Recent Results of High Gradient Superconducting Cavities at Cornell

Transcription

1 Recent Results of High Gradient Superconducting Cavities at Cornell Rong-Li Geng Seminar Brown October Bag Accelerator 8, 2004 Physics Cornell Seminar, University October 8,

2 Contents Background A brief review of high gradients Gradient envelope New geometry approach for higher Eacc Reentrant cavity: concept and performance New fight against field emission Summary Acknowledgement 2

3 Disclaimer Context: High gradient cavities for ILC. Bulk niobium cavities only. Mainly L-Band L ( MHz). 3

4 Background August 19, 2004: ITRP recommended SC RF for ILC ITRP executive summary: The machine will be designed to begin operation at 500 GeV, with a capability for an upgrade to about 1 TeV, as the physics requires. This capability is an essential feature of the design. Therefore we urge that part of the global R&D design effort be focused on increasing the ultimate energy to the maximum extent feasible. Longer linac Higher gradient TESLA type linac 800 GeV: 35 MV/m (Achieved) 1 TeV: 44 MV/m (R&D needed) 4

5 Accelerating Gradient d Accelerating voltage v = c Enter Exit Accelerating gradient 5

6 A brief history Hi field Q-slope 2000 Padamsee Field emission Hi pressure water rinse Hi power pulsed proc Hi purity Nb Thermal breakdown Multipacting 1986 Elliptical cavity shape 1974 Time 6

7 Eacc Envelope Trend First TESLA Workshop at Cornell Envelop defined by single-cell Flat since 1995! Why? 7

8 Multi-cell Cavities catching up quick Multi-cell cavity gradient is about to reach the envelop! 8

9 Eacc important for accelerator Cavity performance determined by EM field on surface No fundamental limit to surface electric field Surface electric field high at iris, Epk Graber et al., MV/m 3GHz Nb 145 MV/m Moffat 210 MV/m Knobloch Surface magnetic field high at equator, Hpk Nb Nb Delayen, Shepard 9

10 Surface magnetic field has physical limit Nb theoretical H crit, RF ~ 2000 Oe Highest achieved Hpk 1880 Oe State-of-the-art 1750±100 Oe 2000 H RF [Oe] HPR H crit,rf 500 TE 011 TM 010 Electropolished Kenji Saito Year

11 Paths toward higher Eacc The maximum feasible Eacc is determined by the RF critical magnetic field H crit,rf. When the surface magnetic field exceeds H crit,rf, superconductivity breaks down into normal conductivity. Eacc max = H H pk crit, RF / Eacc Determined by material Determined by cavity geometry Two paths to raise Eacc: Raise H crit, RF or decrease Hpk/Eacc 11

12 Hpk in Nb cavities is close to theoretical limit, it is apparent new material is required to significantly increase Eacc. situation Nb3Sn is the only candidate having potential of higher H crit,rf. but the new material perspective is still remote. (remind: 40 years R&D on Nb) The high gradient need is imminent. Solution: New geometry for a reduced Hpk/Eacc. Advantage: Cavity is based on well established Nb technology. Disadvantage: Eacc improvement is inherently small (<30%). Bottom line: A better Hpk/Eacc alone would reach 50 MV/m; And 55 MV/m is feasible with final push in H crit,rf of Nb. 12

13 An idea reentrant cavity A reentrant cavity geometry offers a reduced Hpk/Eacc, preserving at the same time the large bore diameter at the iris. Large bore diameter is the inherent advantage of SRF. Regular TESLA shape Reentrant shape 70 mm Hpk/Eacc reduced by 10% H pk E acc =42 Hpk/Eacc in Oe/(MV/m) H pk E acc =

14 TESLA type Reentrant type 14

15 Regular TESLA e = 1 h = 1 e = h = E H pk pk / E 2 / 42 E acc acc r, mm RF optimization Z, mm fabricated Multi-cell: Single cell: Hpk/Eacc=37.8 Hpk/Eacc=37.9 Epk/Eacc=2.4 Epk/Eacc=2.19 A higher Epk is the price for a lower Hpk But no fundamental limit to surface electric field reduction in surface magnetic field ratio increase in surface electric field ratio δe(%) cell-cell coupling δ h(%) k(%) Shemelin, Padamsee, Geng, NIM A 496 (2003)

16 Female die Fabrication Male die Regular deep drawing/coining sufficient to build the reentrant shape reentrancy Half-cell heat treatment with Yt 1200 C 4 hours to improve Nb thermal conductivity RRR increased from 250 to

17 Fabrication E beam half-cell HF+H2SO V cathode half-cell spin bar water magnetic stirrer Electropolish half-cell/beam tube Nb rod Equator EBW with a central rod to shield RF surface from Nb vapor 17

18 Treatment Standard BCP after equator weld to remove residual Nb deposit. Flip cavity between HPR cycles for thorough rising of reentrant surface. Dry cavity horizontally to avoid trapped water in reentrant pocket. Vacuum bake out at C for 48 hour. Vertical electropolish single cell cavity. 18

19 Why we must have EP? BCP = HF + HNO 3 + H 3 PO 4 High-field Q-slope Rapid Q decline >20MV/m Followed by quench ~ 30MV/m Dunk etch of CEBAF cavity 19

20 Why we must have EP? EP C bake cures high-field Q-slope! KEK horizontal EP E. Kako Reproducible 40 MV/m by using EP plus bake 20

21 Acquiring EP Capability at LEPP Single-Cell Vertical EP Half-Cell EP H2SO4(96%):HF(48%)=10:1(vol.), ~0.5 µm/min 21

22 EP: Process and results Suppressed grain boundaries (120µm BCP+30 µm CCO-EP) CCO-EP Continuous current oscillation electropolish 50 µm Geng et al., the 11 th SRF Workshop, Travemunder/Lubeck, Germany, September 8-12, 2003 Tajima Low surface hydrogen 22

23 First EP trials produced high gradients TESLA cavity 1.3 GHz single cell Cornell/CEBAF cavity 1.5 GHz single cell 35 MV/m 36 MV/m 23

24 Reentrant Cavity Vertical RF test Tests run at K, 9 tests to date First test reached Eacc = 27 MV/m Soft multipacting barrier near Epk = 55 MV/m Gas helium processing at Epk > 90 MV/m Highest Eacc = 44.4 MV/m Test stand has MW capability for HPP 24 with short RF pulse

25 Achieved performance 2K X-rays start Q 0 >1E10 at 30 MV/m Q Eacc = 44.4 MV/m Quench limit Eacc [MV/m] 25

26 Reproducible quench field independent of field emission level 26

27 Challenges to materialize the benefit of a lower Hpk/Eacc X-rays start Field emission! Q Eacc [MV/m] Epk=97 MV/m X-ray=60 R/h 27

28 Field Emission Theory QM tunneling theory predicts exponetial Fowler-Nordheim emission current = 2 C C1E exp E j FN 2 28

29 Field Emission Knobloch Acceleration of electrons drain cavity energy. Impacting electrons produce line heating. Impact also produces bremsstrahlung x rays. Nearly all emission is associated with microscopic foreign particles. 29

30 Avoiding Field Emission High Pressure water rinsing Kneisel Pressurized (100 bar) DI water jet to remove surface particles Reschke 30

31 Avoiding Field Emission Clean Room Assembly Pumping and venting without re-contaminating surface TTF 9-cell cavity 31

32 Reentrant cavity faces FE challenges in untested regime Desired low Hpk/Eacc results in a higher Epk/Eacc (remind: traditional wisdom is to lower Epk/Eacc!) Eacc[MV/m] Epk[MV/m] Today Future = 2 C C1E exp E j FN 2 DESY: max. Epk achieved 77 MV/m LEPP: max Epk achieved 70 MV/m 32

33 Reentrant cavity faces FE challenges in untested regime reentrant TTF Shemelin Larger Hi field surface area higher chances to catch particles 33

34 Reentrant cavity faces pocket complexity reentrant Puddle after HPR elliptical Easy drain water jet shadowing restricted drain Solution: Longer HPR time Shake and horizontal drying Need R&D: understand jet/particle interaction improve nozzle design more radical ideas? 34

35 Field Emission in situ processing RF processing Helium processing High Power Processing (HPP) Destroy emitting particles by inducing RF sparking Knobloch Knobloch 35

36 In-situ processing RF (CW and pulsed ) processing at low field effective. But RF processing at high field activates new field emitters. Helium processing at high field is effective, 17% gain obtained. To get best result, He proc. should be followed by partial warm up. LEPP has HPP capability at 1.3 GHz. Past HPP was very successful. Successful HPP requires much higher Epk in short pulse. Max. Epk(pulse) limited by H crit,rf (Hpk already reach 1700 Oe) 1.3GHz, 1 MW 200µs pulse It is worth trying! 36

37 High gradient cavity comparison World high gradient data (regular elliptical shape) 2000 Hpk [Oe] LE1-35 Eacc = 37 1B9 Cornell test Eacc = 40 1B9 DESY test Eacc = 44 July-04 Eacc = 37 July-21 July-16 Eacc = 44.4 Eacc = 42.6 July-14 Eacc = 40 LR1-2 MLP Eacc = 49 Reentrant cavity potential Eacc=49MV/m LR1-2, Feb-04 Eacc = 27 Un-labeled points refer to KEK and DESY cavities Reentrant shape with δh=-10% Eacc in MV/m Rong-Li 50 Geng Epk [MV/m] 37

38 Gradient Status Updated This is a small step but trend line is changed! First TESLA Workshop SRF chosen for ILC Ultimate gradient? Let s explore! 38

39 Summary Achieved Eacc in excess of 44 MV/m in Nb cavity. Established in-house EP procedure providing reproducible Hi Eacc. Demonstrated usefulness of helium processing at Hi gradient. Next step: Keep pushing Eacc by more EP to ~ 50 MV/m. Must reduce field emission: improve HPR nozzle; HPP. Identify/control sources of particulate contamination (facility!). 39

40 Acknowledgement RF design of reentrant cavity by Valery Shemelin In building/testing reentrant cavity, crucial contributions made by Curtis Crawford Joe Kirchgessner Andy Seaman James Sears This work is supported by the National Science Foundation. 40