Preliminary Analysis

1
Preliminary Analysis of GTF Longitudinal
Emittance Experiment D. Dowell SLAC July 18, 2001
2
Talk Outline
I. Description of Experimental Technique II.
Data Analysis III. Experimental Results IV.
Summary and Conclusions
3
Description of the Experimental Technique
Technique similar to quadrupole scan of
transverse emittance
Spectrometer
Booster
Gun
fbooster
Determine Longitudinal f-Space at Exit of Gun
Energy Screen
Longitudinal Measure Energy Spectra Vs.
fbooster Transverse Measure Beam Size Vs. Quad
Current
4
Definition of Initial Phase Space Parameters,
Including Correlations
Longitudinal Beam Ellipse
Longitudinal Beam Matrix
Uncorrelated Bunch Length
Uncorrelated Energy Spread
Include correlated emittance by distorting the
ellipse boundary using quadratic and cubic terms
Randomly populate and ray-trace down the booster

5
May, 2001 Longitudinal Emittance Scan
6
Distortions of the Longitudinal Phase Space
Ellipse
7
Curvature Terms are Needed to Fit Ends of Energy
Spectra
Tail
Head
8
On Crest
Min Energy Spread
9
Fits to Longitudinal Emittance Scan
Fit
Data
10
Phase Space Out of Gun Correlated Length and
Energy Spectrum
Head
Tail
2.9 ps fwhm
11
Uncorrelated Energy Spread
15 keV
12
The Uncorrelated Longitudinal Emittance Grows
Linearly with Surface Charge Density Below the
Space Charge Limit -Results from 1994 Experiment
for 144 MHz Gun-
Ref D. H. Dowell, S. Joly and A. Loulergue, The
Dependence of Longitudinal Emittance Upon
Surface Charge Density in a RF Photoinjector,
Proc. Of 1997 Particle Accelerator Conf.,
Vancouver, BC, Canada
13
In These Experiments Most of the Emittance
Growth Results From Increased Energy
Spread -Results from 1994 Experiment for 144 MHz
Gun-
1s Spread
Ref D. H. Dowell, S. Joly and A. Loulergue, The
Dependence of Longitudinal Emittance Upon
Surface Charge Density in a RF Photoinjector,
Proc. Of 1997 Particle Accelerator Conf.,
Vancouver, BC, Canada
14
Summary and Conclusions
-Curvature Terms Needed to Fit Energy
Spectra -Fits Poor for Large Phases--gt More
careful analysis -Large Energy Correlation for
50 deg Launch Phase (Relative to Peak
Field) -Compression Observed from 4 ps to 2.9
ps -Model-Independent Analysis (Tomography)
Needed