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High Gradients in Dielectric Loaded Wakefield Structures
Manoel Conde
High Energy Physics Division
Argonne National Laboratory
AAC 08 – Santa Cruz, CA
AWA & wakefield generation High gradient excitation Upgrades and future goals Wakefield excitation in SLAC / KEK X-band structure
Outline
Accelerator R&D at Argonne National LaboratoryHigh Energy Physics DivisionAWA Group
Wei Gai Sergey Antipov Manoel Conde Felipe Franchini Feng Gao
Chunguang Jing Richard Konecny Wanming Liu Marwan Rihaoui John Power Zikri Yusof
External Users / Collaborators:
Euclid TechLabs U.Chicago NIU Fermilab
U.Md. IIT APS Yale / Omega-P
Research at the AWA Facility
:
Advanced Accelerating Structures
Current effort has lead to comprehensive knowledge on construction and testing of dielectric based accelerating structures.
High Power Electron Beam (~ GW) and RF Power Generation
Operating a unique facility to study high current electron beam generation and propagation for efficient beam driven schemes, and high power RF generation.
Fundamental Beam Physics and Advanced Diagnostics
High brightness beam generation and propagation, phase space measurements, emittance exchange schemes.
Electron Beam Driven Dielectric Wakefield Accelerator
A high current relativistic electron beam passing through a dielectric structure can generate high gradient field, high power microwaves instantaneously. A new way to power accelerating structures by transporting the power in the electron beam.
Justifications for looking at dielectric structures:
Comparable accelerating properties as metal structures.
More material options; possibly higher gradients.
Simpler geometry, simpler construction and HOM damping.
Applications: Collinear wakefield acceleration Two-beam acceleration
Drive Beam
Dielectrics
Deceleration Acceleration
Accelerated Beam
Wakefields in Dielectric Structures (a short Gaussian beam)
Drive beam is king!
Energy Charge Bunch length Emittance
b a
Q
Cu 21
N
r
)cos(2exp)(2
2kz
a
QzW
n
zZ
Wakefield Amplitude Dependent on a
1
10
100
1000
10000
100000
0.01 0.1 1 10
Inner Radius a (mm)E
z(M
V/m
/10n
C)
AWA Drive Beamline
Drive Gun
Linac & Beam Optics Quads
Wakefield Structure
ExperimentalChambers 4.5 m
GVGV
YAG1 YAG2
Spectrometer
YAG5Dump/Faraday Cup
Slits
YAG4YAG3ICT1
ICT2 BPM
Single bunch operation– Q = 1-100 nC (reached 150 nC)– 15 MeV, 2 mm bunch length (rms), emittance < 200 mm mrad (at 100 nC)– High Current: ~10 kA
Bunch train operation– 4 bunches x 25 nC or 16 bunches x 5 nC (present) – 16 - 64 bunches x 50 - 100 nC 10 - 50 ns long (future)
The Argonne Wakefield Accelerator (AWA)
~ 1 meter
rf-gun
magnetic lenses
8 MeV 15 MeV
Laser Inλ =248nm
Linac
Quads Spectrometer
Faradaycup
YAG2
YAG1
Direction of beam propagation
Experimental Area
Experimental Setup for High Gradient Tests
WF signal
RF field probe (- 60 dB)
43 nC
time (ns)
0 2 4 6
-100
0
100
Monitor for breakdown
Infer Gradients from MAFIA
Q
Cu
Goal: Test breakdown thresholds of dielectric structures under short RF pulses.
Dielectric Loaded Structures Tested
SW Structure #1 C10-102 #2 C10-23 #3 C5.5-28 #4 Q3.8-25.4
Material Cordierite Cordierite Cordierite QuartzDielectric constant 4.76 4.76 4.76 3.75Freq. of TM01n 14.1 GHz 14.1 GHz 9.4 GHz 8.6 GHzInner radius 5 mm 5 mm 2.75 mm 1.9 mmOuter radius 7.49 mm 7.49 mm 7.49 mm 7.49 mmLength 102 mm 23 mm 28 mm 25.4 mmWakefield Gradient 0.45 MV/m/nC 0.5 MV/m/nC 0.91 MV/m/nC 1.33 MV/m/nC
Wakefield Measurements: Structure #1 (C10-102)
10 12 14 16 18
-200
0
200
4 6 8 10 12 14
-200
0
200
12 14 16 18 20
-200
0
200
4 6 8 10 12
-200
0
200
6 8 10 12 14
-200
0
200
time (ns)
0 2 4 6 8 10
-200
0
200
4.0 nC
9.2 nC
11.8 nC
16.8 nC
20.5 nC
25.6 nC
mix
er
ou
tpu
t (m
V)
bunch charge (nC)
0 10 20 30 40 50
pe
ak
mix
er
ou
tpu
t (m
V)
-600
-400
-200
0
200
400
600
46 nC → 21 MV/m
Wakefield Measurements: Structure #2 (C10-23)
Measurement
Simulation
Measurement
Simulation
TM013 (14.1GHz)
TM014
(16.2GHz)TM012 (13GHz)
HEM111 (12.4GHz)
HEM111 (12.2GHz) TM012
(13GHz)
TM013 (14.3GHz) TM014
(16GHz)
HEM112 (14.7GHz)
HEM112 (14.7GHz)
Freq (GHz)
Freq (GHz)
86 nC → 43 MV/m
Measured and simulated Er probe signals
E-field pattern
Wz (V/m)Wz > 1MV/m @ 1nC for 10GHz Structure
28mm
2.5mm
7.5mm
MAFIA Simulation of Structure #3 (C5.5-28)
Structure #1 21 MV/m Structure #2 43 MV/m Structure #3 78 MV/m Structure #4 100 MV/m
Gradients Reached:
Where we go from here:
Once the upgrade is complete, the goal is to achieve:Higher gradient excitation: ~ 0.5 GV/m in structures ( 3 mm apertures)Higher RF power extraction: ~ 1 GW (10 ns)
To improve the drive beam, we are currently upgrading the AWA facility:A new L-band 1.3 GHz RF station to increase the drive beam energy from 15 to 25 MeV.Fabrication of Cesium Telluride high quantum efficiency photocathodes to produce long, high charge bunch trains.A second RF gun to restore two-beam-accelerator capability.
Future AWA Facility (25 MW + 25 MW = 50 MW)
*all distances in cm
D.U.T.
Drive Gun(12 MW)
Linac 1(10.5 MW)
0 225 cm 351.6 cm 581.6 cm29.1 cm 455 cm 650 cm
Linac 2(10.5 MW) D.U.T.7.5 MeV 15.75 MeV 25 MeV
Witness Gun(12 MW)
0 29.1
8 MeV
Single Bunch: 50 - 100 nCBunch Train: 16 – 64, total charge 1 – 2.5 µC
Some goals of the AWA program: Verify that structures can withstand gradients higher than 100
MV/m. Generation of ~ GW level RF power (25 MeV, 130 Amps, 10
ns, 3 GW beam power). Demonstrate high gradient, broad bandwidth, low cost
structures (power extractors and accelerators). Typical parameters:
– f = 10 – 30 GHz, Vg = 5 – 20%– Required power: 0.5 – 5 GW peak– Example (for dielectric based structure):
• 21 GHz, a = 3 mm, b = 4 mm, ε=12, Vg=0.112• For Ez=200 MV/m, it requires P= 1 GW• 16 ns RF pulse, structure length : 30 cm, fill time = 8.4 ns• Beam loading (fundamental mode):0.75 MV/m/nC
Wakefield Observation on SLAC / KEK X-Band Standing Wave Structure
• Single bunch charge: up to 80nC.
Measurements:• Time structure and spectrum of the generated wakefield;• Linearity of the voltage-charge curve.
Dimensions for copper only 3-cell standing wave structure experiment
t
b_c
onv
b_e
nd
b1
b2
a_cp
l
a_p
ipe
5×Rb
Rpipe
5×el
lips_
r5×D
2b_end 23.241249
2b1 22.968
2b2 23.072
2b3 22.965
2b_cpl 22.970
2b_conv 22.86 0.9inch
2a 11.295
2a_pipe 12.7 0.5inch
4×a
a_cpl 5.2715
D 13.116
Rpipe 3.00
Rb 1.00
t 4.5980688
ellips_r 3.398573
Dimensions for 20 deg. C V.A.Dolgashev, 2 March 07
b3
b_c
pl
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
-0.2
-0.1
0
0.1
0.2
0.3
5 6 7 8 9 10 11 12 13 14 150
20
40
60
80
100
120
11.427GHz5500
9.408GHz4600
8.361GHz3600
7.783GHzQ ≈ 6700
t -μs
volta
ge -
Vol
t
f – GHz
volta
ge s
pect
rum
13.711GHz1700
13.825GHz3900
q = 34.6nC
Measured voltage signal
5 6 7 8 9 10 11 12 13 14 150
20
40
60
80
100
120
11.427GHz
9.408GHz
8.361GHz7.783GHz
f – GHz
volta
ge s
pect
rum
13.711GHz
13.825GHz
5 6 7 8 9 10 11 12 13 14 150
5
10
15x 10
4
f – GHz
spectrum of the wakefield in the vacuum break for ICT (simulated)
The wakefield in the ceramic vacuum break for ICT may contribute to some of the peaks
vacuum break
5 6 7 8 9 10 11 12 13 14 150
20
40
60
80
100
12011.427GHz
f – GHz
volta
ge s
pect
rum
The 11.4GHz component is filtered out
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-0.04
-0.02
0
0.02
0.04
t -μs
volta
ge -
Vol
t
q = 34.6nC
IFFT
Q ≈ 5500Vmax
Vmin
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