VELA-EN-20140416-#12022nd April 2014
Calibration of Charge against the Laser Attenuation Wave-plate Angle
VELA Shift Log #120
1.Document History
Version / Date / Authors / Comment1 / 16/04/2014 / James Jones, Julian McKenzie / 1st Issue
2 / 22/04/2014 / James Jones, Julian McKenzie / Correction of charge analysis, additional calculations, grammar, formatting
2.Method
Experimental Notes Used:
VELA-EP-WCM-01
VELA-EP-FCUP-01
VELA-EP-LaserAttenuator-01
VELA-EP-Gun-01
Measurements of the charge were performed using the WCM[VELA-EP-WCM-01] as well as the FCUP [VELA-EP-FCUP-01] in train mode.
The experimental procedure for calibration of the laser attenuator is defined in [VELA-EP-LaserAttenuator-01].
3.Machine Status
We operated at a nominal laser power of 0.5mJ measured at a laser attenuator angle of 55°. The machine was crested using YAG-04 [VELA-EP-Gun-01] as BPMs were unavailable at the time, with an RF phase of -125° at a FWD power of 6.8MW and a ratio of 1.4%.
Parameter / Value / CommentLaser Power / 0.5mJ / Laser Att.=55°
Charge / Variable
Gun Phase – Crest / -125° / Crested using YAG-04
Gun Phase – Experiment / -125°
Gun FWD Power / 6.8MW
Gun Ratio / 1.4%
Snapshot file
4.Results
The data was recorded in an excel spreadsheet located in the shift folder. Additionally, analysis was performed in Mathematica.
The charge data was fitted assuming a Cos2() dependence on transmission through the wave-plate, with the wave-plate angle measured in degrees. The fitting used is defined in equation 1, with the parameter athefitted zero-signal, here assumed to be equivalent to the no-laser condition, parameter b defines the maximum signal, whilst defines the phase offset and n the frequency of the signal in units of 2.
/ (1)a)Wall Current Monitor
The results for the WCM measurement are shown in Figure 1. The fitted parameters are shown in Table 1.
The fit gives a maximum value of 182.9pC at a wave-plate angle of 65.2°.
Table 1 Fitted parameters for the WCM signal against wave-plate angle.
Parameter / Valuea / 21.8
b / 185.55
/ 26.5°
n / 1.96
Figure 1 WCM measured charge against laser attenuator wave-plate angle.
b)Faraday Cup Measurement
The results for the FCUP measurement are shown in Figure 2. The same fitted parameters are found for the FCUP signal, and shown in Table 2.
The calculated fit gives a maximum absolute amplitude of -1534mV at a wave-plate angle of 65.33°, which is in good agreement with the WCM data.
Table 2 Fitted parameters for the FCUP signal against wave-plate angle.
Parameter / ValueA / -51.1
B / -1534
/ 26.03
n / 1.97
Figure 2 FCUP measured charge against laser attenuator wave-plate angle.
Figure 3Comparison of the WCM charge measurement against the calibrated FCUP measurement for varying attenuator wave-plate angles.
The calculated calibration of the FCUP to WCM data is -9.1mV/pC. A comparison of the WCM data with a scaled version of the FCUP data is shown in Figure 3, using this calculated calibration factor.
The calculated calibration is comparable to the expected calibration of 10mV/pC. Differences could arise from the transmission of the beam between the WCM and the FCUP, as well as errors in the calibration of the WCM and/or the FCUP itself.