W.S. Graves
MIT
Presented at High Brightness Electron Beams Workshop
San Juan, PR
March, 2013
W.S. Graves, MIT, March 2013
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MIT
K. Berggren, J. Bessuille, P. Brown, W. Graves, R. Hobbs, K.-H. Hong,
W. Huang, E. Ihloff, F. Kaertner, D. Keathley, D. Moncton, E. Nanni,
M. Swanwick, L. Vasquez-Garcia, L. Wong, Y. Yang, L. Zapata
DESY
J. Derksen, A. Fallahi, F. Kaertner
Jefferson Lab
F. Hannon, J. Mammosser, ...
NIU
D. Mihalcea, P. Piot, I. Viti
SLAC
V. Dolgashev, S. Tantawi
With funding from DARPA AXis,
DOE-BES, and NSF-DMR
W.S. Graves, MIT, March 2013
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Gun
Linac
3 m
Quads ICS X-rays
Cathode laser IR laser or THz ebeam dump
W.S. Graves, MIT, March 2013
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RF
GUN
X-band ICS source with 1 kHz rep rate
ICS X-RAY
GENERATOR
ELECTRON
SPECTROMETER
EMITTANCE
EXCHANGE LINE
LINAC
Equipment cost $3M
X-rays 0.1 – 12 keV
Not shown
- klystron and modulator housed in one 19” X 6’ rack
- instrumentation & power supplies housed in one 19” X 6’ rack
- 10W (10 mJ at 1 kHz) mode locked Ti:Sapp amplifier for photocathode and ICS collision
- x-ray optics
W.S. Graves, MIT, March 2013

X-band ICS source with 1 kHz rep rate
RF
GUN LINAC
EMITTANCE
EXCHANGE LINE
ICS X-RAY
GEN.
ELECTRON
SPECTROMETER
W.S. Graves, MIT, March 2013

Coupler to two adjacent cells
Simulated p
-mode with coupling
Standing wave accelerator structure with distributed coupling
Feed power
Structures by S. Tantawi and V. Dolgashev of SLAC
• Just 3 MW RF power to accelerate 20 MeV in 1 m
• 1 kHz rep rate with 9.3 GHz klystron developed for medical linacs
• 1 kHz solid-state modulator with <.01% stability
• RF gun is 2.5 cell 9.3 GHz structure needing just 2 MW to produce 200 MV/m on cathode
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W.S. Graves, MIT, March 2013

High Repetition Rate ICS with SRF Linac
Superconducting
RF photoinjector operating at 400
MHz and 4K
4 MeV
RF amp
RF amplifiers
RF amp RF amp
Electron beam of ~1 mA average current at 10-30 MeV
30 MeV
Coherent enhancement cavity with Q=1000 giving multi MW cavity power
8 m
Bunch compression chicane
Inverse Compton scattering
30 kW beam dump multi kW cryocooled Yb:YAG drive laser
X-ray beamline
W.S. Graves, MIT, March 2013
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High Repetition Rate ICS with SRF Linac
Emittance exchange beamline
ICS x-ray generator
Niowave Inc
SRF gun
Jefferson Lab
SRF linac
Equipment cost $15M
X-rays 0.1 – 12 keV
W.S. Graves, MIT, March 2013

Superconducting Accelerator R&D for Coherent Light Sources
PI: J. Mammosser, JLab
Goal: develop a low cost, high efficiency SRF solution suitable for compact light sources and other uses
• Compare spoke and elliptical b
=1 cavities
• Evaluate cavity materials, including Nb
3
SN
• Evaluate beam dynamics for highest brightness.
• Develop digital LLRF system for cavity / module testing
• Evaluate options for a low cost versatile cryostat
Beam dynamics
Single cell
CLS concept
Nb
3
Sn
Spoke cavity Elliptical cavity
RF system

ICS (or undulator) emission is not a coherent process, scales as N
Super-radiant emission is in-phase spontaneous emission, scales as N 2
N electrons
Steps
1. Emit array of electron beamlets from cathode 2D array of nanotips.
2. Accelerate and manipulate correlations of beamlet array.
3. Perform emittance exchange (EEX) to swap transverse beamlet spacing into longitudinal dimension. Arrange dynamics to give desired period.
4. Modulated electron beam backscatters laser to emit ICS x-rays in phase. FEL gain appears possible.
W.S. Graves, MIT, March 2013

y
Beamlets from tips
Current x t
Acceleration x’
Energy x t
EEX x’ Energy x t y x
Bunched beam emits coherent ICS
Current t
W.S. Graves, MIT, March 2013

Layout for Super-radiant ICS
RF gun
Linac
Quads
Dipoles
RF deflector
Nanocathode
Emittance Exchange
(EEX)
IR laser or THz
ICS
X-rays ebeam dump
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W.S. Graves, MIT, March 2013

W.S. Graves, MIT, March 2013
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10 nm
30 nm
80°
W.S. Graves, MIT, March 2013
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• Note similarity of stripes to wavefronts.
• Emittance exchange demagnifies pattern and transforms periodicity from ‘x’ to time.
SEMs of tips fabricated by R. Hobbs, MIT Nano Structures Lab
110 nm wide Au lines at 500 nm pitch 18 nm wide Au lines at 100 nm pitch
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W.S. Graves, MIT, March 2013

Cathode spot size maps to pulse length
Cathode stripes
Laser spot
Large laser spot makes long pulse
Current
EEX time y x
W.S. Graves, MIT, March 2013
Number cathode stripes illuminated sets number of micropulses after EEX
Laser spot
EEX
Current time
Small laser spot makes short pulse
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Tune resonant wavelength with quadrupole
Weak quad images cathode at low demagnification
Longer wavelength
Current y
EEX t x
Strong quad images cathode at large demagnification y
Shorter wavelength
Current
EEX x t
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W.S. Graves, MIT, March 2013

Simulation of 300x40 Tip Array through EEX
5M particles tracked, similar to full bunch charge zd slope due to imperfect matching (correctable)
Bunching at 13.5 nm
W.S. Graves, MIT, March 2013
10 fs bunch length

Tests of coherent ICS code
Simulations by NIU grad student Ivan Viti using Lienard-Wiechert solver written by Alex Sell of MIT. Work in progress.
Examine radiation from many nanobunches
Simulations are designed to study coherent radiation opening angle, bandwidth, and electron beam size effects.
Emittance is set unrealistically small to remove its effect. Purpose is to explore radiation properties.
W.S. Graves, MIT, March 2013
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Radiation from many nanobunches
Bandwidth tends to 1/(number bunches) for large numbers of bunches
Opening angle tends to
1

N w
W.S. Graves, MIT, March 2013
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13.5 nm flux vs transverse ebeam size
Bunching factor = 0.2
W.S. Graves, MIT, March 2013
RMS electron beam size (microns)
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13.5 nm GENESIS Simulations
Laser parameters
Pulse energy
Pulse length
Waist size w0
Units
100 mJ
1 ps
7 micron
Pulse shape
A0 at waist flattop
0.3
Wavelength 1.0
Undulator period* 0.5
micron micron
*Undulator period = ½ laser wavelength
Electron parameters Units
Peak current
Pulse length
100
45
A fs
Norm. emittance
Energy
RMS energy spread
0.01
micron
1.7
0.1
MeV
%
Bunching factor 0.2
Beta function at IP 1 mm
• .01 micron emittance is consistent with 150 MV/m cathode field and 5 pC
• 45 fs bunch length contains 1000 periods at 13.5 nm
• Assume uniform bunching factor of 0.2 (not yet a start to end simulation)
• FEL rho parameter = .0012
• FEL gain length = 20 microns
W.S. Graves, MIT, March 2013
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13.5 nm FEL Simulations
Power growth over
300 periods
280 kW peak
Bunching factor
• 14 nJ or 10 9 photons/pulse in 0.15% bandwidth
• Emittance requirement during exponential gain


N

4
 p b x
L g
=50 Very different ratio than cm period undulator
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W.S. Graves, MIT, March 2013

13.5 nm Power and Spectrum Simulations
Spectrum
Radiation RMS size during interaction
0.15% BW
Power vs time
50 fs
280 kW peak
Optical guiding allows larger ebeam size
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W.S. Graves, MIT, March 2013

GENESIS Simulated 13.5 nm Performance
13.5 nm Output
Photons per pulse
Pulse energy
Average flux*
Bandwidth (FWHM) 0.1
Average brilliance* 5 x 10 14
Peak brilliance 3 x 10 25
Opening angle 0.8
Source size
Pulse length
Repetition rate
10 9
1 kHz rep rate
14
10 12 nJ photons/s
1.5
50
1
%
Units photons/(s .1% mm photons/(s .1% mm mrad m m fs kHz
2
2 mrad mrad
2
2 )
)
Avg current 5 nA
*Avg values rise 5 orders of magnitude for SRF linac
• Simulations use aggressive but achievable parameters
• Complete start-to-end simulations in development
W.S. Graves, MIT, March 2013
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Summary
• Nanobunched beam and ICS heading toward tabletop x-ray laser
• Develop accelerator technology specifically for this application
• SRF at 4K with low heat load and modular construction
• kHz rep rate x-band gun & linac using only 6 MW total RF power
• Inexpensive to test and develop
• Compact highly stable RF power supplies are commercially available
• Nanoengineered cathodes likely to have big impact on high brightness beams
W.S. Graves, MIT, March 2013
$3M
~$15M
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