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

2

Outline

Introduction Design of the 7.8GHz power extractor Cold test Beam tests:

i. Single bunch tests

ii. Bunch train tests Summary

3

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

4

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

5

6

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

7

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

8

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 = -0.41dB@7.8GHz

Power coupling efficiency: 91%.

9

Experimental setup

10

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

11

12

Bunch train test – UV laser micropulse train generation

A

B

13

14

Bunch train test – electron bunch train generation

15

Charge Phase Scan

50 degree

16

17

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

18

10ns long RF pulse generation

Beam for ~10ns RF pulse generation769ps

19

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

20

22ns long RF pulse generation

Beam for 22ns RF pulse generation1.538ns

21

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

22

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.