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

WS3 WS4

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 30-35 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 50-300 um

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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

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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 (80-100 um)

• Linac beams were measured to be about 50% larger

than the theoretical limit

Jun 27, 2013 23:36

units are cm-mrad Linac emittance measurements near the end of the run

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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…

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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 usRed has D/4 subtracted in quadrature (exact solution in large-beam limit)

Noise level of 25%

Better ionization more noise

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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

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More things to change20 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 windingsor

Flat ribbon wire

20 μm

100 μm

Courtesy of Nate Lipkowitz

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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

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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

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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