(HP)RF Instrumentation Moses Chung APC, Fermilab MuCool RF Workshop III

Break-down of “Breakdown” 2 1.Vacuum breakdown: Field emission (protrusion)  electrons+RF+B Metal vapor  Metal plasma  Arc Secondary emissions (by electrons, ions, photons) 2.Low pressure breakdown (~ glow): Seed electrons (UV, cosmic ray, artificial source) Electron multiplication by gas ionization Secondary emission (by ion bombardment on cold cathode) 3.High pressure breakdown (~ spark): Electron multiplication by gas ionization Photo-ionization Streamer propagation (faster, independent of cathode) 4.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

Why High Pressure RF ? 3 Muon Beam ? High-pressure hydrogen gas (H 2 ) inside the cavity: 1.To provide an energy absorber (dE/dx) 2.To enable higher accelerating RF field gradient in the presence of the B fields (Paschen’s law, and electron’s m >>  ) 3.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].

HPRF Cavity 4 Metal sealing (use Aluminum gasket) Gas inlet (H 2, N 2, He, SF 6 ) Copper plated stainless steel Semispherical electrode is replaceable (Cu, Al, Sn) Power coupler (Fwd, Ref) Optical port

Highlights of Previous Experiments Run2008 Run 1.We identify gas and electrode breakdown regions. 2.We confirm RF cavity works in the magnetic field. 3.We demonstrate SF 6 can impede electron accumulation. Cu HCC: ~ 3000 psia ~ 20 MV/m H2H2 (I = 15.5 eV) (I = 15.4 eV) + Many mysteries Conditioning

There are no circulator and matched load in our RF system. Pattern of the reflected power appears in the forward signal after ~ 1  s ~ 2 x 150 m / c Phase 1 (Before Breakdown) Phase 2 (Spark) Phase 3 (~stable discharge) Phase 4 (RF Off)  c = R L /Z 0 ~ 1  c < 1 (undercoupled) Behind Physics is Complicated 1/2 Fwd signal even after RF is off Small RF power is steadily absorbed by the plasma PMT signal decays when RF is off Breakdown at lower PU voltage and RF power Voltage recovery & Electron removal  fill = 2Q L /  6

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

What Happens with Beam ? Beam-impact ionization + Ionization by secondary electrons: p + H 2  p + H e - e - + H 2  H e - Fast electrons (< 40 keV, ~ 0.5 MeV   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. H 3 +,H 5 +,H 7 +,… 8

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

Effects of SF 6 Without SF 6 10 With SF 6 p ~1000 psi  r ~ cm 3 /s 32 mA H - ~ 2.5 x10 9 MIP We assume T e = const. in this example. However, T e = T e (V c ) in general. Effects of recomb. = saturation + linear recovery (>> RC) Too much of SF 6 (Z = 70, A = 146) will change electron dynamics. e - + SF 6  SF 6 - e - + SF 6  SF F Effects of recomb.

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

Actual Beam Test mm 67.5 mm H-  95 ~ 10  mm-mrad, I b ~ 32 mA, r b ~ 1 cm 1. Beam commissioning [C. Johnstone et. al.]: MW4MW5MW6 MTA hall Linac HPRF 2. Beam test [MCTF]: - New LabVIEW-based DAQ [A. Kurup] - New coupling loop for magnetic field measurement - New optical (650 nm) diagnostics Long C-magnet 400 MeV, 5 ns

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

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

Spectroscopy 15 HH HH HH HH (c) Copper [CLIC] e - + H 2  H * + H + e - (Dissociative Excitation) H e -  H * + H (Dissociative Recombination, H 3 + ?) e - + H 2  H 2 * + e - (Excitation, Fulcher band) - 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)

Summary and Discussion 16 1.Beam test of the high pressure RF cavity is a high-priority R&D program in MTA. 2.We hope SF 6 can remove electrons with minimal side effects. 3.What is the criteria to evaluate the feasibility of HPRF ? 4.What are the necessary equipments ? 5.Any synergy between vacuum RF and HPRF ?