RESEARCH ON BEAM SIZE MONITORS

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• RESEARCH ON BEAM SIZE MONITORS
• S. Csorna, Vanderbilt U.
• LCCOM2, June 30, 02, Santa Cruz
• Proposal for Non-Intercepting Beam Size
  Diagnosis Using Diffraction Radiation.
  Feng/Gabella/Csorna (Vanderbilt U.)
• Laser Interferometry/Laser Wire Studies.
  Csorna/Ernst/Hartill(Vanderbilt,Albany,Cornell)
• Electro-Optic Technique for beam size
  measurements. Gabella/Feng (Vanderbilt U.)
• Bright Needle Electron Sources. Brau (Vanderbilt
  U.)

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W. M. Keck-Vanderbilt Free Electron Laser Center
Facilities
W. Gabella, B. Feng, J. Kozub and D. Piston
BiOS 2002 Biomedical Optics and Application,
4633B-31

• College of Arts and Sciences
• College of Engineering
• School of Medicine

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FEL pulse structure (Mark-III)

• FEL macropulse
• repetition rate 1-30 Hz
• electron duration 8 ms
• IR pulse duration 3-5 ms
• FEL micropulse
• pulse duration 1 ps
• pulse separation 350 ps (pulse-to-pulse thermal
  confinement)

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Proposal for non-intercepting beam size diagnosis
using coherent diffraction radiation from a slit
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• Advantages
• Non-invasive (beam goes through a slit).
• Easy Extraction of signal since Diffraction
  Radiation (DR) directed
  perpendicular to beam if slit inclined at 45
  degrees.
• Responds rapidly to changes in beam, inherently
  compact, could be installed in many locations.
• Intensity proportional to ?2
• In the limit of zero slit width DR?TR
• Bunch length/bunch shape measurement
• I(?) I1(?)NN(N-1)F(?)
• F(?) Bunch Form Factor ??S(z)expi ? /c
  zdz?2
• Density Distribution

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Asymmetric electron bunch
In case of asymmetric electron bunch
where s is the wave number of radiation, Y(s) the
phase calculated from the observed form factor by
the Kramers-Kronig relation
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Angular distribution from DR
Parallel polarization
Normal polarization
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Martin-Puplett Interferometer
Electron Beam
Beam Dump
Coherent Light
Parabolic mirror
Polarization Splitter
Detector 2
mirror
S1
S2
Movable mirror
Detector 1
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Proposal Studies

• Radiator
• Longitudinal Bunch Length Experiments
• Transverse Beam Dimension Experiments
• Studies of Diffraction Radiation

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BUDGET
1. Design, construction and deployment of
target/slit 2000.00 2. Wire or graphite
polarizer/beam splitter for interferometer

5000.00 3. Parabolic reflector
(mirror) 1000.00 4. Stage for
moving mirror in the interferometer
5000.00 5. 2 Golay Cell Detectors (_at_5000 each)
10000.00   TOTAL
23,000.00  
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Laser Interferometer/Laser Wire Studies  
Transverse beam spot size is determined by using
the laser interference fringe as a physical
scale. Scan the electron beam through the fringes
and measure the Compton scattered g rays from the
photon target.
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The Modulation Depth M (Signal Max Signal
Avg)/Signal Max of the g ray flux is related to
the transverse beam size
MUncorrected½cos q½exp (-(1/2) (2psy /p)2) q
angle between laser beams pfringe pitch l/(2
sin(q/2) p/ky (adjust by changing q or using
higher harmonics) Corrections 1/ Gaussian
laser profile 2/ Power imbalance of laser beams
can affect contrast 3/ Beta Function of e beam
finite width of laser beam along
beam direction, etc. Ref See KEK preprint 96-81
by Tsumoru Shintake (tutorial)  
(PRL 74,2479(95)) SLAC FFTB 706 nm
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WHAT ABOUT BUNCH LENGTH/BUNCH SHAPE ? 1/Don
Hartill is studying the possibility of using
timing information to determine bunch
length/bunch shape. 2/ Longitudinal beam size
can, in principle, be measured by laser
heterodyne techniques. This has not been realized
experimentally.   Principle of Operation   Mix
two lasers of different frequency to create an
intensity modulation at the beat frequency. If
the pitch of the beat wave is longer than the
bunch length, large fluctuations will be seen in
signal due to each pulse. If the pitch is shorter
than the bunch length, the signal will be
approximately constant. So you can determine a
threshold, which is related to the bunch length.
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Scan through relevant beat frequencies, at each
frequency measure the difference M. You
get   M(Ngmax - Ngmin)/( Ngmax Ngmin)
(correction factor for laser radius and
transverse bunch size)
F(wbeat)/F(0)   Here F(wbeat) is the Fourier
spectrum of the bunch and would look like      
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Conclusion It is fairly clear that we need to
use laser based techniques to measure nm size
bunches we hope to report a more detailed plan
at next UCLC meeting. ----------------------------
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Electron-Beam Brightness is More Important than
Current

• by
• Charles A. Brau
• Department of Physics
• Vanderbilt University

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The quantum efficiency for 266-nm pulses
increases sharply at low fields and approaches
unity for high fields
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We have achieved several orders of magnitude
improvement in normalized brightness
Observed currents 100 mA from 0.5-mm tips
produce current densities 1011 A/m2 with an
estimated normalized brightness 1016
A/m2-steradian

• Applications
• Far Infrared (100-500 mm) FEL
• UV/Soft X-ray (10-400 nm) FEL
• Linear Collider

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Conclusion Charley Brau is currently studying
the possibility of getting polarized electrons
from a needle source. A detailed research plan is
to follow