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Optical diffraction - transition radiation interferometry beam divergence diagnostics. R. B. Fiorito and A.G. Shkvarunets Institute for Research in Electronics and Applied Physics University of Maryland, T. Watanabe and V. Yakimenko ATF, Brookhaven National Laboratory

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optical diffraction transition radiation interferometry beam divergence diagnostics

Optical diffraction - transition radiation interferometry beam divergence diagnostics

R. B. Fiorito and A.G. Shkvarunets Institute for Research in Electronics and Applied PhysicsUniversity of Maryland,

T. Watanabe and V. Yakimenko ATF, Brookhaven National Laboratory

J.Power, M. Conde, W. Gei

AWA, Argonne National Laboratory

D. SnyderDept. of Physics, Naval Postgraduate School

slide2

Beam Parameters Accessible to OTR and ODR Diagnostics

  • Incoherent OTR and ODR
  • ‘Near Field’ Imaging (spatial distribution)
        • size (x, y)
        • position (x, y) (offset)
        • Far Field Imaging (angular distribution)
        • divergence (x’, y’)
        • trajectory angle (X’,Y’)
        • energy and energy spread
        • Coherent TR and DR
        • bunch length and shape
slide3

Spectral-angular distribution

q

Radially

Polarized

AD Pattern

Centered on

Direction of

Ve

E

Polarized

OTR pattern

P

q

slide4

Effect of Beam Divergence on OTR:Convolution of single electron OTR

with 2D distribution of beam trajectory angles

x

gs = 0.2

slide5

Forward OTR

backward OTR

Beam

FOTR +BOTR

OTR Interferometry provides greater sensitivity to beam parameters

Formation or coherence length

  • Visibility of OTR interferences is a measure of divergence
  • when gs >> ( DE/E and gsscattering )
  • OTRI visibility measure of divergence in any radial direction
  • >> can be used to separate and measure rms x’ and y’ at
  • an x or y beam waist
  • Center of pattern measures trajectory angle of particle
  • Fringe position measures beam energy (E)

qx

qy

slide6

Extends OTRI diagnostics to low energy and/or low emittance

Perforated first foil overcomes scattering limit of conventional OTRI

mirrored foil (OTR source)

ODR

Beam

perforated foil (ODR source)

ODR+OTR

Optical Diffraction-Transition Radiation Interferometry

slide7

DIFFRACTION RADIATION FROM A SINGLE APERTURE

radiation impact parameter: a-1 = gl/2p ,

a-1is the range of the radial E field of the charge: Ee~ K1 (ar)

DR from a circular aperture with

radius a and position offset r

Forward DR

a-1

e

geometric factor

Backward DR

slide8

Calculation of ODR from a Perforated Foil

Simulation Code needed tocalculate the fields and intensities of ODR produced by an electron passing through a perforated foil at any position in the hole or screen. (No analytic solution is available for multiple aperture radiator)

Huygens-Kirchoff Integral is used to calculate the x, y components of electric and magnetic radiation fields produced by a source field that is proportional to field of the particle passing through the hole or through a solid portion of the screen,

R is the distance from dSf, the differential element of area at the foil to the observation plane, ux,y is the Fourier component of the free space radial field of the electron.

slide9

Calculation of the two ODR component from a grid of rectangular holes in a metal screen:

ODR from Unscattered and Scattered Electrons)

Unit cell used to calculate

DR for a particular electron position

in the beam ( scattered and un-

scattered)

slide10

Closer Look at the Radiation Components in an

ODR-OTR Interferometer

ODR(u) +ODR(s)

backward OTR

Beam

mirror

(Foil #2)

perforated foil #1

ODR(u) + ODR(s) +OTR(u) + OTR(s)

slide12

ODR - OTR Interferences

I. Simulation Code calculates and adds up the intensity distributions at foils 1 and 2 for U and S beam fractions.

generalized phase:

where L is the spacing between the foils, qe is the electron trajectory angle within the beam and q is the observation angle.

II. Convolution of I with Distribution of Beam Angles as for OTR

slide14

OTR and ODTR Interferometers Designed for Electron Beam Divergence Measurements

L ~ g2 l

ODTRI AD Pattern

Electron beam

OTRI AD Pattern

slide15

Experimental Setup for OTRI or ODTRI RMS Emittance Measurement

Experimental Setup for OTR-ODR RMS Divergence Measurements

Far Field Pattern

Camera

Lens focused to

Infinity

Bandpass Filter

Pellicle Beam

Splitter

Qobserv ~ 10/g

Image Plane

Camera

OTR-ODR

Interferometer

Beam magnetically focused to x or y

waist condition

mirror

Beam

Beam magnetically focused to x or y

waist condition at mirror

erms,n = bg<x>1/2<x’>1/2

Far Field Pattern

Camera

Lens focused to

Infinity

Bandpass Filter

Pellicle Beam

Splitter

Qobserv ~ 10/g

Image Plane

Camera

OTRI, ODTRI

mirror

Beam

slide16

Comparison of y divergence measurements at y beam waist

(E = 95 MeV, I = 0.5mA, l =650 x 70 nm)

OTRI (t= 60s)

qy

ODTRI (t =90s)

s1 = 0.57

qx

slide17

Comparison of ODTR and OTR Interference Patterns

at an x waist

OTRI

s = 1.2

ODTRI

s = 1.2

slide19

Comparison of ODTRI and OTRI at ATF

( Tune 1: x = 0.18 mm, y = 0.27 mm, sx = 0.31 mrad and sy = 0.22 mrad )

l = 650 x 10nm )

ODTRI 480s

OTRI 360s

slide20

Comparison of ODTRI and OTRI

( Tune 2: x = 0.18 mm, y = 0.15 mm, sx = 0.37 mad and sy = 0.75 mrad )

slide21

Tune

Method

Scan

Energy

MeV

Comp1

% Tot

s1mrad

Comp2

% Tot

s2mrad

L

mm

sEmrad

1

OTRI

H

50.7

28

0.35

72

1

47

0.31

1

OTRI

V

50.7

38

0.3

62

1

47

0.22

1

ODTRI

H

50

33

0.28

67

1

44.5

0.31

1

ODTRI

V

50

55

0.28

45

1

44.5

0.22

2

OTRI

H

50.3

33

0.5

67

1.6

47

0.37

2

OTRI

V

50.3

33

0.75

67

1.6

47

0.75

2

ODTRI

H

49.3

33

0.4

67

1.6

44.5

0.37

2

ODTRI

V

49.3

33

0.65

67

0.8

44.5

0.75

Fitted beam parameters for ATF beam tunes 1 and 2

slide22

Low Energy (Injector) Diagnostics using ODR and OR from Dielectric Foil

PROBLEM: for low energy beams the inter-foil spacing L required is too small to directly observe backward reflected OTR or ODR from a mesh -metal foil e.g. L ( 8 MeV, 650 nm ) ~ g2l ~ 1 mm

SOLUTION: observe interference between forward directed ODR from mesh and forward dielectric optical radiation(DOR).

dielectric foil

mirror

BEAM

micromesh

ODR+DOR

slide26

Optical Method for Mapping Transverse Phase Space

Unfocussed beam

detector

collimator

optical pepperpot technique
Optical Pepperpot Technique

Bandpass

Filter

OTR, ODR

Interferometer

Farfield Camera

Optical Mask

Polarizer

Profile

Camera

beam

slide28

OTR Phase Space Mapping

Scan with a 1mm Pinhole

Vert. Scan of OTRI passing

through pinhole