(HP)RF Instrumentation

1. (HP)RF Instrumentation 2009. 7. 8. Moses Chung APC, Fermilab MuCool RF Workshop III

2. Break-down of “Breakdown” • Vacuum breakdown: • Field emission (protrusion)  electrons+RF+B • Metal vapor  Metal plasma  Arc • Secondary emissions (by electrons, ions, photons) • Low pressure breakdown (~ glow): • Seed electrons (UV, cosmic ray, artificial source) • Electron multiplication by gas ionization • Secondary emission (by ion bombardment on cold cathode) • High pressure breakdown (~ spark): • Electron multiplication by gas ionization • Photo-ionization • Streamer propagation (faster, independent of cathode) • Beam-induced electron loading • Beam-impact and fast-electron-impact ionization of gas • Ohmic dissipation by electron-gas collisions • Significant reduction in quality factor, Q

3. Why High Pressure RF ? Muon Beam Muon Beam ? • High-pressure hydrogen gas (H2) inside the cavity: • To provide an energy absorber (dE/dx) • To enable higher accelerating RF field gradient in the presence of the B fields (Paschen’s law, and electron’s nm >> W, w) • To achieve ionization energy loss and RF energy regain simultaneously (Key element for HCC) • Effects of beam-induced electrons are of great concern • [A. Tollestrup].

4. HPRF Cavity Gas inlet (H2, N2, He, SF6) Metal sealing (use Aluminum gasket) Power coupler (Fwd, Ref) Optical port Semispherical electrode is replaceable (Cu, Al, Sn) Copper plated stainless steel

5. Highlights of Previous Experiments 2004 Run 2008 Run Conditioning H2 (I = 15.5 eV) (I = 15.4 eV) HCC: ~ 3000 psia ~ 20 MV/m Cu We identify gas and electrode breakdown regions. Weconfirm RF cavity works in the magnetic field. We demonstrate SF6 can impede electron accumulation. + Many mysteries

6. Behind Physics is Complicated 1/2 There are no circulator and matched load in our RF system. Pattern of the reflected power appears in the forward signal after ~ 1 ms ~ 2 x 150 m / c Fwd signal even after RF is off bc < 1 (undercoupled) bc = RL/Z0~ 1 tfill = 2QL/w Breakdown at lower PU voltage and RF power Voltage recovery & Electron removal Small RF power is steadily absorbed by the plasma PMT signal decays when RF is off Phase 1 (Before Breakdown) Phase 2 (Spark) Phase 3 (~stable discharge) Phase 4 (RF Off)

7. Behind Physics is Complicated 2/2 There is uncertain time delay between PMT and Pickup signals. No big Change in Fwd signal ~ 8 cycle ~ 8 cycle ~ 8 cycle ~ 8 cycle Phase shift reflection G(t) = V- / V+ ~ 8 cycle ~ 8 cycle Modulation (AM) ~ 8 cycle ~ 10 % Increase in frequency tdecay ~ 10 ns Q~ 25 ~ Undriven damped oscillation ~ 8 cycle ~ 8 cycle ~ 9 cycle PMT saturation PMT rise time (~ 3 ns) What’s the color ? (Ha or Cu) Adjust according to PMT time delay calibration

8. What Happens with Beam ? Beam-impact ionization + Ionization by secondary electrons: p+ H2 p + H2+ + e- e- + H2 H2+ + 2e- H3+,H5+,H7+,… Fast electrons (< 40 keV, ~ 0.5 MeV  d rays) Most electrons (>90%) are quickly thermalized inside the cavity by elastic and inelastic collisions, and drift with RF until annihilated by recombination, attachment, or diffusion.

9. Effects of Electrons Response of plasma electrons to the RF field is described by complex (Lorentz) conductivity: Equivalent circuit model: Additional driving term by beam itself (LLRF) Additional damping term by beam-induced electrons

10. Effects of SF6 p ~1000 psi br ~ 10-8 cm3/s 32 mA H- ~ 2.5 x109 MIP Without SF6 With SF6 Effects of recomb. We assume Te = const. in this example. However, Te = Te (Vc) in general. Effects of recomb. = saturation + linear recovery (>> RC) Too much of SF6 (Z = 70, A = 146) will change electron dynamics. e- + SF6 SF6- e- + SF6 SF5- + F

11. Simple Test of Theory Thermal energy gain from RF = Elastic & inelastic energy loss to gas Criteria for breakdown: E/p~23 V/cm/torr E/p~15 V/cm/torr E/p~12 V/cm/torr Al electrode run (10% error) E/p~22 V/cm/torr

12. Actual Beam Test 1. Beam commissioning [C. Johnstone et. al.]: Long C-magnet MTA hall 2. Beam test [MCTF]: Linac HPRF 67.5 mm MW4 MW5 MW6 305 mm 400 MeV, 5 ns H- e95 ~ 10p mm-mrad, Ib ~ 32 mA, rb ~ 1 cm • - New LabVIEW-based DAQ [A. Kurup] • - New coupling loop for magnetic field measurement • New optical (650 nm) diagnostics

13. Emittance Measurement 1. Three grid method [C. Johnstone et. al.]  Gaussian beam Multiwire (MW4) BPM8 Phase advance of the particle Tilt angle of the ellipse Beam stop MW4 MW6 MW5 2. Slit-grid method [Mehran Mohebbi (WVU) et. al.]: Probe 4 (750 keV) Vertical • Long scanning time • SEM or Capture ? Slit

14. Optical Diagnostic [with Martin Hu] bending radius > 0.3 m ~ 1 m - 30% - 20% ~ 3 m Teflon sealant 15’ = 180 inch = 4.572 m ~ 1 m NA = 0.22 Acceptance angle = 24.8o Cover range ~ 5 cm 1mm diameter Focus on time-resolved Ha line detection Red-sensitive Rise time = 0.78 ns Transit time = 5.4 ns

15. Spectroscopy e- + H2 H2* + e- (Excitation, Fulcher band) Hb Hg Hd Ha e- + H2 H* + H + e- (Dissociative Excitation) H2+ + e-  H* + H (Dissociative Recombination, H3+ ?) (c) Copper [CLIC] • - Can we have enough light ? (gas breakdown VS beam test) • What would be the required time scale ? (~ns VS ~ms) • What would be the reasonable resolution ? (filters VS grating)

16. Summary and Discussion Beam test of the high pressure RF cavity is a high-priority R&D program in MTA. Wehope SF6 can remove electrons with minimal side effects. What is the criteria to evaluate the feasibility of HPRF ? What are the necessary equipments ? Any synergy between vacuum RF and HPRF ?