1. VELA TDC: Bunch Length and Longitudinal Phasespace Measurements Update
Julian McKenzie
18/07/2016

2. Shifts overview
Problems
Basic method and sources of jitter
What could/should be automated
A few results
Longitudinal phasespace pics – and method
Need for minimising beta at each timeslice for slice momentum spread
Slice emittance?

3. Shifts overview TDC shift runs so far:
Dec First beam with TDC
April 2015 – 1 week – HV issues, no results
June 2015 – 1 week - 1st TDC beam run
Oct 2015 – 3 days – ED measurements with TDC
Oct 2015 – 1 week – 2nd TDC beam run
Bunch length/current profile measurements:
Procedure fully established, not yet automated
Parameter scans completed, eg/ charge, gun phase, solenoid strength
Longitudinal phasespace:
Initial investigations , procedure not yet fully established

4. Technical problems
Electrical noise issues feeding into all other systems (eg/ WCM) and tripping off the RF room power.
Thyratron and transformer failures
TDC chiller failure (cavity set to 57°C for operation)
Circulator issues
LLRF issues
Field flatness ~85% and minimum reflected power ~ 10%
Crosstalk from gun RF into TDC amplifier – this causes RF power when TDC at zero amplitude, and the level jitters
At certain RF phases see spikes in klystron power

5. Example of current dominating issue

6. Example images
Vertical projections
TDC on
TDC off

7. Example distributions of beam centroid and size over 100 shots
Example phasescan of the TDC giving the linear fit used as the calibration factor.
Vertical beamsize:
Calibration factor:
Bunch length:
Any jitter on laser position, gun-laser timing, gun gradient, TDC phase, TDC gradient, will lead to jitter in vertical position on screen

8. Bunch charge scans Low charges for ED with Andor camera
Simulations use free parameters – very sensitive to initial transverse profile
Remember: bunch length measured only at TDC, it changes a lot down the beamline! Eg/1pC simulation

9. Gun phasescan 100 pc (blue) and 10 pC (red)
Dotted lines are ASTRA simulations
(Laser profile free parameter in simulations, but kept constant for both charge regimes)

10. Longitudinal Phasespace
Eg/ Behaviour with gun phase:
-20
-10
+10
TDC phasescan to get time axis on vertical scale
Dipole scan to get momentum axis on horizontal scale
Need to use post-dipole quads to get any noticeable affect from TDC
Pre-TDC quads affect shape of sub-structure and should be used to minimise horizontal beta for each time slice for slice momentum spread measurements
Detailed study/procedure not yet taken place