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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)
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