Considerations on laser-p+ beam merging for AWAKE@CERN

CB, BG, PM

Some info/issues/questions• Minimum distance of mirror to IP is about 20 m, maximum distance is

about 50 m • Laser beam size as a function of S from plasma cell (done) – can change by

altering the laser b* at the plasma cell centre• Additional tolerances for position adjustment, stability etc. should be

small (maybe ~1 mm)• Entrance window between p+ and laser vacuum can be separated from

the p+ beamline and away from the turning mirror• Co-linearity is assumed between laser and p+ beam (could be angled to

help with merging but would then need more powerful laser – keep as ‘2nd choice’ option)

• Volume of the turning mirror and positioning system will need maybe 50 cm longitudinal free space in the lattice

• (Also of note: we should update the beam loss estimates to include instruments, commissioning, setting up and accident cases)

First ideas: resuse p+ beamline switch between TT40 and TT41

• Rotate everything 90 degrees to make vertical bend• In TT40, 8x MBSG magnets needed (3.0 m, ½ MBB length)• Total ‘merging bend’ length is 40 m (inside the quads). Too

long

• Combine merging and bends and triplet...• Separate vertical bends for RP considerations

QD

Basic ideas

MBA/B?MBB?

QF

Laser

QFMBA?QD

Triplet

Sagitta in MBB chambers

• With 2.1 T we have already a 25 mm offset at the exit of one MBB, 100 mm after two MBB, so at maximum only two MBA/B magnets possible (by offsetting the axis)

First attempt at merging region layout*• Use 3.0 m quadrupoles, 6.3 m MBA/B, 0.5 m inter-magnet

drifts, and 1.0 m drift for turning mirror• All quadrupoles centred on p+ beam axis• 0.6 degree total bend (need 3 more MBA/Bs for 2 degrees)• 22 m from mirror to exist of last triplet quad...

-100

-50

0

50

100

150

200

250

300

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

Y [m

m]

S [mm]

merging bend MBA+MBB at 6 mrad (1.5 T)

QFMBB

QD MBA

QF

MBB

MBBTriplet

Turning mirror

*upside down and back to front

P+ beamLaser beam

~40 mm

~40

mm

~30

mm

In Central triplet QD

In MBA

~30

mm

~100 mm

At turning mirror

~30

mm

~110 mm

Next steps

• Optics with such a triplet!– Required spot size– Drift distance to plasma cell entrance– Beam envelopes, apertures– Element strengths

• Add real laser envelope function and determine beam sizes along the line

• Will certainly need some iteration(s)

Other possibilities

• MBS type magnets – no H apertue limit for laser beam

• Problem is that only one spare exists – but anyway will need to design and build new ones for the switch from TT60!

MBS(G) magnets used to switch between beamlines – open C-core

MBS magnet cross-section

56 mm

MQA quadrupoles?• If needed, more aperture vertically• Also coil window between 190 and 300 mm, so passage at about 220 mm

comfortable – easier than QTL in terms of merging angle.• Horizontal gap between coils about 50 mm (45 inside chamber)