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ASTA rf gun measurements with solenoid scans

G. Stancari, D. Edstrom, D. Crawford, J. Ruan
Fermi National Accelerator Laboratory
March 23, 2015

Summary

Solenoid scans at low charge, together with models of beam dynamics in the rf gun, yield estimates of the peak rf gradient, laser-gun relative phase, beam emittance, and beam energy. The beam spot at the first YAG screen is recorded as a function of solenoid settings.

Apparatus

Schematic diagram of the apparatus: electron gun with cathode, main and bucking solenoids, YAG screen.

Experimental procedure and data collection

The beam spot at the first YAG screen is recorded as a function of solenoid current for different values of the rf power and phase. The solenoid settings are kept at a fixed ratio, so that the magnetic field at the cathode is negligible.

Measurements are taken at low charge (< 5 pC/bunch), so that emittance growth due to self fields is small (examples in Stephan's et al.), and with short pulses (< 4 ps), to minimize the energy spread.

The data sets acquired at ASTA are reported in the following Table.

Solenoid scan data sets: experiment date; type of cathode; set gradient; gun power (average of N:GCVFP over the course of the scan); set rf phase; rms laser spot measured at the virtual cathode; number of bunches (set at the seed laser); charge per bunch (from the Faraday cup current); number of YAG screen images, one for each value of the solenoid current in the scan.
Date Cathode Set gradient N:GRESPA [MV/m] Set rf phase N:GRESPP [deg] Gun power N:GCVFP [MW] Laser spot h, v [mm] Number of bunches Charge/bunch [pC] Number of data points
28 Jan 2014 Mo 25 -20 (+59?) 2.840 0.43, 0.58 100 0.10 16
29 Jan 2014 Mo 20 -20 (+59?) 2.217 0.43, 0.58 100 0.12 18
14 Feb 2014 Mo 25 192 (+65.5) 2.787 0.98, 2.2 100 0.19 10
14 Feb 2014 Mo 20 192 (+65.5) 2.173 0.98, 2.2 100 0.17 10
3 Apr 2014 Cs2Te 26.6 187 (+60.5) 2.870 0.41, 0.76 100 0.81 32
4 Apr 2014 Cs2Te 23 187 (+60.5) 2.472 0.42, 0.69 20 0.47 26
14 Apr 2014 Cs2Te 45 200 (+61.5) 3.463 0.53, 0.67 250 2.3 23
14 Apr 2014 Cs2Te 37 200 (+61.5) 2.299 0.57, 0.61 250 2.3 25
9 Mar 2015 Cs2Te 42 28 (+50.8) 3.083 0.403, 0.436 60 2.6 28
10 Mar 2015 Cs2Te 37.5 28 (+53.0) 2.428 0.403, 0.436 30 2.9 43
10 Mar 2015 Cs2Te 45 24 (+47.5) 3.623 0.403, 0.436 30 1.5 25

Data analysis

Electron gun power

Gun power is the average of N:GCVFP over the course of the scan. Directory ./10-gun-power/ contains an R script to do these calculations automatically from a file with ACNET data. This variable is our reference for gun power settings.

Relative phase between rf and laser

The rf phase is set through parameter N:GRESPP. The definition of this variable has also changed with time, and it can easily fluctuate by several degrees due to laser-rf synchronization (at 1.3 GHz, 1 degree corresponds to 2.1 ps), so it is an arbitrary number. The numbers in parentheses indicate the phase with respect to the extinction phase, obtained from a phase scan, which is a beam-based absolute calibration with an accuracy of a few degrees. After a phase-scan, N:GKIW0M[11] (Faraday cup) vs. N:GRESPP (phase) data is downloaded from the D44 ACNET datalogger. The phase scan analysis is stored in directory ./20-phase-scan/.

Bunch charge

The charge per bunch is measured by analyzing the time evolution of the Faraday cup data over several pulses, to identify the background level, the dark current, and the beam current. Beam current is multiplied by bunch spacing (333 ns) to obtain the charge per bunch.

Faraday-cup data (current vs. time) is in the ./30-beam-intensity/data directory. Parameters are described in the ./30-beam-intensity/fcup-descr.txt file: file names and time intervals for the calculation of current offset, dark current, and beam current. ./30-beam-intensity/fcup.R is the analysis script. Outputs are saved in the ./30-beam-intensity/results directory as PDF plots (such as this) with diagnostics and final numbers.

Laser spot size at the cathode

The laser spot size is recorded by a video camera at the so-called virtual cathode, which is set up to have the same optical path as the laser light that goes to the actual photocathode. This is a sample image.

Image processing is launched by ./40-imageproc/laser-spots.R, which is based on the routines defined in ./40-imageproc/imageproc.R. The rms spot size is calculated from 3 estimates: interquantile, statistical rms, and Gaussian fit. Results are in ./40-imageproc/diag-plots (diagnostic plots such as this) and ./40-imageproc/results/.

Beam spot sizes at YAG screen

Same ./40-imageproc/imageproc.R, driven by ./40-imageproc/process-images.R

Model

Transport from the cathode to the screen is assumed to be linear, with constant emittance. Matrix elements depend on beam energy, rf fields, and solenoid settings.

For a given set of parameters, matrix elements are calculated * by integration of trajectories in time (Astra) or * by slicing the longitudinal coordinate using the Gulliford-Bazarov approach () as implemented by G. Stancari's code tramalargu.

A best fit of the model to the data is used to estimate * peak rf gradient and phases, and therefore final beam energy * initial beam emittance

Results