Design and testing of the Beam Delivery System collimators for the International Linear Collider J. L. Fernandez-Hernando STFC/ASTeC Daresbury Lab

The International Linear Collider (ILC) will accelerate 2E10 electrons on one side, and 2E10 positrons on the other up to a center of mass energy of 1 TeV (500 GeV in the first period of operation) in their collision. The ILC will be complementary to the LHC and will allow to study “in detail” the new particles discovered at the LHC. The ILC collimation system is composed of a spoiler and an absorber. The collimator mission is to clean the beam halo from e- or e+ off orbit which could damage the equipment, but mainly to clean the beam from photons generated during the bending of the beam towards the Interaction Point. These photons, if not removed, would generate a noise background that would not allow the detectors to work properly.

a 0.6c0 Starting point Long, shallow tapers (~20mrad?), reduce short range transverse wakes High conductivity surface coatings Robust material for actual beam spoiling Long path length for errant beams striking spoilers Large c0 materials (beryllium…, graphite, ...) Require spoilers survive at least 2 (1) bunches at 250 (500) GeV Design external geometry for optimal wakefield performance, reduce longitudinal extent of spoiler if possible Use material of suitable resistivity for coating Design internal structure using in initial damage survey seems most appropriate.

[Details, see Eurotev Reports 2006-015, -021, -034] Spoilers considered include… 0.6c0 2.1010 e-, Ebeam=250 GeV, sxsy=1119mm2 also, Ebeam=500 GeV 2mm 335mrad 10mm Option 1: Ti/C, Ti/Be As per T480 Ti, Cu, Al Graphite regions Option 3: Ti/C Option 2: Ti/C, Cu/C, Al/C 0.3 Xo of Ti alloy upstream and downstream tapers 0.6 Xo of metal taper (upstream), 1 mm thick layer of Ti alloy [Details, see Eurotev Reports 2006-015, -021, -034]

∆Tmax = 870 K per a bunch of 2E10 e- at 500 GeV 2 mm deep from top Full Ti alloy spoiler 810 K 405 K 270 K 135 K ∆Tmax = 870 K per a bunch of 2E10 e- at 500 GeV σx = 79.5 µm, σy= 6.36 µm

Ti / Graphite Spoiler Ti C beam Temperature data in the left only valid the Ti-alloy material. Top increase of temp. in the graphite ~400 K. Dash box: graphite region. 540 K 405 K 400 K 270 K ∆Tmax = 575 K per a bunch of 2E10 e- at 500 GeV σx = 79.5 µm, σy= 6.36 µm 2 mm deep from top Ti alloy and graphite spoiler

Temperature increase from 1 bunch impact Summary of simulations Temperature increase from 1 bunch impact Exceeds fracture temp. 2mm depth 10mm depth 250 GeV e- 1119 µm2 500 GeV e- 79.56.4 µm2 Solid Ti alloy 420 K 870 K 850 K 2000 K Solid Al 200 K 210 K 265 K 595 K Solid Cu 1300 K 2700 K 2800 K 7000 K Graphite+Ti option 1 325 K 640 K 380 K 760 K Beryllium+Ti  option 1 - 675 K Graphite+Ti option 2 290 K 575 K 295 K 580 K Graphite+Al option 2 170 K 350 K 175 K 370 K Graphite+Cu option 2 465 K 860 K 440 K Graphite+Ti option 3 300 K Exceeds melting temp.

A final collimator design should minimise this effect. Wakefields deteriorate the beam quality. A final collimator design should minimise this effect. Studies on wakefields generated by different collimator geometries. Comparison to analytic predictions and simulations in order to improve both methods.

Beam Parameters at SLAC ESA and ILC Repetition Rate 10 Hz 5 Hz Energy 28.5 GeV 250 GeV Bunch Charge 2.0 x 1010 Bunch Length 300 mm Energy Spread 0.2% 0.1% Bunches per train 1 (2*) 2820 Microbunch spacing - (20-400ns*) 337 ns *possible, using undamped beam

~40 m 2 doublets ~15 m 2 triplets BPM BPM BPM BPM Vertical mover Wakefield measurement: Move collimators around beam (in steps of 0.2 mm, from -1.2 mm to +1.2 mm, being 0 mm the centre of the collimator). Measure deflection from wakefields vs. beam-collimator separation

~40 m 2 doublets ~15 m 2 triplets BPM BPM BPM BPM Vertical mover Wakefield measurement: Move collimators around beam (in steps of 0.2 mm, from -1.2 mm to +1.2 mm, being 0 mm the centre of the collimator). Measure deflection from wakefields vs. beam-collimator separation

Col. 1 Col. 6 Col. 3 L=1000 mm Col. 12 a = 324 mrad r = 2 mm (r = ½ gap) a = 166 r = 1.4 mm Col. 6 Col. 12 a = 166 mrad r = 1.4 mm L=1000 mm Col. 3 a = 324 mrad r = 1.4 mm

1Assumes 500-micron bunch length Coll. Measured4 Kick Factor V/pC/mm (c2/dof) Linear + Cubic Fit Analytic Prediction1 V/pC/mm 3-D Modelling Prediction2 1 1.2 ± 0.3 (1.0) 2.27 1.7 ± 0.37 2 1.2 ± 0.3 (1.4)/ 1.3± 0.6 (1.0) 4.63 3.1 ± 0.84 3 3.7 ± 0.3 (0.8) 5.25 7.1 ± 0.94 4 0.5 ± 0.4 (0.8) 0.56 0.8 5 4.9 ± 0.2 (2.6) 4.59 6.8 6 0.9 ± 0.3 (1.0) / 0.7 ± 0.2 (0.9) 4.65 2.4 ± 1.14 7 2.2 ± 0.3 (0.5) 2.7 ± 0.53 8 1.7 ± 0.3 (2.2) 2.4 ± 0.89 10 1.1± 0.2 (2.2) 11 2.5± 0.3 (0.9) 12 1.5± 0.2 (1.1) 14 2.6 ± 0.4 (1.0) )/ 2.3± 0.3 (1.0) 1Assumes 500-micron bunch length 2Assumes 500-micron bunch length, includes analytic resistive wake; modelling in progress 3Kick Factor measured for similar collimator described in SLAC-PUB-12086 was (1.3 ± 0.1) V/pC/mm 4Still discussing use of linear and linear+cubic fits to extract kick factors and error bars L=1000 mm

Material damage test beam at ATF x y reference pin hole guide channels low mass mounting Cu Ti target area 10mm The purpose of the first test run at ATF is to: Make simple measurements of the size of the damage region after individual beam impacts on the collimator test piece. This will permit a direct validation of FLUKA/ANSYS simulations of properties of the materials under test. Allow us to commission the proposed test system of vacuum vessel, multi-axis mover, beam position and size monitoring. Validate the mode of operation required for ATF in these tests. Ensure that the radiation protection requirements can be satisfied before proceeding with a second phase proposal. Assuming a successful first phase test, the test would be to measure the shock waves within the sample by studying the surface motion with a laser-based system, such as VISAR (or LDV), for single bunch and multiple bunches at approximate ILC bunch spacing. sample holder Bunch sxsy (mm2), material Estimated damage region, x Estimated damage region, y Estimated damage region, z 1.90.5, Ti alloy 11 (14) mm 4 (5.6) mm 5 (8) mm 202, Ti alloy 45 (90) mm 5 (9) mm 2 (7) mm 202, Cu 65 (100) mm 7 (10) mm 3 (7) mm

A similar test done in SLAC FFTB gave the results that can be seen in the bottom left plot of this section. Results of a FLUKA simulation using same beam and target specification can be seen in the bottom right plot of this section. There is a systematic divergence of ~100 µm2 but both plots agree in the slope. [Measurements c/o Marc Ross et al., Linac’00]

Resulting mechanical stresses examined with ANSYS3D Summary & Future Plans Resulting mechanical stresses examined with ANSYS3D Continue study into beam damage/materials Experimental beam test to reduce largest uncertainties in material properties Study geometries which can reduce overall length of spoilers while maintaining performance Means of damage detection, start engineering design of critical components Combine information on geometry, material, construction, to find acceptable baseline design for Wakefield optimisation Collimation efficiency Damage mitigation