FACET Machine Performance and Expectations M. Sullivan for the FACET Accelerator group SAREC Review July 25, 2013

2 Improvements from last year New optics for the sector 20 chicane were found that feature reduced beta functions in the chicane and larger beta functions at the IP compared to last year’s lattice. This discovery was made near the end of the last FACET run. This greatly reduced the non-linear lattice terms in the chicane which lowered the beam sensitivity to small variations in position, energy and charge. The beam energy spread is very large due to the high compression needed to get a short beam bunch Resulted in a significant improvement in beam stability and reproducibility

3 Improvements from last year (2) Sector 20 chicane hardware improvements Added two skew quadrupoles (X-Y coupling) Added two more sextupole movers Now have a total of four movers Installed a transverse RF cavity for dedicated bunch length measurements Upgraded selected bpms for shot-to-shot readout Two bunches time X 60 μm

4 Improvements from last year (3) Linac Increased the damping ring RF voltage shorter bunch Improved orbit control in the Linac Interaction Region Added more wire scanners (now have four) Improved the OTR screens Developed and improved operational procedures New and better beam measurement software packages Greatly enhanced beam reproducibility and decreased recovery times from accesses or other off times

5 Interaction Region WS1 WS2 WS3WS4

6 “Typical” wire scans X Y I believe these were taken with no gas present We measured smaller numbers many times but this method has limitations

7 Bunch length measurement We achieved smaller bunch lengths when fully compressed but stopped measuring the bunch length in order to prevent vacuum damage (more soon)

8 Ability to easily change the Interaction Region waist location Center of the plasma chamber Wire scanner #2 Main IP Excellent software program that let the operator select the Z location of the IR waist, then calculates the necessary changes to the final focus optics and implements the new settings. This was asked for by the experimenters and quickly became a valuable tool for general machine setup

9 Beam performance Eleven primary IP parameters X position, angle, size, dispersion, dispersion slope Y position, angle, size, dispersion, dispersion slope Z length Consistently able to get measured transverse spot sizes down to um (beam size is smaller than this) and a bunch length well below 50 um (not measured under full compression). Also able to measure and control (minimize) the beam dispersion and slope of the dispersion Improved software package to measure dispersion was of great benefit here together with the ability to move the sextupoles Sextupole moves to correct dispersion were um

10 Beam performance (cont.) We were able to shoot the high compression beam through very small dielectric tubes (400 um inner dia.) that were 10 cm long without hitting the tube wall For reference, the wire used in an ordinary paper clip has an 800 um diameter This was done consistently over a period of hours (with intermediate checks) Correcting dispersion is critical here because with dispersion, a klystron that drops out of the Linac will significantly change the beam energy thereby shifting the beam position at the IP

11 Linac emittances Four general Linac configurations were developed Pencil beam low charge (minimal compression – 500 um) Low charge maximum compression High charge maximum compression High charge two bunch operation (using notch collimator) longer bunch ( um) Linac beams were measured to be about 50% larger than the theoretical limit Jun 27, :36 units are cm-mrad Linac emittance measurements near the end of the run

12 Damage to vacuum components Beam power density was high enough to damage Optical Transition Radiators – punched holes through Be foils used for vacuum isolation – drilled holes Care had to be taken to ensure the beam waist was far enough away from the OPRs and Be windows Be window Succeeded in ionizing He which would require a bunch size closer to 10x10x20 um

13 Plans going ahead Work on improving wire scanner information The measured values are overestimates The beam size is getting too small to measure very well with the wire size we are using (next few slides) Smaller wires do not make enough of a signal especially when a gas is being ionized -The scanning signal is scattered beam particles as the beam goes through the wire Make the scanning step size smaller (will be done) Try using wire strips -Instead of this try this Try to reduce backgrounds Under study…

14 Realistic beam size estimation Really quite difficult to resolve any difference below about 20 μm. In 10 x 100 cm optics, σ ≈ 10 μm. Simulation Courtesy of Nate Lipkowitz Blue is what the wire scan GUI gives us Red has D/4 subtracted in quadrature (exact solution in large-beam limit) Noise level of 25% Better ionization  more noise

15 Things to change Same data, now with 500 nm steps (10x finer) Easy change: add more points. Still 60 um wire. Currently there is a software limit of 5 um for step size. We have requested it to be removed. Simulation Courtesy of Nate Lipkowitz

16 More things to change 20 micron “wire” with 500 nm steps 20 μm wire with small step size gives much more believable results. However in practice we don’t see the small wire (signal < noise) Propose to use instead a small aspect ratio wire: 20 μm Multiple wire windings or Flat ribbon wire 20 μm 100 μm Courtesy of Nate Lipkowitz

17 Positron System Start commissioning the positron system Initial work to see what parts still work and what parts are broken or need replacement Last used in spring 2008 Steps to getting positrons to the IR dump Make e+ on target (have been doing this) Capture Accelerate to about 200 MeV Make 180 deg turn up to the ceiling of the vault Transport back to the front of the Linac Make a 180 deg turn and inject down into the Linac Accelerate up to 1 GeV Extract and transport to the entrance of the SDR

18 Positrons Still more to do Compress the positron bunch prior to injection into the ring Inject into the SDR Store Extract the bunch from the ring Compress the bunch after extraction Inject back into the Linac Go through the sector 10 chicane Accelerate up to 20 GeV Go through the S20 chicane with all magnet polarities reversed Considerable amount of work and effort

19 Summary The last run delivered a steady and reproducible beam to users with good specifications A lot of the current success rests on the work done during the previous FACET run where much of the difficulties in both hardware and in lattice design were uncovered Positron commissioning will start this coming fall with delivery of positrons planned for calendar 2015 We will be working on our diagnostics to improve spot size measurements We plan to deliver at least as good a (and hopefully better) beam as we had this last run to upcoming users