Activities at LANL in the Field of High Power Superconducting Proton Linac Structures

1. Activities at LANL in the Field of High Power Superconducting Proton Linac Structures T. Tajima and F.L. Krawczyk Los Alamos Neutron Science Center (LANSCE) LA-UR-05-3224 HPSL Workshop, Naperville, May 22-24, 2005

2. Outline • Accelerator Production of Tritium (APT) Project • 100-170 MW (1.0-1.7 GeV, 100 mA) Proton Linac • Measurements of b=0.64, 5-cell 700 MHz elliptical cavities • Advanced Accelerator Applications (AAA) Project (Transmutation of Nuclear Waste) • 8 MW (600 MeV, 13.3 mA) Proton Linac • Measurements of b=0.29 (ANL) and 0.175 (LANL) spoke cavities • Q0 disease with low-b cavities HPSL Workshop, Naperville, May 22-24, 2005

3. Accelerator Production of Tritium (APT) Project • Purpose: Replenish Tritium Inventory for Nuclear Weapons (3 kg/year) • Method: Using spallation neutrons produced by a proton accelerator, split Lithium into Tritium and Helium. HPSL Workshop, Naperville, May 22-24, 2005

4. 700 MHz SRF Section Specs Numbers in parentheses are for 1.7 GeV case HPSL Workshop, Naperville, May 22-24, 2005

5. APT b = 0.64, 700 MHz, 5-Cell Cavity Cavity length : 116 cm Cell diameter : 40 cm Beam aperture : 13 cm Ep/Eacc= 3.38 Hp/Eacc= 69.6 Oe/(MV/m) HPSL Workshop, Naperville, May 22-24, 2005

6. Facilities for SRF Activities at LANSCE 2 Inserts for 0.96 m diameter, 3 m deep cryostat 240 m2 Class-100 Clean room Ultra-pure water system with 5700 liter storage tank 43 m High-pressure rinse in a clean room. Cryostat with movable radiation shield 30 m Control, tuning Pumps to pump down the cryostat to < 2 K HPSL Workshop, Naperville, May 22-24, 2005

7. Results of 6 Prototype Cavities at 2 K. Defects, field emission and MP(?) limited performance. Half the cavities were measured by Jlab as well (J. Mammosser). b=0.64 835 Oe 40 Epk APT Goal Defect at the middle cell equator Calculated MP band by W. Hartung HPSL Workshop, Naperville, May 22-24, 2005

8. Fitting of an Rs-T curve with the theoretical formula gives you Rres.In this case, Rres was determined to be 6.4 nW.Cavity: Sylvia HPSL Workshop, Naperville, May 22-24, 2005

9. Comparison with TESLA Cavities • Difference of Ep/Eacc and Hp/Eacc. • Difference of b. APT(0.64) vs. TESLA (1). • It might be more susceptible to multipacting (MP) due to the squeezed shape APT cavity shape is not well optimized! HPSL Workshop, Naperville, May 22-24, 2005

10. Comparison with TESLA Cavities APT results (Eacc) were converted to TESLA equivalent numbers for comparison. Best TESLA 9-Cell Cavity 2 K TESLA 500 TESLA 800 APT Eacc*: Hp/Eacc=42.6 was used. HPSL Workshop, Naperville, May 22-24, 2005

11. Issues that need to be addressed • The APT Goal was achieved, but we need to achieve higher gradients for future machines to further reduce costs. • Q0 drops at medium and high gradient due to Multipacting (?) and Field emission. • Measures to take • MP free design of the cavity with MP codes. • Thermometry and other diagnostics for detecting heating spots to localize and identify the problem. • Improve contamination control during assembly to reduce field emission due to migrated particles. HPSL Workshop, Naperville, May 22-24, 2005

12. Advanced Accelerator Applications (AAA) Project • Purpose: Transmutation of Nuclear Waste, i.e., reduce life of nuclear waste by about 2 orders of magnitude to <500 years. • Method: Accelerator-Driven System. HPSL Workshop, Naperville, May 22-24, 2005

13. Spoke Cavity Development for Low-Energy Sections of Proton Linacs • Studies have been carried out at ANL since late ’80s (Jean Delayen, Ken Shepard) • Some advantages over elliptical cavities • Compact (~1/2 diameter of elliptical cavities for the same frequency) • Due to the lower frequency it can be operated at 4 K • Mechanically stable • Larger velocity acceptance HPSL Workshop, Naperville, May 22-24, 2005

14. Tests on ANL b = 0.29, 340 MHz, 2-gap Spoke Cavity Cell diameter : 44.2 cm Cavity length : 30 cm Beam aperture : 3.2 cm Ep/Eacc : 3.18 Hp/Eacc : 85 Oe/MV/m Previous tests at ANL showed field emission and limited at Eacc~5 MV/m BCP ~100 micron and 1000-psi (~70 bar) HPR in a class-100 clean room were performed at LANL HPSL Workshop, Naperville, May 22-24, 2005

15. Vertical Test Results of ANL b=0.29, 340 MHz, 2-Gap Spoke Cavity 2 K 4K AAA Goal Ep 40 MV/m Hp 1063 Oe HPSL Workshop, Naperville, May 22-24, 2005

16. AAA b=0.175, 350-MHz, 2-Gap, Spoke Cavity Two cavities were fabricated at ZANON, Italy. Cell diameter : 39.2 cm Cavity length : 20 cm Beam aperture : 5 cm Ep/Eacc : 2.82 Hp/Eacc : 69 Oe/MV/m HPSL Workshop, Naperville, May 22-24, 2005

17. Vertical Test Results of LANL b=0.175, 350-MHz, Spoke Cavities With Nb blank flanges on large radial ports Thoroughly cleaned with high-pressure water rinse after BCP. The second EZ02 test was done after disassembly and high-pressure rinse. 4 K 4 K Eacc, max = 13.5 MV/m Ep, max= 38.0 MV/m Hp, max = 994 Oe Details in PAC03 Proceedings. HPSL Workshop, Naperville, May 22-24, 2005

18. Very high gradients are not useful for low-b structures due to longitudinal envelope instability [T.P. Wangler and K.R.Crandall, Spoke Cavity Workshop, Los Alamos, Oct. 7-8, 2002.] • Our longitudinal beam-dynamics design approach has been to keep s0<90° and minimize the focusing period. • The cryomodules form piecewise constant FODO lattices where each period contains one cavity and one solenoid. • For 350-MHz proton linac in b range of 0.2 to 0.5 (20 to 150 MeV) we could use cavity gradients up to about 8 MV/m without longitudinal beam-dynamics problems. HPSL Workshop, Naperville, May 22-24, 2005

19. Some questions that need to be answered. • What is the experimentally (practically) achievable Hp at 4 K? It has been ~1800 Oe at 2 K with TESLA cavities. (Due to the decrease of Hc with lower temperature, i.e., ~14 % from 4 K to 2 K, and the better thermal conductivity of liquid He at T<2.17 K (Lambda point), it is expected that the achievable field is higher at 2 K. ) • Are there any difference between elliptical and spoke cavities in terms of the field limitations? It has been shown for spoke cavities that the maximum fields are almost the same at 4 K and 2 K, whereas some elliptical cavities have shown much better results at 2 K than at 4K. HPSL Workshop, Naperville, May 22-24, 2005

20. 3. Q0 disease with low-frequency cavities (tests with 350 MHz spoke cavities) • Q0 disease is a degradation of Q0 due to niobium hydrides (Tc~1.2 K?) precipitated on the surface from bulk during slow cooldown by holding the cavity at medium temperatures (70-150 K, worst at ~100 K). It had been said before our tests that low frequency cavities (<500 MHz) do not show it. • A systematic Q disease tests have been carried out with our 350 MHz spoke cavities • Spoke cavities made of RRR~250 Nb. • Original H contest is unknown. • No high temperature heat treatment. (<150 °C) • Standard BCP (HF:HNO3:H3PO4=1:1:2) ~150 mm at <15°C HPSL Workshop, Naperville, May 22-24, 2005

21. Q0 – Eacc Curves were taken after every 12 hours of holding at 100 K HPSL Workshop, Naperville, May 22-24, 2005

22. Q0 Dependence on the Holding Time at 100 K HPSL Workshop, Naperville, May 22-24, 2005

23. Additional Surface Resistance Due to Q disease HPSL Workshop, Naperville, May 22-24, 2005

24. Summary of the Q disease tests • With our 350-MHz spoke cavities, little Q disease occurs after 150-mm BCP, up to ~24 hours of holding time at ~100 K. • Holding the cavity for longer time causes Q disease. • The additional surface resistance due to the Q disease increases linearly after the increase starts. • Q disease disappears if the cavity is warmed up to 180 K. HPSL Workshop, Naperville, May 22-24, 2005

25. Summary • Six b=0.64 700-MHz 5-cell cavities were prototyped and all of them exceeded APT specs, but needs better design in terms of optimum Hp/Eaccand MP, and further particulate contamination control to get higher gradients • Two b=0.175 350-MHz single-spoke spoke cavities were prototyped and both of them showed similar results and exceeded AAA specs. • It was found that Q0 disease occurs with our 350-MHz spoke cavities, but it does not occur if cooled down within 24 hours. HPSL Workshop, Naperville, May 22-24, 2005