Chad Mitchell Lawrence Berkeley National Laboratory LCLS-II Accelerator Physics Meeting Aug. 19, 2015

Transcription

1 Layout Op*ons and Op*miza*on for LCLS- II Injector Design Chad Mitchell Lawrence Berkeley National Laboratory LCLS-II Accelerator Physics Meeting Aug. 19, 2015

2 Mo*va*on The distance between the second solenoid and the first accelera*ng cavity was increased (in May) by 21 cm to make room for standard cryomodule endcaps, resul*ng in a deteriora*on of beam emijance to ε xn > 0.65 μm (for 300 pc). This, together with a small stay- clear ra*o HA/σ x < 3, mo*vated the considera*on of shorter layouts with larger apertures, leading to a redesign of solenoids/bpms. Both manual tuning and global op*miza*on indicate that solu*ons can now be obtained with projected emijance ε xn < 0.5 μm with acceptable HA/σ x > 4. Ques*ons to be resolved: a choice between short layout op*ons, tradeoff between beam emijance and peak cavity fields/final beam energy. All solu*ons shown are for 300 pc with 1 μm/mm thermal emijance at the cathode.

3 Recent LCLS- II Injector Layout Op*ons 1. Baseline layout with APEX solenoids (as of 6/17/2015) increased 21 cm 2. Short layout with LBNL large- bore solenoids (as of 7/2/2015) 46 cm shorter than baseline. 3. Revised layout with LBNL large- bore solenoids (as of 7/31/2015) 15 cm shorter than baseline.

4 LBNL Large- Bore Solenoid Design New large bore design similar to Cornell field profile length: 17.5 cm shorter than exis*ng aperture diameter: mm B 0 = B z max I 1 (cm) I 2 (m -1 ) q f (m -2 ) I 1 = z2 z 1 2 B z dz B 0 APEX nominal Cornell nominal I 2 = z2 z 1 1 B 0 B z z 2 dz APEX: LB with caps APEX: LB no caps q f = f = I 2 2I 1

5 Predicted EmiJance Contribu*on (in SOL1) Predicted contribu*on to slice emijance in solenoid 1. effect of removing the endcaps Predic*on: ε x,n = ε NOSC x,n 1 (1 ν) q f a 2 where 3s 8(1+ s 2 ) 5 / 2 s = γl /2a ν = f 0 / f beam aspect ra*o space- charge detuning NOSC ε x,n βγ = κασ x 4 = κq f f 0 4 σ x predicted emijance without space- charge

6 Pareto Fronts at 300 pc: Short Layouts 2&3 are Preferred Pareto front (10K): - energy constraint removed - cavi*es 2-3 off - constrained bunch length - final energy varies MeV emi$ances near 0.45 μm at high resolu6on

7 Example: Layout 2 Solu*on (250K resolu*on) Final emi$ance: 0.44 μm Final beam energy: 84.5 MeV Peak field in CAV1: 15.9 MV/m Maximum field in CAV 4-8: 30.5 MV/m HOM spread: 11.4 kev/c Beam energy can be increased by increasing the fields in CAV 4-8. HA/σ x > μm

8 Considera*ons Regarding Cavity Accelera*ng Gradients Op*miza*on pushes for solu*ons with cavi*es 2, 3 off with a low peak field in cavity 1 (15-16 MV/m) for emijance compensa*on. To achieve 100 MeV under these condi*ons requires cavi*es 4-8 to be run on- crest near the maximum allowed peak field (33.5 MV/m corresponding to 16.8 MV/m average accelera*ng gradient). Possible op*ons: 1) Run with CAV 2-3 off and with CAV 4-8 at the maximum gradient. 2) Run with CAV 2-3 off and reduce peak fields in CAV 4-8, reducing beam energy. 3) Run with CAV 3 on and reduce peak fields in CAV 4-8. All of these are being explored.

9 EmiJance Compensa*on: Matching into Cavity 1 Criteria for matching onto the invariant envelope at the booster entrance*: 0 x,y =0, x,y = 2 0 waist Parameters: I pk =27 A I A = ka γ = σ x 2 mm apple Ipk hgi 3 I A geometrical factor g 1 E 0 15 MV/m 1/2 0 ee 0 2mc 2 sin = /2 on crest A preferred gradient in cavity 1 is evident among the Pareto- op*mal solu*ons for each layout. SoluAons on the Pareto front *L. Serafini and J. Rosenzweig, Phys. Rev. E 55, 7565 (1997); M. Ferrario et al., SLAC- PUB (2000).

10 Low Gradient is Preferred in Cavity 3 Baseline (Layout 1) + Short (Layout 2) x Revised (Layout 3) x Op*miza*on is pushing toward low peak field in cavity 3 (< 5 MV/m). SoluAons on the Pareto front Pareto front (10K): - energy constraint removed - cavity 3 tunable - constrained bunch length

11 Pareto Fronts at 300 pc: Sensi*vity to Peak Field Sepng in Cavi*es 4-8 Baseline layout: E pk = 30.5 MV/m + Baseline layout: E pk = 33.5 MV/m x MeV Modifying field gradients in CAV 4-8 by 10% has small effect on Pareto front, modifying emijance values < 5% MeV Pareto front (10K): - energy constraint removed - cavi*es 2, 3 off - vary peak field in CAV constrained bunch length

12 Exploring Solu*ons with Accelera*on in Cavi*es 2-3 Peak field in CAV 2 (MV/m) Peak field in CAV 3 (MV/m) Final beam Energy (MeV) Final horizontal beam emijance (μm) SoluAons with higher fields in cavity 3 are being explored. E pk 30.5 MV/m 90 MeV solu6on Final emi$ance: 0.46 μm

13 Summary and Remaining Ques*ons Op*miza*on confirms that effec*ve emijance compensa*on requires a moderate peak accelera*ng field in cavity 1 (~15 MV/m) and low fields in cavi*es 2, 3 (< 5 MV/ m). Op*miza*on confirms that both short layouts using the LBNL large- bore design can produce much smaller emijances (< 0.45 μm) than the long baseline design. Exis*ng low- emijance solu*ons have either 1) peak accelera*ng fields near the maximum value allowed, or 2) final beam energies below 100 MeV. How important is it to find solu*ons with smaller fields in cavi*es 4-8? While op*miza*on indicates that near- zero accelera*ng field in CAV 3 is preferred, emijance values may not be too sensi*ve to this value. Can the field in cavity 3 be scaled up to provide 100 MeV without deteriora*on of emijance?

14 LCLS-II Injector Optimization Parameters (6/23/2015) Layout based on H. Alvarez "LCLSII_Injector_Layout_ jpg". INITIAL DISTRIBUTION Transverse profile: truncated Gaussian (1 sigma) Longitudinal profile: plateau Thermal emittance: 1 micron/mm Sigma_x (mm) [0.05,2] FWHM (ns) [0.01,0.075] Rise time (ns) LATTICE Data File Midpoint (m) Peak Field (MV/m, T) RF Phase (deg) Length (m) RF Gun 0 20 [-20,20] Solenoid 1 (APEX) [0.01,0.2] N/A 0.8 Buncher (2-cell) *[0,4] [-150,-30] Solenoid 2 (APEX) [0.01,0.2] N/A 0.6 Cavity [0,30.5] [-60,60] Cavity Cavity Cavity [0,30.5] [-60,60] Cavity Cavity Cavity Cavity [0,30.5] [-60,60] LCLS- II Injector opqmizaqon parameters: Baseline layout (nominal APEX solenoids) CAV 2,3 off OBJECTIVES RMS emit_x (micron) Sig_z (mm) Evaluated 15 m downstream from cathode. CONSTRAINTS Max. emit_x (micron) 3 Max. z_rms (mm) 4 Max. E_rms (kev) 200 Max. HOM spread (kev) (relaxed) Relative energy error 4.00E-03 Target beam energy: 100 MeV

15 Pareto Fronts (300 pc): Baseline and Shortened Layouts nominal HOM < 15 kev/c relaxed HOM < 20 kev/c fully converged (10K resoluaon)

16 EmiJance Compensa*on: Matching into the Booster Slice rms envelope equa*on with accelera*on and focusing: ˆ00 + apple 2 2 ˆ apple s ˆ 2 n ˆ3 =0 ˆ = apple s = I/2I A x,y p invariant envelope ˆ = r apples apple, ˆ0 =0 Criteria for matching onto the invariant envelope at the booster entrance: 0 x,y =0, x,y = 2 0 waist apple Ipk hgi 3 I A geometrical factor g 1 1/2 0 ee 0 2mc 2 sin = /2 on crest L. Serafini and J. Rosenzweig, Phys. Rev. E 55, 7565 (1997); M. Ferrario et al., SLAC- PUB (2000).

17 EmiJance Compensa*on: Matching into the Booster Accelera*ng from 750 kev to 100 MeV with CAV2, CAV3 off (6 cavi*es only) requires: 0 = 6Lacc = m 1 E MV/m All cavi*es must be operated nearly on- crest at maximum gradient, and at the entrance to CAV1, matching requires: I pk =27 A I A = ka γ = x,y = 2 0 apple Ipk hgi 3 I A 1/2 =0.98 mm waist. This requires a strongly converging beam out of solenoid 2; typical beam size awer solenoid 2 is closer to 2 mm, which would require in CAV1: 0 = 2 x,y apple Ipk hgi 3 I A 1/2 = m 1 E MV/m L. Serafini and J. Rosenzweig, Phys. Rev. E 55, 7565 (1997); M. Ferrario et al., SLAC- PUB (2000).