Bunch Length Measurements in the E167

Experiment

Ian BlumenfeldE167 Collaboration

SLAC/UCLA/USC

2

Contents

Introduction Theory

CTR and Autocorrelation Practice

Interferometry Simulation and Measurement

Future

3

Introduction to Bunch Length Measurements

Short Bunch in past not important for Particle Physics experiments, so not measured directly

Important for plasma experiment due to need for high peak current

4

Linear Plasma Theory According to linear plasma theory the wake amplitude is:

This is optimized for if yielding:

In reality, we are no longer in this regime, but simulations show that this scaling still holds

€

eE linear[eV /cm] = n0

nb

n0

2π kpσ ze−

kp2σ z

2

2

1+1

kp2σ r

2

€

kpσ z ≈ 2

€

kpσ r <<1

€

eE linear = 240 MeV /mN

4 ×1010

⎛

⎝ ⎜

⎞

⎠ ⎟

0.6

σ z (mm)

⎛

⎝ ⎜

⎞

⎠ ⎟

2

∝N

σ z2

5

Previous Methods

E167 Efforts Streak Camera Pyroelectric Detectors Phase space matching

Also: E/O’s, transverse deflection cavities (LOLA), etc.

Desire direct measurement

6

CTR and Bunch Length Radiation generated when charged particles moves from one

dielectric medium to another Longitudinal profile of CTR is the same as that of the beam Coherent for wavelengths longer than bunch length

E field

electronbunch

metallic foil

7

CTR and Bunch Length (cont’) Analytically, the radiation intensity is related to the

Fourier Transform of the electron number density Coherence due to interference of electrons in the bunch

Thus CTR spectrum yields information about the bunch

€

I tot(λ ) = I inc(λ )• [1+( N −1)• f (λ )]

f (λ ) = e2πiz

λ∫ ρ (r r )dz

2

8

CTR and Bunch Length

We need only measure the longitudinal profile of the CTR for the bunch length

Problem: We have short bunches, ~10 microns

or ~30fs

9

CTR and Bunch Length

This means no time resolved measurement

Must use interferometry Like in femtosecond laser pulse

measurements Despite disadvantages of symmetric

measurement and averaging

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Autocorrelation and CTR Autocorrelation

function gives information on the pulse shape

Width of this function is correlated to the width of the original pulse

€

S( t) = S0e−

t 2

2σ 2

G(τ ) = S(t)• S( t −τ )dt−∞

∞

∫

G(τ ) = S02 e

−t 2

2σ 2

−∞

∞

∫ e−

(t −τ )2

2σ 2dt

G(τ )∝ e−

τ 2

4σ 2

thus :

σ S =1

2σ A

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CTR Properties CTR differential energy angular

distribution obeys the Ginzburg-Frank Formula

€

U(θ )∝ β 2 sin2(θ )

[1− β 2 cos2(θ )]2

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CTR Properties

CTR energy peaks at 1/gamma off the axis of propagation

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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The Michelson Interferometer

Chose Michelson Interferometer for autocorrelation due to small opening angle

Can easily adjust delay arm with micron precision

)(2)(2)(

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ττ

τττ

ττ

GdttII

dttEtEdttEdttEI

dttEtEI

+=

−+−+=

−+=

∫

∫∫∫

∫

∞

∞−

∞

∞−

∞

∞−

∞

∞−

∞

∞−

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The Michelson Interferometer: First

Results

First results Translates to bunch of ~18 microns

0

4.0

8.0

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01

02

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noitalerrocotuA ?z

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gniretliFw

0

4.0

8.0

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)mµ(noitisoPeniLyaleD

z= mµ9

= sf06

z= mµ9

z= mµ81

:noitalerrocotuA

naissuaGhcnuB

ro

91-esahpSNB6-2,m/VM8.14:egatlovrosserpmoc

=9µmz

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Simulation: Understanding the Results

Simulations show that ideal trace does not contain dips apparent in measured spectrum

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Material Effects

Turns out materials in interferometer have large effect on trace

E.g. loss of long wavelength generates large dips

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Material Effects

Measurements using Bruker interferometer at LBNL in M/FIR show material transmission characteristics

Special Thanks to Michael Martin and Zhao Hao of LBNL and Walt Zacherl of Stanford University for making this happen

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Material Properties

Measurements done from 16 microns to ~320 microns Mylar and TPX appear to have uneven response in this

range

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Material Properties (cont’)

HDPE possibly good for long wavelength Silicon has flattest response

~50% transmission means could be used as beam splitter

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Simulation Results

Material effects distort our expected signal The silicon appears to cause less distortion

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The New setup

Used Silicon beam splitter and Vaccum Window, as well as gold coated mirrors

As Silicon is opaque to visible light, had to align with 1.5micron laser

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Results

Dips now reduced, more features in trace Experimental Method still rough

m m

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Results (cont’)

Features indicate head or tail on beam As well trace width scales with r56

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Next Steps: Further response Studies

Will return to LBNL Take FIR measurements out to mm

range Take reflectivity measurements Calibrate pyro detectors and energy

meter vs. Bruker

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Far Future: Improvements and Single Shot

Next beam access, run current setup in Nitrogen or Helium purge environment

Acquire THz camera, look at radiation properties Study feasability of single-shot

measurement

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Conclusion

Have improved bunch length measurement with study of material properties

Will continue to develop this until it is a useful diagnostic tool

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Presented by the E167 Collaboration

U C L A

M. Berry, I. Blumenfeld, F.-J. Decker, P. Emma, M.J. Hogan*, R. Ischebeck, R.H. Iverson, N. Kirby, P. Krejcik, R.H. Siemann, and D. WalzStanford Linear Accelerator Center

C.E. Clayton, C. Huang, C. Joshi*, W. Lu, K.A. Marsh, W.B. Mori, and M. ZhouUniversity of California, Los Angeles

S. Deng, T. Katsouleas, P. Muggli* and E. OzUniversity of Southern California

Work supported by Department of Energy contracts DE-AC02-76SF00515 (SLAC), DE-FG03-92ER40745, DE-FG03-98DP00211, DE-FG03-92ER40727, DE-AC-0376SF0098, and National Science Foundation grants No. ECS-9632735, DMS-9722121 and PHY-0078715.