FACET II Project – User Area

Christine Clarke 26th June 2015

Introduction to FACET II

• FACET II will use central 1/3 SLAC

linac to deliver electrons to the

experimenter area in Sector 20

• Project CD1 is October (still very

conceptual at the moment!)

• CD2+CD3 September 2016 for

construction in 2017

Facility for Advanced Accelerator Experimental Tests

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Multi-GeV Plasma Wakefield Acceleration (PWFA)- Apparatus

• The main purpose for

FACET II will be to

continue PWFA studies

• FACET II’s high power

electron beam ionises

alkali vapour or a gas

and interacts with the

plasma

• Wakefields accelerate

part of the bunch

http://phys.org/news/2014-11-physicists-milestone-particles-plasma.html

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PWFA in alkali metal vapours

• Currently we use thin beryllium vacuum tight windows

upstream and downstream (50 um thick)

• Fill with helium to ~5 Torr

• Heat the lithium to ~900 degrees C

• Cooling jackets and a wicking mechanism keep a flat top lithium vapour density

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PWFA in room temperature gases

• We use H2, Ar, He

• Up to 30 Torr is typical

• Currently beryllium windows form boundary for the gas

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The problems…

1. Scattering through beryllium window

contributes to beam size:

• 25-30 micron contribution to the spot size for

50um window

• FACET II beam size is 10um so this is a

significant growth

2. High peak current of beam is capable of

destroying beryllium windows

• Usually multiple shots needed but single shot

has been seen

• The peak current for FACET II is 5 times larger

than the peak current we currently operate with

Therefore we have incentive to use

differential pumping.

Normal window

Leaky window due to beam burn-thru

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The sector

• Sector 20 does not include accelerating structures but it does

include an x-band deflective cavity at z = 1968.57 m

• Vacuum requirement for TCAV is ~10^-8 Torr (needs to be verified)

• “Gas section” is in the area we call IP Area between z = 1991.66 m

and z = 1996.88 m

• No vacuum requirement for after the gas section

• Usually aim for 10^-6 Torr compatibility in user area (we have lots of

motors, cabling, laser optics etc)

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The happy FACET team