Ion Linac: Comparison and Optimization March 30, 2015

2. Content • Economics of NC and SC linacs • Peak and accelerating fields in NC and SC structures • Review of pulsed ion Linacs • Main difference between NC and SC structures in application to multi-ion linacs • High-performance QWRs and HWRs • Cost-efficient design of the SC section: reduce number of cavities and cryomodules • Lorentz detuning and its compensation • Conclusion P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

3. MEIC Requires an Multi-Ion Linac P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

4. What Do We Know about NC and SC Linacs? • Linac cost includes 3 major components • Accelerating structure • RF system • Cryoplant and helium distribution system (SC linac) • Everything else (building, vacuum, water cooling, controls, diagnostics,…) • Normal Conducting Pulsed Linac • The most expensive component is the RF system • High power (many MWs) RF systems are cost efficient • Multi-gap, long accelerating structures are cost efficient • Accelerating gradients are limited by breakdowns which is known as Kilpatrick limit • Superconducting Pulsed Linac • Accelerating structure is very expensive • Cryoplant is considerable cost • RF is low-cost especially for low power linacs P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

5. Peak Fields in Accelerating Cavities Accelerating gradients are limited by peak fields • Normal conducting structures made from copper • Kilpatrick limit was introduced in 1950s, empirical formula • In modern pulsed structures, electric field exceeds Kilpatrick limit by a factor of ~2 • Superconducting structures • Peak magnetic field is limited by quench, theoretical value is ~200 mT at 2K • Peak electric field is limited by the surface quality. ~120 MV/m can be achieved • Peak fields can not be measured • These ratios are known from the simulations of the resonator design • EACC can be obtained experimentally from the stored energy • EPEAK can reach ~60-70 MV/m in pulsed oepration P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

6. Autophasing, Synchronous Phase in Multi-gap Standing Wave Structure Lc  = 0, maximum acceleration  =-30, stable acceleration Q is ion charge, A is ion mass number P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

7. Acceleration of Ion Beams • Due to limited extraction voltage in ion sources, RF acceleration is applied at very low velocities • To have practical length of the acceleration period, the RF frequency must be low • Transverse dimensions of accelerating structures are large, • RFQ is the first accelerating structure after the ion source • IH type structures are popular right after the RFQ • Alvarez structure is used at GSI after the IH structure P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

8. RF Linacs Protons, H Synchrotron Injectors (FNAL, KEK, CERN, IHEP….) MMF (Moscow) SNS LANSCE Heavy Ions Synch. Injectors at GSI LEIR - CERN BNL injector Carbon therapy synchrotrons SNS CERN SPL ESS ISAC-I RIKEN inj. LEDA RFQ SARAF RFQ ATLAS RFQ ATLAS ISAC-II INFN SARAF SPIRAL-2 FRIB Project X EURISOL RAON ADS projects *Low-energy, several MeV/u Heavy-ions P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

9. CW Linac: NC or SC ? • Required wall plug power to create accelerating field • Typical example: 1 GeV CW linac • Superconducting CW linac is much more economic than NC • Both pulsed or CW SC linacs require NC front end • Multi-gap cavities are cost efficient due to fast change of velocity • ~0.3 to 10 MeV/u depending on q/A and duty factor P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

10. Low Duty Cycle Pulsed Linacs, Normal Conducting • For injection into synchrotrons we need higher  and  to control space charge in the ring • For acceleration of ions we need A/Q times higher total voltages compared to protons to reach the same velocity • Acceleration of ions with low Q and high A requires much more voltage then for protons • For pulsed linacs it is cost efficient to have multi-gap accelerating structure and pulsed high-power RF system, several MW pulsed power • GSI Linac, UNILAC – 11.4 MeV/u • RFQ (36 MHz), Q/A  4/238 • Alvarez (108 MHz), Q/A  28/238 • Linac-3, CERN - 4.2 MeV/u, Q/A  27/208 • RFQ (100 MHz), IH structure (3 tanks), 100 MHz, 200 MHz • New BNL injector, 2 MeV/u, Q/A 1/6 • RFQ, IH structure (1 tank), 100 MHz P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

11. Drift Tube Linac (DTL) for Protons • Protons, f=200 MHz, resonator diameter is ~1 meter • From 0.75 to 200 MeV/u P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

12. J-PARC DTL • F=325 MHz, H-minus, 3 MeV to 50 MeV P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

13. GSI Linac • IH structure, 36 MHz Alvarez, 108 MHz up to 1.4 MeV/u up to 11.4 MeV/u P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

14. Normal Conducting IH structure • U. Ratzinger (Frankfurt) Group has built it for the BNL EBIS injector project Courtesy of J. Alessi (BNL) P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

15. Room Temperature Linac at BNL • RFQ and IH structure EBIS 4-rod RFQ IH-structure P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

16. Fixed Velocity and Variable Velocity Structures Normal Conducting Beam β/2 Normal or Super Conducting Beam βOPT/2 P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

17. Front End of Multi-Ion Linac • For low duty cycle linac, NC front end up to ~5 MeV/u is cost effective • The cost of pulsed RFQ and IH structure is low: much less than the cost of CW structures • For this section of NC linac, the main cost is in RF system • Much more cost-effective than SC structure due to large range of velocity change • Multi-gap SC structure – high cost • Or many short SC cavities are required - expensive P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

18. Linac for MEIC: from Proton to Lead • For heavy ions we need • Assume, we need 100 MeV/u for lead ions • Higher energy for light ions is preferable to control space charge in the ring (light ion beams are more vulnerable to space charge) • In low duty cycle linac NC front end is more cost-effective, ~5 MeV/u • For protons we need effective voltage • For lead ions we need effective voltage P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

19. Options • NC pulsed Linac for lead ions up to 100 MeV/u • We need 417 MV total voltage • If linac is based on long multi-gap cavities it can be used for protons too • The phase velocity is given by the geometry of the linac • Lower the voltage and power to maintain synchronous motion of light particles • Protons will gain total voltage 115 MV and can be accelerated up to 100 MeV • If linac is based on 2-3 gap cavities like SC linac • Protons can be accelerated to higher energies, ~280 MeV • Peak fields are limited by breakdown voltage (Kilpatrick limit), this volatge is lower for low frequencies • Factor of 5 larger number of cavities as compared to SC option will be necessary • SC pulsed Linac • Provides higher accelerating voltage per cavity than NC structures • About factor of 4 higher accelerating voltages per cavity than for NC cavity • Number of 2-gap cavities is 107 to accelerate lead ions from 5 MeV/u to 100 MeV/u • Provides 100 MeV/u lead ions and 280 MeV/u protons P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

20. Basic Parameters of the Linac • Linac layout Normal conducting Superconducting P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

21. Focusing • Focusing with SC solenoids is cost-efficient • Return coils are used to minimize fringe fields, no iron or mu-metal shielding is required 9-Tesla SC solenoids in helium vessel ATLAS Cryomodule 6-Tesla SC solenoids in helium vessel PXIE Cryomodule P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

22. Quarter Wave Resonators • This data for CW mode, even higher fields are possible for pulsed mode P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

23. HWR, Very High EPEAK was Demonstrated P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

24. Cryomodule Assembly 2009, OPT=0.15 2013, OPT=0.077 P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

25. Design evolution of the injector linac: Cost Reduction • Number and type of SC cavities is reduced since 2010 design • Footprint is reduced significantly • Based on SRF technology development for ATLAS facility P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

26. Lorentz Detuning • Lorentz force detuning coefficient • 72 MHz QWR: -1.6 Hz/(MV/m)2 • 170 MHz HWR: -1.5 Hz/(MV/m)2 • Simplest method is to provide high RF power in overcoupled mode like SNS • Expensive way, high power is not required for EIC injector • Piezoelectric tuner is a common way to compensate for Lorentz detuning in pulsed SC cavities • There is also issue for LLRF to provide stable phase-locked operation P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting

27. Summary • SC Linac is the best option for acceleration of multi-ion beams • Protons – 280 MeV • Lead ions -100 MeV/u • Performance of SC cavities is being improved every year • Cost of a SC linac is stable in “cost of living” dollars • Multi-gap NC Linac for 100 MeV/u lead ions requires 417 MV effective voltage • Protons can reach to 100 MeV only • NC linac composed from 2-gap (or 3-gap) cavities requires large cavity count, ~500 cavities • This results in very high cost of both cavities and RF system P.N. Ostroumov, Warm and Cold Ion Linacs Collaboration Meeting