7.8GHz Dielectric Loaded High Power Generation And Extraction F. Gao, M. E. Conde, W. Gai, C. Jing,...
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Transcript of 7.8GHz Dielectric Loaded High Power Generation And Extraction F. Gao, M. E. Conde, W. Gai, C. Jing,...
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7.8GHz Dielectric Loaded High Power Generation And Extraction
F. Gao, M. E. Conde, W. Gai, C. Jing, R. Konecny, W. Liu, J. G. Power, T. Wong and Z. Yusof
Advanced Acceleration Concept Workshop, Santa Cruz, 2008
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Outline
Introduction Design of the 7.8GHz power extractor Cold test Beam tests:
i. Single bunch tests
ii. Bunch train tests Summary
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Dielectric loaded two beam acceleration
Power: >100MW Frequency: up to 30GHz RF pulse length: a few nanoseconds to a few tens of nanoseconds
lower group velocity for RF pulse compression
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The power extractor using a circular DL waveguide
dielectric
dielectric
vacuum
Dielectric loaded waveguide and the power extractor
transverse view e-field of the TM01 mode (vp = c)
Ez of the TM01 mode (vp = c)
wakefield
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Design of the power extractor: the deceleration waveguideDL deceleration waveguide
TM01-TE10 coupler
rf output port
2
2
0
201
4
b
L
g
zt T
e
Q
rkqP
Frequency: 7.8GHzInner diameter: 12.04mmOuter diameter: 22.34mmDeceleration section length: 266mmGroup velocity: 0.23cGenerated power (Gaussian bunch length = 2mm): Single bunch: 79MW @100nC per bunchBunch train (Tb = 769ps): 100MW @30nC per bunch 280MW @ 50nC per bunch 1.1GW @ 100nC per bunch
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Bunch Length MeasurementsExperimental Results*
*Preliminary Results0
1
2
3
4
5
6
7
8
0 20 40 60 80
charge (nC)
rms
bu
nc
h l
en
gth
(m
m) 1.5 Cell PC Gun
original 1/2 cell gun
PARMELA old AWA gun
new AWA gun
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mode launcherinside
-20
-15
-10
-5
0
7.6 7.7 7.8 7.9 8
frequency (GHz)
inse
rlotio
n lo
ss S
21 (d
B)
simulated
measured
-30
-25
-20
-15
-10
-5
0
7.6 7.7 7.8 7.9 8
frequency (GHz)
refle
ctio
n S
11 (d
B)
simulated
measured
Design of the power extractor: the RF output coupler
S21 = [email protected]
Power coupling efficiency: 91%.
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Experimental setup
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Single bunch test
0 10 20 30 40 50 60 700
10
20
30
40
simulated
measured
Pow
er P
s -
MW
q - nC
0 2 4 6 8 10-2
-1
0
1
2
t - ns
volta
ge -
Vol
t measured
0 2 4 6 8 10-2
-1
0
1
2
t - ns
volta
gear
bitr
ary
unit simulated
6 7 8 9 100
50
100
150
200
f - GHz
volta
ge s
pect
rum
measured
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Bunch train test – UV laser micropulse train generation
A
B
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Bunch train test – electron bunch train generation
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Charge Phase Scan
50 degree
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Check UV laser bunch train with photodiode & energy meter
0
0. 005
0. 01
0. 015
0. 02
0. 025
0. 03
0 5 10 15 200
0. 005
0. 01
0. 015
0. 02
0. 025
0. 03
0 5 10 15 20t - ns t - ns
v -
Vo
lt
v -
Vo
lt
train A train B
voltage signals from photodiode
bunch charge comparison with energy meter
0 1 2 3 4 5 6 7 8 90
0.5
1
1.5
2
train A
train B
index
pea
k vo
ltag
e -
mV
0
0. 5
1
1. 5
2
2. 5
3
0 1 2 3 4
volta
ge
- m
V
t - ms
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10ns long RF pulse generation
Beam for ~10ns RF pulse generation769ps
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2.4MW, 10ns long RF pulse generation
0 5 10 15 20 25-1.5
-1
-0.5
0
0.5
1
1.5
t - ns
volta
ge -
Vol
t
2.4MW generated,2.2MW extracted.
t - ns
v -
Vo
lt
7 7.5 8 8.5 90
50
100
150
200
250
300
f - GHz
voltage spectrum
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22ns long RF pulse generation
Beam for 22ns RF pulse generation1.538ns
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4-bunch test for high power generation
Simulation shows the power reach “flat-top” saturation level when the drive bunch contains 4 or more consecutive bunches spaced by 769ps.
To maximize this power level the UV laser bunch was only split into 4 bunches.
44MW generated40MW extracted26.5nC per bunch
f - GHz t - ns
769ps
the 4th bunch
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Summary
Dielectric loaded power extraction has been demonstrated. 30MW of power has been generated in single bunch tests and 44MW in bunch train tests. 10ns and 22ns RF pulses have been observed.
Currently the limitation for higher power generation is the beam current, which is further limited by the QE of the magnesium photocathode (~10-4).
A new cesium telluride photocathode with much higher QE (~10-2) has been developed, yet to be installed and tested in a new gun (AWA G3). 280MW of output power are expected to be generated by electron charge of 50nC per bunch.
For much higher charge with the new photocathode, space charge effect will be much stronger. Thus beam confinement with quadruple magnets may be needed.