(HP)RF Instrumentation

Moses Chung

APC, Fermilab

MuCool RF Workshop III

2009. 7. 8.

1

Break-down of “Breakdown”

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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 ?

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Muon Beam

Muon Beam Muon BeamMuon Beam

eHH 22

dx

dE

RFfield B

?

High-pressure hydrogen gas (H2) 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

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Metal sealing (use Aluminum gasket)Gas inlet (H2, N2, He, SF6)

Copper plated stainless steel

Semispherical electrode is replaceable (Cu, Al, Sn)

Power coupler(Fwd, Ref)

Optical port

0 200 400 600 800 1000 1200 1400 1600798

800

802

804

806

808

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Superfish CalculationRe

son

an

ce f

req

ue

ncy

(M

Hz)

Pressure (PSI)

Experiments with H2

Cu electrode Al electrode Sn electrode

6.1~

Highlights of Previous Experiments

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2004 Run 2008 Run

1. We identify gas and electrode breakdown regions.2. We confirm RF cavity works in the magnetic field.3. We demonstrate SF6 can impede electron accumulation.

CuHCC:~ 3000 psia ~ 20 MV/m

H2 (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 = RL/Z0~ 1 c < 1 (undercoupled)

Behind Physics is Complicated 1/2

Fwd signaleven after RF is off

Small RF power is steadilyabsorbed by the plasma

PMT signaldecays when RF is off

Breakdown at lower PU voltage and RF power

Voltage recovery &Electron removal

fill = 2QL/

6

Behind Physics is Complicated 2/2

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~ 8 cycle~ 8 cycle ~ 9 cycle

~ 8 cycle ~ 8 cycle ~ 8 cycle ~ 8 cycle

~ 8 cycle ~ 8 cycle ~ 8 cycleModulation (AM)

~ 10 % Increase in frequency

~ Undriven damped oscillation

Phase shift

ns 10

decay ~ 10 nsQ~ 25

No bigChange inFwd signal

reflectiontV- / 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 ?

31000/cmeV 35proton 1

~)(

1

)(

)/(2 srW

sdxdEn

bi

e

Beam-impact ionization + Ionization by secondary electrons:p + H2 p + H2

+ + e- e- + H2 H2+ + 2e-

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.

H3+,H5

+,H7+,…

eegaegDAeregie n

DnnknnknnnkS

dt

dn2

2

-2 HHHe )(v

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Effects of Electrons

Response of plasma electrons to the RF field is described by complex (Lorentz) conductivity:

Equivalent circuit model:

9

01

2,

)(21

)()(21

1 000

2000

20

Q

ffff

dVrE

dVrEr

Q mV

V DC

22

2

22

2

m

m

mm

mDC j

][102 11 psiafrequency collsionpm

me

eDC m

en

2

dt

dI

Q

R

dt

dV

QV

dt

d

QQdt

d bF

ec

L

22

11 002002

2

Additional damping term bybeam-induced electrons

Additional driving term by beam itself (LLRF)

Effects of SF6

Without SF6

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-10 -5 0 5 10 15 20 25 30 35 400.0

0.2

0.4

0.6

0.8

1.0

1.2

RF offRF on

Different beam intensity

1010 protons/bunch

109 protons/bunch

108 protons/bunch

107 protons/bunch

Pic

kup

sig

na

l (A

rb.)

Time (s)

Beam on

Beam off

With SF6

-10 -5 0 5 10 15 20 25 30 35 400.0

0.2

0.4

0.6

0.8

1.0

1.2

RF offRF on

Different SF6 dopant fraction

0% 0.001% 0.01% 0.1%

Pic

kup

sig

na

l (A

rb.)

Time (s)

Beam on Beam off

p ~1000 psir ~ 10-8 cm3/s

32 mA H- ~ 2.5 x109 MIP

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

Effects of recomb.

E/p~23 V/cm/torr

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Criteria for breakdown:

E/p~15 V/cm/torr

,)()()( eaeDAei TkTkTk )(2

3)(357.0

71.0

tTp

tEe

Thermal energy gain from RF = Elastic & inelastic energy loss to gas

0 5 10 15 20 25 30 35 4010-18

10-16

10-14

10-12

10-10

10-8

10-6

ki

kDA

(v=0)

kDA

(v=0) + 0.01% ka

Att

ach

me

nt

rate

co

eff

icie

nt

(cm

3 s-1)

E/p (V/cm/Torr)E/p~12 V/cm/torr

E/p~22 V/cm/torrAl electrode run (10% error)

Simple Test of Theory

Actual Beam Test

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305 mm

67.5 mmH- 95 ~ 10 mm-mrad, Ib ~ 32 mA, rb ~ 1 cm

1. Beam commissioning [C. Johnstone et. al.]:

MW4 MW5 MW6

MTA hallLinacHPRF2. 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

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Tilt angle of the ellipse

Phase advanceof 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 timeSEM or Capture ?

Optical Diagnostic

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15’ = 180 inch = 4.572 m

~ 1 m

~ 3 m

~ 1 m

bending radius> 0.3 m

Red-sensitiveRise time = 0.78 nsTransit time = 5.4 ns

HPRF TM010 805 MHz Pillbox Cavity F = 804.93825 MHz

C:\LANL\EXAMPLES\RADIOFREQUENCY\CFISH\HPRF.AF 2-20-2009 16:38:12

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NA = 0.22Acceptance angle = 24.8o Cover range ~ 5 cm1mm diameter

Teflon sealant

- 30%

- 20%

Focus on time-resolvedH line detection

[with Martin Hu]

Spectroscopy

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H H H H

(c) Copper [CLIC]

e- + H2 H* + H + e- (Dissociative Excitation)

H2+ + e- H* + H

(Dissociative Recombination, H3+ ?)

e- + H2 H2* + 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

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1. Beam test of the high pressure RF cavity is a high-priority R&D program in MTA.

2. We hope SF6 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 ?