1. Progress of high pressure hydrogen gas filled RF cavity K. Yonehara APC, Fermilab 11/02/111 Joint MAP & High Gradient RF Workshop, K. Yonehara

2. Outline Idea of high pressure gas filled RF (HPRF) cavity HPRF cavity with magnetic field Application of HPRF cavity Recent beam-induced plasma loading test Recent HPRF breakdown study Dielectric loaded (HP) RF cavity Conclusion 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 2

3. Idea of gas filled RF cavity 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 3 Vacuum cavity Gas filled cavity Surface electrons appear on asperity Electrons are accelerated by E Electrons smash on other end of wall and deposit their ε Repeat heating ↔ cooling makes surface damage due to fatigue Secondary electron Electrons are cooled by a buffer gas RF cavity wall A buffer gas resists electron dynamics even in strong B

4. Demonstration of HPRF cavity in magnetic field 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 4 Maximum electric field in HPRF cavity Schematic view of HPRF cavity High Pressure RF (HPRF) cavity has been successfully operated in strong magnetic fields Metallic breakdown Gas breakdown: Linear dependence Governed by electron mean free path Metallic breakdown: Plateau Depend on electrode material No detail study have been made yet Gas breakdown P. Hanlet et al., Proceedings of EPAC’06, TUPCH147 No field degradation due to B field

5. Apply HPRF cavity for muon cooling & acceleration 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 5 Dense hydrogen gas can be used as an ideal buffer to suppress breakdown GH2 cools down RF windows Schematic drawing of HPRF cavity in frontend pre- cooler channel Simulation of muon emittance in hybrid front end channel Hybrid: LiH (various widths (6~10 mm) in simulation) + 10 atm GH 2 Be pressure safety window is included J.C. Gallardo & M.S. Zisman et al., Proceedings of IPAC’10, WEPE074 Results are comparable with vacuum front end channel

6. Apply HPRF cavity for 6D cooling channel 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 6 Simulation of muon emittance evolution in helical cooling channel CAD drawing of helical cooling channel HPRF cavity Helical solenoid coil Particle tracking in helical cooling channel Apply HPRF cavity (p = 200 atm) in helical 6D cooling channel 6D cooling factor > 10 5 in 300 m - Short & Compact - It reduces μ beam loss due to decay Transmission efficiency 60 % K. Yonehara et al., Proceedings of IPAC’10, MOPD076 Dense hydrogen also works an ideal muon ionization cooling absorber

7. Estimate beam-induced plasma loading in HPRF cavity 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 7 Conditions are in the plot Estimated RF power consumption by single electron: 7.8 10 -17 J/cm/RF cycle Required beam in IC channel: ~10 14 μs/s * - It corresponds to 4 MW proton beam Example beam repetition rate - NF: 50 Hz - MC: 10 Hz RF power consumption - NF: 0.37 J/cm/RF cycle with 2 10 12 μs - MC: 1.86 J/cm/RF cycle with 10 13 μs No recombination process included in this plot Gas filled RF cavity μ beam Beam-induced plasma Electric field = 16 MV/m RF frequency = 200 MHz Gas pressure = 200 atm @ Room Temp Average μ momentum = 200 MeV/c Estimated # of electrons per μ = 2.4 10 3 electrons/μ RF period * MAP proposal http://map.fnal.gov/proposals/pdfs/MAPproposal-R6d.pdf

8. Mucool Test Area (MTA) & work space NuFact'11 - K. Yonehara8 Multi task work space to study RF cavity under strong magnetic fields & by using intense H - beams from Linac Compressor + refrigerator room Entrance of MTA exp. hall MTA exp. hall SC magnet 200 MHz cavity Workstation 400 MeV H - beam transport line

9. Study interaction of intense beam with dense H2 in high gradient RF field Beam signal (7.5 μs) RF power is lost due to plasma loading RF power is recovered when beam is off RF pulse length (80 μs) p + H 2 → p + H 2 + + e - Ionization process 1,200 e - /cm are generated by incident p @ K = 400 MeV ν= 802 MHz Gas pressure = 950 psi Beam intensity = 2 10 8 /bunch 11/02/119 Joint MAP & High Gradient RF Workshop, K. Yonehara Plasma loading in pure H2 gas Equilibrium condition Electron production rate = Recombination rate Detail beam profile study will be presented by Mukti

10. Preliminary estimation of plasma loading effect in HPRF cavity for cooling channel ν= 802 MHz Pure H 2 gas Gas pressure = 950 psi Beam intensity = 2 10 8 /bunch 11/02/1110 Joint MAP & High Gradient RF Workshop, K. Yonehara From RF amplitude reduction rate, RF power consumption by plasma can be estimated Joules/RF cycle in Δt = 200 ns Hence, energy consumption by one electron is (including with initial beam intensity change) Joules/RF cycle/e/cm electrons in Δt = 200 ns Experimental result is a half of model estimation Joules/RF cycle/e/cm Model from electron plasma dynamics in hydrogen gas

11. Study electronegative gas effect 11/02/1111 Joint MAP & High Gradient RF Workshop, K. Yonehara SF6 removes residual electrons

12. Compare with muon beam structure 11/02/1112 Joint MAP & High Gradient RF Workshop, K. Yonehara E/p in helical 6D cooling channel is 1.6 V/cm/mm Hg Bunch gap is 5 ns Electron capture time looks to be fast enough for real application

13. Optical measurement 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 13 Time dependent hydrogen spectra in 540 psi t = -4 ns t = 0 ns t = 5 ns t = 10 ns t = 20 ns t = 50 ns λ = 503 (red) and 656 (blue) nm t = 0 ns at peak in 503 nm K. Yonehara, Fermilab Tech Note

14. Breakdown plasma dynamics in HPRF cavity 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 14 Plasma temperature Plasma density K. Yonehara, Fermilab Tech Note

15. Equivalent circuit analysis 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 15 Electron density from RF pickup signal analysis Current can be estimated from L, R and drift velocity of electrons in hydrogen plasma P = 1350 psi Equivalent resonance circuit Resonance circuit of normal RF cavity consists of L and C Breakdown makes streamer It produces additional L and R Resonance frequency is shifted by them A. Tollestrup et al., Fermilab Tech Note, FERMILAB-TM-2430-APC

16. Comparison HPRF breakdown with vacuum one Typical parameter of breakdown current in vacuum cavity – A few kA – Size of plasma sheath a few hundreds of μm HPRF cavity breakdown – Plasma density 10 20 cm -3 – Total plasma 10 15 – Plasma sheath ~100 μm – Current ~ 1.6 10 -19 10 15 /10 -8 ~ 16 kA Both breakdown parameters looks similar (or probably both are the same) 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 16

17. Dielectric loaded HPRF cavity 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 17 Empty cavity: -f = 1003.87 MHz -Q = 22772.3 Experimental results: SMALL CYLINDER BIG CYLINDER f = 897.96 (±0.97) MHz Q = 11823.1 (±1037.1) f = 814.30 (±0.81) MHz Q = 9415.89 (±828.6) ε = 8.925 (±0.125) tgδ = 7.28E-5 (±4.92E-5) ε = 9.595 (±0.134) tgδ = 8.17E-5 (±5.52E-5) Al 2 O 3 ring E field direction Goal: Measure dielectric constant and loss tangent in room & cryogenic temperature Study breakdown with dielectric material in HPRF cavity Make a compact HPRF cavity to incorporate into a compact magnetic structure Al 2 O 3 99.5% D IELECTRIC RINGS J. Cenni, MAP Friday meeting, https://indico.fnal.gov/conferenceDisplay.py?confId=4661 SF simulation

18. Conclusion Great progress of HPRF cavity R&D – Study magnetic field dependence – Study plasma effect Beam induced plasma loading effect can be an issue – Electronegative gas works well as one possible solution Present goal – More R&D to make a practical HPRF cavity & 6D cooling demo channel with HPRF cavity 11/02/11 Joint MAP & High Gradient RF Workshop, K. Yonehara 18

19. Collaboration C. Ankenbrandt, A. Bross, J. Cenni, M. Chung, G. Collura, G. Flanagan, B. Freemire, P. Hanlet, R. P. Johnson, D. Kaplan, G. Kazakevitch, A. Kurup, A. Moretti, J. Mukti, M. Neubauer, D. Neuffer, M. Notani, G. Pauletta, M. Popovic, G. Romanov, A. Tollestrup, Y. Torun, R. Sah, T. Schwarz, + AD external beam division + AD mechanical design + Machine shop + Rad/Hydrogen safety committees + Director/Division Heads + Operators & Technicians Supported for many years by the DOE HEP SBIR-STTR program 11/02/1119 Joint MAP & High Gradient RF Workshop, K. Yonehara Muons, Inc.