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Prospects for a very high power CW SRF Linac

Prospects for a very high power CW SRF Linac. Bob Rimmer, JLab AHIPA 2009 workshop 10/19/09. Outline. Challenges in high power CW SRF for protons State of the art Cost drivers and technical risks Choices/optimization The SNS mystery Structure examples Some suggested R&D topics

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Prospects for a very high power CW SRF Linac

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  1. Prospects for a very high power CW SRF Linac Bob Rimmer, JLab AHIPA 2009 workshop 10/19/09

  2. Outline • Challenges in high power CW SRF for protons • State of the art • Cost drivers and technical risks • Choices/optimization • The SNS mystery • Structure examples • Some suggested R&D topics • Conclusions

  3. Challenges in high power SRF for Protons • CW SRF requires a lot of cryogenic capacity • High average currents require a lot of RF power • E.g. 10 mA x 2 GeV = 20 MW • High RF power requires robust couplers • E.g. 20 MW ÷ 200 cavities = 100 kW/coupler (SNS couplers good for ~200 kW, 1 MW windows OK) • Modest average gradient requires longer tunnel • High average current may require BBU control • ADS user operations require stability, low trip rate • All these things could cost a lot of money • Beam losses could spoil everything

  4. State of the art The good news: • Large CW SRF facilities work (CEBAF, LEP, Dalinac, etc...) • Many “proton driver optimized” cavities are in development • ANL, Triumf, JLab, KEK, … • Cavity performance continues to improve (Eacc and Qo) • BBU limits and mitigations are well understood/tested • Large 2K cryogenic plants are getting more efficient • New CW RF sources continue to be developed • IOTs, multi-beam tubes, magnetrons(!) • Science case for new machines is very strong

  5. CEBAF • CEBAF is a 5-pass 6 GeV* CW recirculated linac based on SRF. • Three experimental halls for nuclear physics research • Nuclear structure, Gluonic excitation etc… • 421/4 original cryomodules assembled in-house. • (Then) world’s largest 2K Cryo plant. • Worlds Largest operating installed base of SRF. * originally 4 GeV design specification, soon to be upgraded to 12 GeV

  6. Stuart Henderson ALCPG09 Medium beta cavity High beta cavity SNS Linear Accelerator 2.5 MeV 87 MeV 186 MeV 386 MeV 1000 MeV H- RFQ DTL CCL SRF,b=0.61 SRF,b=0.81 Reserve • World’s first high-energy superconducting linac for protons • 81 independently-powered 805 MHz SC cavities, in 23 cryomodules • Space is reserved for additional cryomodules to give 1.3 GeV

  7. SNS Beam Power Performance History 1 MW beam power on target achieved in routine operation Stuart Henderson ALCPG09 Power on Target [kW]

  8. Some typical CW parameters (JLab upgrade) • Frequency 1.5 GHz (could be lower?) • 15-20 MV/m CW (~10 MV/m real estate gradient) • Qo ~ 1010 at 20 MV/m (has been demonstrated) • CM Cost ~$2.6M*/100 MeV (Jlab upgrade module) • RF ~$1.7M/cryomodule (8x13kW RF stations) @~1mA • 2K cryogenic plant ~$30M/GeV (CHL2) excluding distribution • ~7.3 cents/volt or $73M/GeV (excluding tunnel costs) • ~$73/watt electron beam power (1ma @ 1GeV =1MW ) *FY08 loaded dollars, actual 12 GeV project costs will be known soon

  9. 2K and 4K JLab Technology Development Areas • Large machines are getting more efficient • Difference between 2K and 4K does not make up for BCS losses 2008 12 GeV PED Dana Arenius 3x Power Reduction 20-25% Power Reduction

  10. NASA-JSC 2008 Plant Test Results Ganni cycle allows good efficiency at high turn-down ratio Original 3.5kW Plant Modified 3.5kW Plant to Floating Pressure Planned13kW Plant

  11. Cost drivers & technical risk 50 • Linac cost drivers: • Cryomodules • RF • Cryogenics • tunnel • Technical risks • Field emission • BBU • Beam loss (heating & activation) Matthias Liepe, ERL 2009 Cornell University, Ithaca New York 100 mA ERL 40 30 relative cost [%] 20 10 0 Tunnel RF power Cryomodules Cryogenic plant

  12. Choices (optimization) • Current ≤ 30mA? • Optimum Frequency (depends on Rs & beam physics ) • Operating temperature, depends on Rs • Gradient depends on Qo (Rs) • Number of cells per cavity • Number of different b values? • Complex optimization, sensitive to assumptions • E.g. M. Liepe ERL 2009, Cornell ERL • NLS project outline design report 7/09) (P. Macintosh)

  13. Effect of operating temperature • Cryogenic losses strongly depend on temperature below Tc • Optimum operating temp ultimately set by residual resistance NEW LIGHT SOURCE (NLS) PROJECT SCIENCE CASE AND OUTLINE FACILITY DESIGN www.newlightsource.org Editors: J Marangos, R Walker and G Diakun, Science and Technology Facilities Council (STFC)

  14. Matthias Liepe, ERL 2009 Cornell University, Ithaca New York Cooling Power for Dynamic Cavity Losses (f,T) for given Eacc Optimum frequency and temperature depend on residual resistance Benefit from lower temp & freq. No advantage to lower freq? (dream…) (still quite optimistic)

  15. Example: NLS (UK) frequency study • Found no benefit to going to lower frequency • But, assumed constant residual resistance?, fixed temp? P. Macintosh

  16. Jlab data: SNS MB 805 MHz G. Ciovati A lot of scatter, average ~8 nΩ

  17. Jlab data: SNS HB G. Ciovati A lot of scatter, average ~8 nΩ

  18. Jlab data: CEBAF 5-cells 1.5 GHz G. Ciovati A lot of scatter, average ~12 nΩ Implies that residual resistance not constant with frequency?

  19. Matthias Liepe, ERL 2009 Cornell University, Ithaca New York Optimal Field Gradient 11 1 10 30 • Q0-value has significant impact on cost (high impact and risk parameter) • Construction cost changes only moderately for gradients between ~16 and ~27 MV/m • Operating cost / AC power increases with gradient • Select gradient at lower end: 16.2 MV/m construction case 1 cryo AC power Cavity Q0 case 2 10 yr operation 25 0.8 20 0.6 Construction and operation cost 0 10 normalized cost Q 10 15 Cryo AC power [MW] 0.4 10 0.2 5 9 0 10 0 10 15 20 25 30 10 15 20 25 30 10 20 30 field gradient [MV/m] field gradient [MV/m] field gradient [MV/m]  Less risk for same cost!

  20. SNS mystery SNS cavity processing data Typical SNS cavity performance in vertical test at JLab [J. Ozelis. J. Delayen, PAC05]

  21. SNS cavity data VTA vs CMTF • Gradient at Q0 = 5 x 109, as measured in the VTA and CMTF, for the medium-β (triangles) and high-β(squares) cavities • Individual cavity tests only • Not all CM’s were tested at JLab [J. Ozelis. J. Delayen, PAC05]

  22. SNS mystery • SNS high beta cavities do not reach average spec. • Almost all cavities exhibit high electron activity/radiation • Many look like early onset field emission • Others exhibit signs of multipacting in FPC and HOMs • Radiation does not follow Fowler-Nordheim • Fast radiation monitors see spikes during fill and decay • Diodes detect heating in HOM cans • DC bias on FPC’s helps • Multipacting barrier in cells is also frequently observed • Ensemble average is lower than individual limits due to dark current transport

  23. Stuart Henderson ALCPG09 Helium Vessel Fast Tuner Slow Tuner SNS Cavities and Cryomodules b=0.81 Specifications: Ea=15.8 MV/m, Qo> 5E9 at 2.1 K b=0.61 Specifications: Ea=10.1 MV/m, Qo> 5E9 at 2.1 K Medium beta (b=0.61) cavity High beta (b=0.81) cavity Field Probe HOM Coupler HOM Coupler Fundamental Power Coupler 11 CMs 12 CMs

  24. Stuart Henderson ALCPG09 Cavity Gradient Performance History: August 2006: 7 cavities off-line; 850 MeV; 5 Hz Large fundamental frequency coupling through HOM coupler Tuners out of range Cold-cathode gauge/ turn-on issues

  25. H06 back to service Irregular dynamic detuning H01 out of service for repair Stuart Henderson ALCPG09 Noisy FP HOMB Additional HVCM; enough RF power for design current H01 repaired and put in the slot of CM19 HOMB 25

  26. Individual and Collective Cavity Limits Stuart Henderson ALCPG09 Individual; powering one cavity at a time Collective; powering all cavities in a CM at the same time CM19; removed Large fundamental power through HOM coupler Design gradient Average limiting gradient (collective) Field probe and/or internal cable (control is difficult at rep. rate >30 Hz) Average limiting gradient (individual)

  27. SNS mystery • Possible explanations • Inadequate HPR at equator of cavity or in couplers • Multipacting barrier modified by surface condition or geometry • Electrons from multipacting in couplers seeding cells https://www.slac.stanford.edu/~lge/acd_multipacting_small.mov • Electron cross talk between cavities lowers usable gradient • End group low RRR exacerbates problems • What to do next? • Try EP of SNS cavities with ILC processes • Test cavities with hooks removed • Evaluate alternative designs

  28. EP after BCP on 1.5 GHz cavities • EP tests on existing 7-cell cavities with prior BCP • Dramatic improvement with final light EP • High field Q slope removed (after 120°C baking) • Lower residual resistance (suggests BCP Q-slope is due to small scale surface roughness)

  29. Candidate Structures: Low Beta SSR1 Low beta cryomodule Thomas Nicol. PX collab mtg. 9-11-09 ANL TSR ANL 345 MHz =0.5 Triple-spoke P.N. Ostroumov ANL Physics Division PX collab. Mtg. September 11, 2009 ANL 345 MHz =0.62 Triple-spoke

  30. E.g. high beta: JLab high-current cavity • Development of electron cavity for ≥100 mA F. Marhauser PAC09

  31. E.g. high current cryomodule • JLab 700 MHz ERL module (based on modified SNS layout) • Could be economical if can operate in BCS dominated regime • Very large apertures (halo!) Very high BBU threshold • Use TV band RF sources

  32. High beta: XFEL module converted to CW • Larger cryogenic piping (chimney, 2-phase line) • Gas return pipe OK (sized for ILC!) • Higher-power FPC (e.g. Cornell, Daresbury) • Modified HOM probes (temp. stabilized)

  33. Some suggested R&D topics • Qo (surface resistance, materials, processing, mag shielding) • Elimination of field emission, next generation processing • Microphonics (sources, response (stiffness), feedback, tuners) • Beam optics: Halo and losses • DemonstrateProton Driver optimized cavities with beam • Coupler power, tuner, HOM spectrum, microphonics • BBU/HOM damping (real couplers and loads) • Fast reset (automated trip recovery << thermal time constant)

  34. Conclusions • Large scale CW SRF is viable for proton drivers • Prototype cavities exist for all beta range • No show stoppers to running CW • Robust high-power couplers must be used • Main challenges may be halo / beam loss / trip rate • Qo (residual resistance) is significant cost driver • Poorly understood but under active investigation • Cost “optimization” depends strongly on assumptions • Full multi-variable optimization worthwhile using best available world scaling data

  35. Back up material

  36. SNS HOM multipcating • 3D simulation by SLAC ACD group

  37. The Beam Power Frontier for Protons Courtesy J. Wei • Central challenge at the beam power frontier is controlling beam loss to minimize residual activation • 1 nA protons at 1 GeV, a 1 Watt beam, activates stainless steel to 80 mrem/hr at 1 ft after 4 hrs Stuart Henderson ALCPG09

  38. Storage ring SRF cavites • Cornell CESR 500 MHz cavity, KEK B cavity • High average power delivered to beam • High reliability for user operations

  39. capital total cost capital cost 10 Yr operating cost 1.5 operation 0.8 0.4 RF total tunnel cryo 0.6 0.3 linac 1 RF normalized cost normalized cost normalized cost 0.4 0.2 cryo 0.5 0.2 0.1 0 0 0 10 15 20 25 30 10 15 20 25 30 10 15 20 25 30 field gradient [MV/m] field gradient [MV/m] field gradient [MV/m] cavity Q tunnel length number of cavities 0 11 1500 800 10 1000 600 10 length [m] 0 # 10 Q 500 400 9 0 200 10 10 15 20 25 30 10 15 20 25 30 10 15 20 25 30 field gradient [MV/m] field gradient [MV/m] field gradient [MV/m] IOT peak power cryo AC power cryo power fractions 20 15 10 15 10 power [MW] power [MW] power [kW] 10 5 5 5 0 0 0 10 15 20 25 30 10 15 20 25 30 10 15 20 25 30 field gradient [MV/m] field gradient [MV/m] field gradient [MV/m] Example: Dependence on Accelerating Field Gradient Matthias Liepe, ERL 2009 Cornell University, Ithaca New York cav. dyn. HOM input C static

  40. New Hall Add 5 cryomodules 20 cryomodules Add arc 20 cryomodules Add 5 cryomodules Enhanced capabilities in existing Halls SCOPE OF 12 GeV UPGRADE • Upgrade is designed to build on existing facility: • vast majority of accelerator and experimental equipment have continued use Scope of the proposed project includes doubling the accelerator beam energy, a new experimental Hall and associated beamline, and upgrades to the existing three experimental Halls.

  41. SNS Downtime by System Stuart Henderson ALCPG09 SNS Availability FY07: 66% FY08:72% FY09:80%

  42. Example: JLAB HC Cryomodule Development High-current cavity developed for high-power ERL/FELs HC optimized cell shape, 5-7 cells, WG FPC, WG HOMs Aiming for beam test in JLab FEL in 2010 two-phase He return header line 50 K heat station HOM waveguide with load HOM end group Cavity He vessel He fill line high power rf window “dogleg” chicane fundamental power couplers F. Marhauser ERL09 Conceptual design of a cavity-pair injector cryomodule (L=2.6m)

  43. JLAB HCCM Broadband HOM Damping Efficiency • Parasitic HOMs measured on warm model (bead-pull method) • Simulations performed with MAFIA & Microwave Studio (MWS) • HOM damping requirements can support Ampere-level of current ideal absorbing boundaries at waveguide ports • Simulation and measurement in good agreement CST MAFIA model CST MWS model Qext with beam tube and waveguide ports Bead-pull HOM measurement setup F. Marhauser ERL09

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