High Brilliance X-rays from Compact Sources

Report
High Brilliance X-rays
from Compact Sources
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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People
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
W.S. Graves, MIT, March 2013
With funding from DARPA AXis,
DOE-BES, and NSF-DMR
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Basic Layout for ICS
3m
Gun
Linac
Cathode laser
W.S. Graves, MIT, March 2013
Quads
IR laser or THz
ICS
X-rays
ebeam
dump
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X-band ICS source with 1 kHz rep rate
ICS X-RAY
GENERATOR
ELECTRON
SPECTROMETER
EMITTANCE
EXCHANGE LINE
LINAC
RF
GUN
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
W.S. Graves, MIT, March 2013
EMITTANCE
EXCHANGE LINE
ICS X-RAY
GEN.
ELECTRON
SPECTROMETER
Optimized X-band SW Structure
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
W.S. Graves, MIT, March 2013
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High Repetition Rate ICS with SRF Linac
RF amplifiers
Superconducting
RF photoinjector
operating at 400
MHz and 4K
RF amp
RF amp
Inverse Compton
scattering
RF amp
30 kW
beam dump
Bunch compression
chicane
X-ray
beamline
4 MeV
30 MeV
Electron beam of ~1 mA
average current at 10-30 MeV
Coherent enhancement
cavity with Q=1000
giving multi MW cavity
power
multi kW cryocooled Yb:YAG
drive laser
8m
W.S. Graves, MIT, March 2013
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High Repetition Rate ICS with SRF Linac
Emittance exchange
beamline
Niowave Inc
SRF gun
ICS x-ray
generator
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 Nb3SN
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
Nb3Sn
Spoke cavity
Elliptical cavity
RF system
Superradiant X-rays via ICS
ICS (or undulator) emission is not
a coherent process, scales as N
Super-radiant emission is in-phase
spontaneous emission, scales as N2
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
Emittance Exchange (EEX)
Beamlets
from tips
y
Current
x
t
Acceleration
x’
Energy
x
t
EEX
x’
Energy
x
y
x
W.S. Graves, MIT, March 2013
t
Bunched
beam emits
coherent ICS
Current
t
Layout for Super-radiant ICS
RF gun
Linac
Quads
Dipoles
RF deflector
ICS
X-rays
Nanocathode
Emittance Exchange
(EEX)
ebeam dump
IR laser or THz
W.S. Graves, MIT, March 2013
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Nanostructured Cathodes
W.S. Graves, MIT, March 2013
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Au Nanopillar Array Geometry
10 nm
30 nm
80°
W.S. Graves, MIT, March 2013
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Nano Stripes
• 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
W.S. Graves, MIT, March 2013
18 nm wide Au lines at 100 nm pitch
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Cathode spot size maps to pulse length
Cathode stripes
Large laser spot
makes long pulse
Laser spot
Current
EEX
time
Number cathode stripes illuminated sets
number of micropulses after EEX
Laser spot
Current
EEX
time
y
Small laser spot
makes short pulse
x
W.S. Graves, MIT, March 2013
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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
Shorter wavelength
y
EEX
x
W.S. Graves, MIT, March 2013
Current
t
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Simulation of 300x40 Tip Array through EEX
5M particles tracked, similar to full bunch charge
z-d slope due to imperfect
matching (correctable)
Bunching at 13.5 nm
10 fs bunch length
W.S. Graves, MIT, March 2013
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
W.S. Graves, MIT, March 2013
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 Nw
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13.5 nm photons/shot
13.5 nm flux vs transverse ebeam size
Bunching factor = 0.2
RMS electron beam size (microns)
W.S. Graves, MIT, March 2013
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13.5 nm GENESIS Simulations
Laser parameters
Pulse energy
100
Pulse length
1
Waist size w0
7
Pulse shape
flattop
A0 at waist
0.3
Wavelength
1.0
Undulator period*
0.5
Units
mJ
ps
micron
micron
micron
Electron parameters
Peak current
100
Pulse length
45
Norm. emittance
0.01
Energy
1.7
RMS energy spread 0.1
Bunching factor
0.2
Beta function at IP 1
Units
A
fs
micron
MeV
%
mm
*Undulator period = ½ laser wavelength
• .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
280 kW peak
Power growth over
300 periods
Bunching factor
• 14 nJ or 109 photons/pulse in 0.15% bandwidth
• Emittance requirement during exponential gain
 N  bx


4p Lg
=50
W.S. Graves, MIT, March 2013
Very different ratio than
cm period undulator
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13.5 nm Power and Spectrum Simulations
Radiation RMS size
during interaction
Spectrum
0.15% BW
280 kW peak
Power vs
time
Optical guiding allows larger ebeam size
50 fs
W.S. Graves, MIT, March 2013
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GENESIS Simulated 13.5 nm Performance
13.5 nm Output
Photons per pulse
Pulse energy
Average flux*
Bandwidth (FWHM)
Average brilliance*
Peak brilliance
Opening angle
Source size
Pulse length
Repetition rate
Avg current
1 kHz
rep rate
109
14
1012
0.1
5x1014
3x1025
0.8
1.5
50
1
5
Units
nJ
photons/s
%
photons/(s .1% mm2mrad2)
photons/(s .1% mm2mrad2)
mrad
mm
fs
kHz
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
$3M
W.S. Graves, MIT, March 2013
~$15M
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