(HP)RF Instrumentation
2009. 7. 8.
Moses Chung
APC, Fermilab
MuCool RF Workshop III
1
Break-down of “Breakdown”
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
2
Why High Pressure RF ?
B field  RF
Muon
Beam
dE
dx
H2  H2  e 
Muon Beam
?
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 nm >> W, w)
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].
3
HPRF Cavity
Gas inlet (H2, N2, He, SF6)
Metal sealing (use Aluminum gasket)
Power coupler
(Fwd, Ref)
Resonance frequency (MHz)
814
Experiments with H2
812
Cu electrode
Al electrode
Sn electrode
810
808
806
804
802
Superfish Calculation
800
798
0
200
400
600
800
1000 1200 1400 1600
Pressure (PSI)
~  1 .6
Optical
port
Semispherical electrode is
replaceable (Cu, Al, Sn)
Copper plated stainless steel
4
Highlights of Previous Experiments
2004 Run
2008 Run
Conditioning
H2
HCC:
~ 3000 psia
~ 20 MV/m
(I = 15.5 eV)
(I = 15.4 eV)
Cu
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.
+
Many mysteries
5
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
Phase 1
(Before Breakdown)
Phase 2
(Spark)
Small RF power is steadily
absorbed by the plasma
Phase 3
(~stable discharge)
PMT signal
decays when
RF is off
Phase 4
(RF Off)
6
Behind Physics is Complicated 2/2
There is uncertain time delay between PMT and Pickup signals.
~ 8 cycle
~ 8 cycle
~ 8 cycle
~ 8 cycle
Phase shift
~ 8 cycle
~ 8 cycle
No big
Change in
Fwd signal
reflection
G(t) = V- / V+
Modulation (AM)
~ 8 cycle
~ 10 % Increase in frequency
~ 8 cycle
~ 8 cycle
~ 9 cycle
tdecay ~ 10 ns
Q~ 25
~ Undriven damped oscillation
PMT saturation
PMT rise time (~ 3 ns)
What’s the color ? (Ha or Cu)
Adjust according to
PMT time delay calibration  10 ns
7
What Happens with Beam ?
Beam-impact ionization + Ionization by secondary electrons:
p + H2  p + H2+ + eH3+,H5+,H7+,…
e- + H2  H2+ + 2e-
Fast electrons (< 40 keV, ~ 0.5 MeV  d rays)
ne
 (dE / dx)s
1
3


~
1000/cm
1 proton Wi ( 35 eV) (rb2 s)
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.
dne
D
= S  ki ng ne  b r ne2  k DA ng ne  k aang ne  2 ne
dt

e  H2 (v)  H  H-
8
Effects of Electrons
Response of plasma electrons to the RF field is described
collsionfrequency
by complex (Lorentz) conductivity:
 DC
ne e 2
=
men m

 =  DC 
n
2
m
n m  w
2
2
j

w n

n m n m2  w 2 
2
m
n m  2 1011 p[psia]
1
2

(
r
)
E
(r )dV
DC
0

1 V2
f w   1 
  
, f = f  f 0  0  0      0
2 n m   Q 
 Q  w0 1  0 E02 (r )dV
V 2
Equivalent circuit model:
2


 1
 1  d
w0 dVF w0  R  dIb
d
2





w



w
V
=
2

 2
0
0 c
 Q  dt
 Q   dt
dt
Q
Q
dt
2




 
e
 L



Additional damping term by
beam-induced electrons
Additional driving term
by beam itself (LLRF)
9
Effects of SF6
p ~1000 psi
br ~ 10-8 cm3/s
32 mA H- ~ 2.5 x109 MIP
Without SF6
With SF6
1.2
1.2
Beam on
Beam on
Different beam intensity
10
10 protons/bunch
9
10 protons/bunch
8
10 protons/bunch
7
10 protons/bunch
0.8
0.6
0.4
0.2
0.0
-10
RF on
Effects of
recomb.
Beam off
-5
0
5
10
15
20
25
30
35
Beam off
1.0
Pickup signal (Arb.)
Pickup signal (Arb.)
1.0
0.8
0.6
Different SF6 dopant fraction
0.4
0%
0.001%
0.01%
0.1%
0.2
0.0
-10
40
Time (ms)
RF off
RF on
-5
0
5
10
15
20
Time (ms)
25
30
35
40
RF off
We assume Te = const. in this example.
However, Te = Te (Vc) in general.
Too much of SF6 (Z = 70, A = 146) will
change electron dynamics.
Effects of recomb.
= saturation + linear recovery (>> RC)
e- + SF6  SF6e- + SF6  SF5- + F
10
Simple Test of Theory
Thermal energy gain from RF =
Elastic & inelastic energy loss to gas
 E (t ) 

0.357 
 p 
0.71
Criteria for breakdown:
3
 Te (t )
2
ki (Te )  kDA (Te )  ka (Te )a ,
E/p~23 V/cm/torr E/p~15 V/cm/torr
3
-1
Attachment rate coefficient (cm s )
-6
10
Al electrode run (10% error)
ki
-8
10
kDA (v=0)
kDA (v=0) + 0.01% ka
-10
10
-12
10
-14
10
-16
10
-18
10
0
5
10
E/p~12 V/cm/torr
15
20
25
30
35
40
E/p (V/cm/Torr)
E/p~22 V/cm/torr
11
Actual Beam Test
1. Beam commissioning [C. Johnstone et. al.]:
Long C-magnet
2. Beam test [MCTF]:
MTA hall
Linac
HPRF
MW4 MW5 MW6
400 MeV, 5 ns
67.5 mm
H-
95 ~ 10 mm-mrad, Ib ~ 32 mA, rb ~ 1 cm
305 mm
- New LabVIEW-based DAQ [A. Kurup]
- New coupling loop for magnetic field measurement
- New optical (650 nm) diagnostics
12
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
MW5
MW6
2. Slit-grid method [Mehran Mohebbi (WVU) et. al.]:
Probe 4 (750 keV) Vertical
Long scanning time
SEM or Capture ?
Slit
13
Optical Diagnostic
[with Martin Hu]
- 30%
~ 1m
- 20%
bending
radius
> 0.3 m
~ 3m
15’ = 180 inch = 4.572 m
Teflon sealant
HPRF TM010 805 MHz Pillbox Cavity
F = 804.93825 MHz
11
11
10
10
NA = 0.22
Acceptance angle
= 24.8o
Cover range ~ 5 cm
1mm diameter
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7
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5
5
4
4
3
3
2
2
1
1
0
0
0
2
4
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10
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C:\LANL\EXAMPLES\RADIOFREQUENCY\CFISH\HPRF.AF
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2-20-2009
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16:38:12
Focus on time-resolved
Ha line detection
Red-sensitive
Rise time = 0.78 ns
Transit time = 5.4 ns
14
Spectroscopy
e- + H2  H2* + e- (Excitation, Fulcher band)
Ha
e- + H2  H* + H + e(Dissociative Excitation)
Hb
Hg
Hd
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)
15
Summary and Discussion
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 ?
16