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

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?

Shifts overview TDC shift runs so far: Dec 2014 - 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

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

Example of current dominating issue

Example images Vertical projections TDC on TDC off

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

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

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)

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