Transcript Development of RF Undulator-Based Insertion Devices for

Development of RF UndulatorBased Insertion Devices for
Storage Rings and Free
Electron lasers
Sami Tantawi, Jeff Neilson, Robert
Hettel, Gordon Bowden
Outline
•
•
•
•
Background
Motivation
Approach
Work Plan
Undulators
• Transverse acceleration of relativistic electron
beam produces synchrotron radiation
• Usually constructed from static periodic
magnets
• Main undulator parameters are the period and K
– K typically 2 to 3 for static undulators
– Typical λu for existing static undulators few cm
RF Undulator Background
• Only one referenced construction of
undulator Shintake* (1983)
• All practical designs to date produce
too low of K value to be of much
interest
• RF limitations
– Excessive field level/loss on metallic
surfaces
– Power levels exceeding available sources
*T. Shintake,K.Huke, J. Tanaka,I. Sato and I. Kumabe,”Development of Microwave Undulator”,
Japanese Journal of Applied Physics, May 1983
Why RF Undulator?
• Many desirable features
– Fast dynamic control of
• Polarization
• Wavelength
• K
– Large aperture (cm vs mm for static undulator)
– No issue with permanent magnet damage by
radiation
– Economic considerations
– Potential use as LCLS “After Burner”
– Dynamic undulator for storage ring
Available Resource - NLCTA
•
3 x RF stations
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–
•
2 x pulse compressors (240ns - 300MW
max), driven each by 2 x 50MW X-band
klystrons
1 x pulse compressors (400ns – 300MW
/200ns – 500MW variable), driven by 2
x 50MW X-band klystrons.
1 x Injector: 65MeV, ~0.3 nC / bunch
*
In the accelerator housing:
–
*
*
2 x 2.5m slots for structures
Shielding Enclosure:
suitable up to 1 GeV
For operation:
–
Can run 24/7 using automated
controls
(Gain = 3.1)
7/16/2015
Page 6
TW RF Undulator in Circular
Guide
TE11 Mode at 11.424 GHz for K=1
• Research initiated by Claudio Pellegrini* at UCLA
• Undulator K parameter of 1 requires > 1 GW
• K = 0.5 of some interest
– power level achievable 250 MW
– surface fields (80 MV/m) would limit pulse length to < 200 ns
• Substantial enhancement of K parameter can be obtained with
resonant structures
*S. Tantawi, V. Dolgashev, C.Nantista, C.Pellegrini,J.Rosenweig, G. Travish,” A Coherent Compton Backscattering High Gain Fel
Using An X-band Microwave Undulator”,Proc. of 27intl FEL Conf, Aug 2005
Circular waveguide mode TE11
• Fundamental mode, easily
excitable
• Has very strong RF field
on the axis where the
electron bunch travels
• Not the least lossy mode
Electric field over waveguide cross-section
Effect of Power losses
Because the e-beam and the em wave are traveling in
opposite directions one can tailor the rf pulse to compensate
for errors in the waveguide and also to taper the undulator
field
Waveguide Undulator
e-
RF Power
RF
Time
1
80
72.737
Waveguide High Gradient Study
Maximum breakdown electric fields
for different geometries and materials
M2 sort 
2
100
64.828
3
100
60.133
4
200
53.528
5
500
53.066
6
500
45.742
7
800
45.255
8
1· 103
38.385
9
1.3· 103
36.693
100
71.313
10
Low
magnetic field
60
40
High
magnetic field
20
0
11
80
Max. surfac e electric field [MV/m]
Max. surface ele ctric field [MV/m]
80
60
Copper
150
67.3
12
300
60.08
13
400
55.656
14
500
54.787
15
700
47.531
K~1
40
Stainless
steel
Gold
20
0
500
1000
Time [ ns]
1500
0
0
500
1000
Time [ ns]
Initial Design point K~0.4
1500
Waveguide Types
• Rectangular (Square)
– Supports TE and TM modes
– High Attenuation
– Hard to build
• Circular
– Supports TE and TM modes
– Lower Attenuation
– easier to build (I think) but hard to taper
• Open Rectangular
– Supports TM modes
– High Attenuation
– Easier to build and taper
•
Traditional ridge waveguides etc., will not work because of surface peak
fields
The use of Higher order modes not only reduces needed power but also reduces
surface fields.
10
5
E Field Along
The
waveguide
surface
E Field
along the
x-axis
4
5
3
0
2
1
5
0
15
10
10
5
0
5
10
5
0
5
10
15
10
TE12 in circular guide (E field Lines)
5
0.6
4
0.4
3
0.2
0
E Field
Along The
waveguide
surface
E Field
along the
x-axis
2
0.2
1
0.4
0.6
0
1
0.5
0
0.5
1
sTE12 in elliptical guide (E field Lines)
1.5
1
0.5
0
0.5
1
1.5
Hz along the surface
0.6
1
0.4
0.2
0.8
0
0.6
0.2
0.4
0.4
0.6
0.2
25
1
0.5
0
cTE12 mode
0.5
1
E Field
Along The
waveguide
surface
E Field
4
along the
x-axis
3
2
1
0
1.5
1
0.5
0
0.5
1
1.5
50
75
100
125
Angle (degrees)
150
175
Power from the dual
Moded SLED-II pulse
compressor (500 MW)
Open Elliptical Waveguide undulator
Mode Launcher
Low Loss Overmoded waveguide
Because of the integration of RF pulses in a resonant ring the rf pulse in the
undulator can be smoothed. Further, the ring can have a multiplication factor
of more than 10, resulting in 5 GW of RF power through the undulator
waveguide.
Spherical Cavity
E
TM113
H
Cavity Parameters for an Equivalent of 1 T
Peak Field at 11.424 GHz
• TE011
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–
–
–
Diamter :1.477” (3.753 cm)
Power: 2.67 MW
Maximum H on Surface: 0.26MA/m (0.320 T)
Maximum E on surface: 0 MV/m
Cavity Parameters for an Equivalent of 1 T
Peak Field at 11.424 GHz
• TM112
– Diamter :2.01159” (5.109 cm)
– Power: 2.98 MW
– Maximum H on Surface: 0.197MA/m (0.248 T)
– Maximum E on surface: 24.35 MV/m
The Perl String Undulator
• Even with optimized diamter and operating frequency, For a
waveguide resonators, the end is a problem. Its losses
dominates
• Also We need to reduce the average power for a near CW
undulator operation. (However, it could be done as
superconducting device)
• Spherical cavities offer about 35% increase in Q factor
over circular cylindrical ones.
• Also highly overmoded spherical cavities can have a very low
surface fields in comparison with the center field.
S-band Undultor Characteristics
• K~1
• Average power for 1% duty cycle 13 kW
• Mode TM111
Resonant Waveguide Undulator
Cutoff taper
Cutoff taper
Lu=1-2 m
•Instead of increasing the field by operating close to the cutoff frequency of the
waveguide undulator one creates a resonant Line
•Tuning the radiation wavelength is done through tuning K between 0.5 to 1
for example:
Undulator frequency= 2.856 GHz
Mode: TE11
Waveguide Radius=3.22 cm
Radiation spectrum from 705 eV -940eV (K~ 0.5-1)
Power/feed @ K=11.1 MW (two feeds for two polarizations)
The circulating power within the undulator ~569 MW
•With the use of nonstandard frequency (~1.905 GHz) one can reduce the
power/feed to about 5 MW. This is done by using TE12 mode and choosing the
diameter of the waveguide such that the line losses is much smaller than the end
losses
Resonant RF Undulator in Circular
Guide
TE12 Mode at 11.424 GHz for K=1
Power MW/m
Power MW/m
TE11 Mode at 11.424 GHz for K=1
• Resonant structures reduce required power by order of
magnitude
• Power requirements reduced further by use of higher
order modes
Filling time
TE11 Mode at 11.424 GHz for K=1
TE12 Mode at 11.424 GHz for K=1
• Higher-order modes require more stored energy,
hence longer filling times
• Same issue applies to other overmoded structures
that have been proposed
HE11 Mode in Corrugated Guide
P ~ λ/2
b –a ≈ λ/4
2a
2b
P
• Inspired by our work on a previous LDRD project which
involved corrugated feed horns for CMB applications
• Lowest order mode (HE11) is a combination of primarily
TE11 and TM11 modes
• Magnetic field is extremely low on waveguide walls –
attenuation can be less than that of smooth wall cylindrical
TE01 mode
• Field configuration ideal for beam interaction
Power and Fill Time for HE11 Mode
HE11 Mode at 11.424 GHz for K=1
Power MW/m
HE11 Mode at 11.424 GHz for K=1
• Power requirements reduced substantially
• Fill time large but sources available for required
power levels
• K of 2 or higher is achievable for existing sources
• Longer undulator wavelengths are easier as
frequency and losses decrease
Superconducting RF Undulator
HE11 Mode at 11.424 GHz for K=1
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•
•
Surface magnetic field is less than the quenching field of niobium
Allows application of RF undulator to storage ring applications, where
CW or quasi-CW operation are required
RF power needed is only few hundred watts to kilo watts – sources
readily available
Comparison of Existing SPEAR3 Static
Undulator to RF Undulator Designs
EPU
TE11
TE12
HE11
Photon Flux Ratio
1
0.2
0.2
0.2
K
1.07
0.71
0.68
0.68
RF Power Loss (MW/m)
5.1
1.6
0.32
RF Frequency
2.63
2.35
2.35
Guide Radius (cm)
7.4
57.5
38.0
• RF undulator design produces 1/5 of flux of existing EPU in
SPEAR3
• Superconducting version would only require 10s of watts
Hybrid Mode Optimization( Undulator wavelength
~1.3 cm)
Hybrid Mode Optimization( Undulator wavelength
~1.3 cm)
Energy/pulse produced by one
SLAC X-band Klystrons
At the beginning of this talk we
showed that the straight forward
fundamental mode in a waveguide
required 4 klystrons and a very
advanced
pulse
compression
system to produce an undulator
with a factor of 0.4!
Hybrid Mode Fields
Radial Field
Electric Field
Axial Field
Magnetic Field
Azimuthal Field
Axial Field
Resonant Ring Configuration
Load
Miter
Bend
Coupler
RF Input
Power
Corrugated
Waveguide
Undulator
Light
RF
Particle Beam
• A closed ring with length nλg
• Tune by adjusting ring length
• Considerable development for relevant components
(miter bend, couplers) has been done (ITER
transmission lines)
Future Work
• Prototype design
– Refine corrugated waveguide parameters for optimal performance
– Beam impedance calculations
– Resonant ring / resonator design
• RF feed
• Particle beam port
• Low power testing of critical components
• Construction and test of undulator
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–
–
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HOM damping design as necessary for storage ring applications
Mechanical design
Construct and test at NLCTA
If successful apply for more funds for testing either with LCLS or SSRL
Test at NLCTA
• Injector Parameters
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–
–
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50 MeV beam energy
200 A peak current
Normalized emittance 2mm mrad??
Relative energy spread 5 X 10-4
• RF system
– 11.424 GHz
– Peak power 600 MW at 400 ns or 150 MW 1.5 us
• Accelerator
– Up to 120 MeV
Conclusions
• Use of HE11 mode provides key to first
practical application of RF undulators
• Successful development will enable
design of undulators with capabilities
not possible with current static
undulators
• Could lead to a new class of FEL and
storage ring undulators