Updated MEIC Ion Beam Formation Scheme Jiquan Guo Nov. 5. 2015

Ion Complex (originally presented at fall coll. meeting) ~8GeV H+ ~2.7GeV Pb82+ 130MeV H- 42MeV Pb61+ <=100GeV H+ <=40GeV Pb82+ ion sources SRF Linac Booster with DC cooler collider ring with BB cooler Strip Multi-turn strip injection Bucket-to-bucket transfer Major bottleneck: with given aperture, the beam current in the booster ring or collider ring is limited by space charge tune shift, especially at the lowest energy (right after injection from the linac or the booster) Higher linac/booster extraction energy helps mitigating the SC tune shift and reduces the geometric emittance, but associated with higher cost. The efficiency of strip injection also has dependence on linac energy. Need to optimize to reach the performance goal with minimum cost Coasting beam bunched beam

collider ring with BB cooler collider ring with BB cooler Ion Complex (updated) ion sources SRF Linac Booster with DC cooler collider ring with BB cooler 130MeV H- ~42MeV Pb61+ (strip at 8.2MeV) ~8GeV H+ ~1.8GeV Pb61+ Multi-turn injection Strip for H No strip for ions Bucket-to-bucket transfer Ion Strip <=100GeV H+ <=40GeV Pb82+ Change of ion stripping Pb tuneshift in booster ↓ collider ring ↑ 1.8GeV energy might be too low H Strip ion sources SRF Linac Booster with DC cooler collider ring with BB cooler 130MeV H- ~40MeV Pb67+ (strip at ~13MeV after 2 QWR CMs) ~8GeV H- ~3.3GeV Pb67+ (12 GeV H equivalent) <=100GeV H+ <=40GeV Pb82+ Multi-turn injection Strip injection Pb tuneshift in booster ↓ collider ring → Optional changes

Ion Beam Formation Cycles (original) Step 7/8. Bucket-to-bucket transfer to the collider ring and BB cooling, repeat 26-36 times 2. Accumulating coasting beam 4. Accelerate to 7.9GeV and cool DC cooler BB cooler up polarized long bunches 2 empty buckets for gaps down polarized long bunches DC cooler 5. Bunch compression 3. Capture to bucket Eject the used beam from the collider ring, cycle the magnets Multi-turn injection of up polarized ion from linac to booster (non-polarized for heavy ions) Capture beam into a bucket (~0.7× booster ring circumference) Ramp to an intermediate energy (~2.0GeV proton), perform DC cooling, ramp to 7.9GeV (proton) Compress the bunch length to ~0.7/Nh0× of the collider ring circumference (Nh0 is the harmonic # for long bunches) Bucket-to-bucket transfer the long bunch into collider ring Repeat step 2-6 for (Nh0/2-1) times, each cycle ~1 min Switch to down polarized ions and repeat step 1-5 for another (Nh0/2-1) times, total cycles ~Nh0-2 Ramp collider ring to collision energy, perform bunch splitting to harmonic # Nh (~3400-3800) If needed, manipulate the beam to create several extra empty buckets (476 MHz) in the gap, as required by beam synchronization.

SC Tune Shift in the Booster at Different Injection Energy Particle in booster Pb82+ Pb67+ Pb61+ Booster ring circumference (m) 226.7 Collider ring circumference (m) 2267 (Nh=3600) Collider ring beam current (A) 0.5 1.0 Collider ring charge (µC) 3.78 7.56 Linac extraction energy (MeV/u) 42 53 100 39 69 58 Booster cycles 36 28 Booster nucleon charge (µC) 0.11 0.14 0.27 Normalized emittance, step 3 (µm) 0.95 1.07 1.47 0.92 1.23 0.96 1.13 Booster SC tune shift, step 3 0.15 6σ aperture, step 3 (mm) 39.8 39.6 39.9 40.0 Step 3 ΔνSC limited at 0.15 (won’t be affected by booster circumference) Beam stay-clear (6σ aperture) limited at 40mm. Required linac energy is much lower for Pb61+

Ion Collider Ring SC Tune Shift Particle in booster Pb82+ Pb67+ Pb61+ Booster ring circumference (m) 226.7 Collider ring circumference (m) 2267 (Nh=3600) Collider ring beam current (A) 0.5 1.0 Collider ring charge (µC) 3.78 7.56 Booster cycles 28 Booster extraction energy (GeV/u) 2.65 2.46 3.20 2.39 3.39 Equiv H extraction energy (GeV) 7.9 9.25 11.6 10.0 13.5 Booster ring charge (µC) 0.14 0.27 Normalized emittance, end of step 8 (µm) 1.47 0.92 1.23 0.96 1.13 Collider ring SC tune shift, end of step 8 0.12 0.17 0.15 6σ aperture, step 6-8 (mm) 11.7 14.2 11.5 12.0 11.9 11.4 Assume step 8 (after the collider ring is filled) emittance is no larger than step 3(after the booster ring bunch captured in one RF bucket).

Upgrade path With Pb67 from linac and no strip injection to booster, 0.5A beam current requires 40/130MeV linac and 9.25GeV proton equivalent energy for the booster. To upgrade to 1.0A, we need 70/212MeV linac and 11.6GeV booster. Ramp ratio: 130MeV to 9.25GeV=19.9 212MeV to 11.6GeV=18.8 130MeV to 11.6GeV=24.5? Design convention is 20X ramp ratio for magnets. Can the static field range ratio be higher? Increase # of booster bends by 25% when upgrading to 1.0A with linac energy upgrade to 70/200MeV? How about quadrupoles?

Pre-split in the booster 27 or 32×24 split scheme in the collider ring needs too many stages and requires too many cavities in the collider ring, may cause problems like impedance budget; some low frequency cavities (especially10-40MHz) might be too big. We might consider pre-split the bunch in the booster Split coasting/rectangular beam into any Nh is comparably easy; Gaussian beam needs multi-step splitting, 2× or 3× each time; 5× split is very hard to achieve.

Pre-split in the booster (option 0) booster ring DC cooler Collider ring Harmonic number: 10-90 BB cooler 49-405 up polarized long bunches 2-90 empty buckets for gaps 49-405 down polarized long bunches Capture/pre-split coasting beam into 10-90 long bunches, ~0.7C/Nh long ea 10× bucket-to-bucket transfers Example: booster ring 226.8 m, collider ring 2268 m, ring circumference ratio 10 booster cycles 10, booster ring Nh=90 (10 as shown), booster cycles 10 (-2×0.5 empty), collider ring Nh0=90x10=900 (32 as shown), after 2x2 splitting, Nh=3600 No compression, can’t increase booster cycles beyond 10, which limits the total beam current in the collider ring (~0.2A with 42MeV/u Pb61+ or 130MeV proton), unless we increase the linac energy to 285MeV for 0.5A, or 450MeV for 1A collider ring beam current.

Pre-split in the booster (option 1): barrier bucket booster ring DC cooler Collider ring BB cooler 420 up polarized long bunches 60 empty buckets for gaps 420 down polarized long bunches Compress coasting beam with barrier bucket to C/3 rectangular long bunch booster ring DC cooler 28× bucket-to-bucket transfers ×30 split Nh=90, Nb=30 booster ring 226.8 m, collider ring 2268 m, ring circumference ratio 10 Long bunch compressed to 1/3 circumference, then perform 30× split (Nh=90) booster cycles 30 (-2 empty), collider ring Nh0=30×30=900 initially, after 2×2 splitting, Nh=3600 Only need 2 extra frequencies (119MHz, 238MHz).

Pre-split in the booster (option 2): binary splits booster ring DC cooler Collider ring BB cooler 144 up polarized long bunches 12 empty buckets for gaps 144 down polarized long bunches After compression, ~0.7C/4 long bunch, switch Nh from 1 to 4 booster ring DC cooler 36× bucket-to-bucket transfers 2×2×2 split Nh=32, Nb=8 booster ring 242 m, collider ring 2268 m, ring circumference ratio 9.375 (300/32) Long bunch compressed to ~1/3 circumference, then perform 2×2×3 split to 12 bunches (Nh=36) booster cycles 37.5 (-1 empty), collider ring Nh0=12×25=300 initially, after 2×2×3 splitting, Nh=3600 Still needs the 6-7 RF frequencies for all the splitting

Pre-split in the booster (option 3) booster ring DC cooler booster ring DC cooler Harmonic number: 10 After capture, 70% After compression, 70% / 3 1st transfer 2nd transfer 3rd transfer One of 10 sections of the collider ring Length same as the booster ring circumference Yuhong Example: booster ring 226.8 m, collider ring 2268 m, ring circumference ratio 10 booster cycles 10x3 = 30 (minus 2 for the gap) booster ring Nh=10, collider ring Nh=30x10=300 initially, after 2x2x3 splitting, HM=3600 How to implement the 3× transfers without disturbing the previously injected beam? Slip stacking(SPS scheme ~50ns bucket size)? strip injection?

SPS Slip stacking Procedure Argyropoulos Two super-batches injected into the SPS: PS batch: 4 bunches spaced by 100 ns 6 PS batches injected into the SPS (batch space of 100ns) SPS super-batch: 24 bunches spaced by 100 ns (2.3 μs) The two super-batches are captured by the two pairs of 200 MHz TWC  independent beam controls are needed fRF variation to accelerate the first batch and decelerate the second Let the batches slip Bring them back by decelerating the first and accelerating the second Once the bunches are interleaved they are recaptured at average RF frequency

Pre-split in the booster (option 4): slip compression booster ring DC cooler booster ring DC cooler Harmonic number: 5 Capture with Nh=5, bunch length compressed to 0.7C/5 After compression, bunch length 0.7C/5/3 booster ring DC cooler BB cooler ~75 up polarized long bunches empty buckets for gaps ~75 down polarized long bunches Harmonic number: 15 28 Bucket to bucket transfers Slip compression in the booster Transfer to collider ring Example: booster ring 226.8m, collider ring 2268m, ring circumference ratio 10 booster Nh0=5, after slip compression Nh=15 booster cycle 30(-2empty), collider ring long bunch Nh0=5×30=150 (~50ns ea) after 3×23 splitting, Nh=3600 Can we achieve Nh=90 (30 before compression) in the booster?

Summary of Updates Strip injection of heavy ions into the booster is not practical. Booster extraction energy of lead ions will be lowered due to lower charge state in the updated scheme, space charge lower in the booster but worse in the collider ring. Needs to be compensated by stronger bending magnets with larger ramping ratio in the booster, or partially by increase linac charge state to 67. Change the booster to racetrack may help w/o increasing cost. 27 bunch split in the collider ring sounds very difficult. We may pre-split in the booster. Several options are available, but either have limitations or need extensive R&D.