Linac optimisation for the New Light Source NLS source requirements Electron beam requirements for seeded cascade harmonic generation LINAC optimisation (2BC vs 3 BC) CSR issues energy chirp issues jitter (preliminary studies at 200 pC and 500 pC) FEL simulations (time dependent SASE cascade harmonic) R. Bartolini Diamond Light Source and John Adams Institute, Oxford University FEL Beam Dynamics Meeting DESY, 13 July 2009 NLS Source requirements (Dec 08) 1. Photon energy range and tunability: Three FELs: FEL1 @ 50-300 eV; FEL2 @ 250-850 eV; FEL3 @ 430-1000 eV 2. Polarisation: FEL1&FEL2: complete polarisation control (arbitrary elliptical and rotatable linear). FEL3: at least horizontal and circular (R/L) polarisation over the full range 4301000 eV. 3. Pulse length & pulse energy: 20 fs FWHM photon pulse length at all photon energies with 1011 photons/pulse at 1 keV (upgrade path to sub-fs pulses) 4. Repetition rate: 1 kHz with an upgrade path to 100 kHz (or more) 5. Transverse and longitudinal coherence

Electron beam requirements Operation of an X-ray FEL requires extremely high quality electron beams; Energy Emittance (normalised) relative energy spread Peak current 1-few GeV 106 m 104 few kA for seeded cascade harmonic generation constant slice parameters on a length of 100 fs and more? to accommodate the seed pulse and the jitters without accidental good slices that would spoil the contrast ratio no jagged current distribution no slice offset and angle not too sensitive to jitters no residual energy chirp (or very limited) Carefully optimised RF photocathode gun and LINAC with magnetic bunch compressors can provide high brightness electron beam, but satisfying all the requirements is non trivial Layout and parameters of a 2.2 GeV Linac based FEL 2.2 GeV; 200-500 pC

optimised L-band gun: 20 ps FW, 0.33 m (proj. nor. emit.) see J-H. Hans talk LINAC L-band gradient 20 MV/m and 3HC @ 3.9 GHz 15 MV/m Magnetic bunch compressors Beam spreader see D. Angal-Kalinins talk Undulator train see also N. Thompsons talk on FEL harmonic schemes Gun A01 A02 A03 Gun A01 A39 BC0 A02 A03 BC1 A04 A05 A06 A07 A08 Gun A01

A39 BC0 A02 A03 A39 BC1 Gun A01 A02 A39 A39 BC1 BC1 A04 A05 A06 A07 A08 BC2 A09 A10 A11 A12 A13

BC2 A04 A05 A06 A07 A08 A03 A04 A05 A06 A07 A08 A14 A09 A10 A11 A12 A13 BC2 undulator s DL A14 A09 A10 A11 A12 A13 A09 A10 A11 A12 A13 A14 DL

undulator s DL A14 undulator s DL undulator s FEL working point Working point defined by: minimum allowable undulator gap at 430 eV (8 mm) saturation length at 1 keV requirements on K (K > 0.7) and beam quality Using Xie parameterisation with Ipeak = 1 kA, n = 1m, = 5104 430 eV 1 keV u = 32 mm; g = 8 mm u = 32 mm; g = 14.4 mm

K = 1.56; < > = 5 m K = 0.67; < > = 10 m Numerical optimisation of a 2.2 GeV LINAC (I) Gun A01 A02 A03 A39 BC1 A04 A05 A06 A07 A08 Astra Gun A01 BC2 A09 A10 A11 A12 A13 DL GENESIS

elegant A39 BC0 A02 A03 BC1 A04 A05 A06 A07 A08 Layout 1 (2BCs) undulator s BC2 A09 A10 A11 A12 A13 A14 DL undulator

s Layout 2 (3BCs) Tracking studies to optimise the beam quality at the beginning of the undulators peak current, slice emittance, slice energy spread, etc elegant simulations include CSR, longitudinal space charge, wake-fields in TESLA modules Parameters used in the optimisation Accelerating section and 3HC amplitude and phase, Bunch compressors strengths Validation with full start-to-end simulation Gun to FEL (time dependent) Numerical optimisation of a 2.2 GeV LINAC (II) Main issues and guidelines for the optimisation compression should not introduce space charge and CSR issues avoid strong compression at lower energy linear optics tailored to reduce CSR (e.g. minimum horizontal beta at 4th dipole) compression aided by linearisation of longitudinal phase space with a 3HC

analytical formulae for 3HC setting, microbunching gain curves, beam quality optimisation driven directly by the required FEL performance We have devised a multi-parameters multi-objectives optimisation of the LINAC working point based on the Xie parameterisation (semi-analytical expressions) for the gain length and the FEL saturation power. We target gain length, FEL saturation power as in a SASE FEL and due to the additional complication with seeding simultaneous optimisation of many slices to achieve a flat portion of the bunch (length 100 fs) with constant slice parameters Numerical Optimisation of a 2.2 GeV LINAC (III) We have adopted a multi-objectives multi-parameters optimisation of the LINAC based on a fast numerical computation of the Xie gain length vs LINAC parameters Xie gain length L3 D L3 D ( x , , x ,...) Compute the slice properties x ,, x, from elegant L3 D L3 D (V1 ,...,Vn , V3 HC , 1 ,...,n , 3 HC , 1 , 2 ,...) Objectives: L3D, Psat AND length of the good slice region (compression strength, )

Parameters: amplitudes and phases of RF accelerating sections, compressors, Parallel Search Algorithms (Genetic Algorithm SPEA) Multi-objective multi-parameter optimisation 18000 runs with 100K particle each 2 objectives: minimise Xie Length and maximise and Psat 4 parameters: phase of ACC02; ACC4-7, BC1, BC2 Manual optimisation (red dot) xie length 1.24 (m) avg sat power 2030 (MW) (2, 3, 2, 4) = (17.5, 6.1, 7.75, 25) Multi-objective optimisation xie length 1.15 (m) (8%) avg sat power 2220 (MW) (+11%) (2, 3, 2, 4) = (17.54, 4.92, 8.96, 29.91) The Xie length is averaged over the slices covering a portion of 100 fs of the bunch Optimised beam from the L-band injector L-band NC gun optimisation ongoing Longitudinal phase space ASTRA to deal with space charge issues 200 pC 20 ps FW 130 MeV Normalised slice emittance below 410-7 m Slice relative energy spread below 2 10-5

I peak 15 A Slice normalised emittance Longitudinal current distribution Beam properties along the LINAC: layout 1 (2BCs) Gun before 3HC A01 A02 A03 A39 BC1 A04 A05 A06 A07 A08 BC2 A09 A10 A11 A12 A13 DL undulator s before undulators

after BC1 50 fs (rms) 200 fs BC2 6.95 deg; best slices Ipeak 1.7 kA, n 0.4 m, 104; 50 fs (rms) 180 8000 160 7000 140 peak current (A) bunch length rms (fs) CSR issues and compression optimisation (BC2) 120 100 80

60 40 6000 5000 4000 3000 2000 1000 20 0 0 6.4 6.6 6.8 7 7.2 7.4 7.6

7.8 BC2 compressor angle (deg) 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 BC2 compressor angle (deg) normalised H emittance (um) 120 over compressed

100 80 60 under compressed working point 40 20 0 6.4 6.6 6.8 7 7.2 7.4 BC2 compressor angle (deg)

7.6 7.8 10 e- bunches superimposed FEL Time dependent S2E simulations u = 31.9 mm; g = 14.4 mm; K = 0.67 These offsets in position and angle kill the gain 70 m and 30 rad Trajectory of centre of charge corrected Trajectory of each slice corrected Lsat 50 m Lsat 36 m Issues and alternative layouts The main issue with the 2BC layout is the need to reduce 3HC gradient (40 MV/m): Several attempts were made

generating a shorter (15 ps FW) and more linear bunch from the gun and/or add on more 3HC module (FLASH Type with 4 cavities per modules) Gun A01 A02 A03 A39 A39 BC1 A04 A05 A06 A07 A08 BC2 A09 A10 A11 A12 A13 A14 DL undulator s or add on more (weak) bunch compressor

Gun A01 A39 BC0 A02 A03 BC1 A04 A05 A06 A07 A08 or other locations of BCs BC2 A09 A10 A11 A12 A13 A14 DL undulator s

Beam properties along the LINAC: layout 2 (3BCs) Gun A01 A39 BC0 A02 A03 135 MeV 110 MeV 18 ps 10 ps BC1 A04 A05 A06 A07 A08 BC2 A09 A10 A11 A12 A13 440 MeV

1.2 GeV 2 ps 300 fs A14 DL undulator s 3HC at 3.9 GHz can work at 15 MV/m; No need of a second 3HC before second BC; nx 4.310 7 Numerical issues with CSR 100k; 500 bins 1M; 500 bins Significant Modulation with period 7 fs 2 um Compression factor 70

500 fs at the cathode 100k; 100 bins 1M; 100 bins 200 pC optimised case; 100k particles; NBINS = 500; jagged longitudinal phase space Still OK for SASE but too spiky(?) for HGHG Numerical issues with CSR Sources of density fluctuations shot noise; external causes (e.g. laser flat top,) Energy chirp issues (I) Energy chirp should be smaller than the SASE intrinsic BW ( = 10-3) 200 100 0 Time (fs) 100

/ = 3.2 10-3 3 times larger than the SASE BW 200 Comparing to numerical simulations at LCLS-FERMI wakefields were crucial in flattening the residual chirp. Larger energy helps (large and longer linacs) Energy chirp issues (II) use wakefields in Lband structures wakefields are weaker than in S-band 200 pC too small use the main RF to reduce chirp accelerating beyond crest bunch too short after 3BC (RF slope sampled is too small) L-band has a smoother curvature than S-band use less chirp from the beginning / = 1.6 10-3 reduced but larger compression ! 200 100 0 Time (fs) 100

200 C-type chicanes 3D gain length with beam offset and angle Using genesis SS simulations adding H offset, V offset, H angle, V angle Trying to parameterise as per Xie formalism (normalis. emitt. x = 0.4 um, relative energy spread 2e-4, I peak = 2 kA) Angle dependence is very critical ! (tbc) S-type chicanes 200 pC best optimisation 100 fs Cascade Harmonic Scheme output Power @19.3m Spectrum @19.3m z3.2m Power @19.3m

/4e-4 Time-bandwidth product 1 Contrast ratio 15 Jitter studies The FEL performance can be severely spoiled by jitter in the electron beam characteristics To understand this issue we have started a numerical investigation of the sensitivity of the beam quality to various jitter sources: phase and amplitude of RF sections bunch compressor power supplies Adding the jitter sources one-by-one and all together as random noise Jitter in the GUN was also included gun phase and voltage solenoid field charge laser spot position jitter FEL time dependent simulations with 3BC layout (multi-parameter optimisation) overcompressed Best slice: x = 4107 m ; = 8105 ; Ipeak = 1.6 kA;

along 100 fs or more with a flat plateau (typical for overcompressed bunches) Lsat 45 m 200 pC jitter Gun Jitter Parameters (standard deviations) Solenoid Field Gun Phase Gun Voltage Charge X Offset 0.02e-3 0.15 0.05 1% 0.025 T degrees MV/m mm Arrival Time Jitter at end of LINAC (standard deviation, fs) Phase (P) Bunch Compressor (B)

Voltage (V) Gun (G) P+V+B P+V+B+G 0.01 degrees 5e-5 fractional 1-e4 fractional 9 11 14 23 18 28 200pC Beam profiles with jitter Beam jitter causes from the LINAC RF Applying jitter errors in the amplitude and phase of the RF station one by one we can determine the impact of each accelerating section on the beam parameters 100 Jitter The plots refer to the extreme case of 0.1 degrees phase jitter and 0.1% voltage jitter to

RMS energy [MeV] 10 RMS projected emittance [mm.mrad] RMS time of flight [fs] 1 0.1 0.01 14 13 12 11 10 9 8 BC3

7 6 5 4 BC2 3 2 BC1 RMS energy [MeV] 100 RMS projected emittance [mm.mrad] RMS time of flight [fs] 10 1 0.1

0.01 RF station 14 13 12 11 10 9 BC3 8 7 6 5 4

BC2 3 0.001 2 RF station BC1 RF stations near the start of the linac contribute a large amount to jitter Concentrate on minimising jitter in RF stations between first two bunch compressors but cannot neglect contribution from structures between bunch compressors 2 and 3 Cavities after 3BC play little role Jitter

0.001 Conclusions and future work (I) We have produced a promising layout of a single-pass LINAC which can deliver an electron beam with the required properties with modest improvements on the present technology requirement on 3HC and RF jitter to be further analysed SASE FEL performance achieved exceeds the 1011 ppp @ 1 keV Alternative FEL operating mode under consideration (Single Spike; different charge) Wakefield effects and jitter issues under consideration along with a thorough tolerance analysis; Finalise layout choice: continue the evaluation of the 3BC vs 2BC layouts collaboration with LBNL (revisiting our 1BC design) Acknowledgments: C. Christou, J.H. Han, I. Martin, J. Rowland (DLS) D. Angal-Kalinin, F. Jackson, B. Muratori, P. Williams (ASTeC) and the NLS Physics and Parameters Working Group Optimisation for the operation at 500 pC We consider also a longer electron pulses with constant slice parameters with at least 200 fs flat top (in view of jitter results) Exit of spreader section (500 pC) > 200 fs Effect of the laser heater (500 pC) The jagged longitudinal current profile is smoothed by the laser heater Laser heater on

Still < 2104 Emittance is unaffected 500 pC jitter Gun Jitter Parameters (standard deviations) Solenoid Field Gun Phase Gun Voltage Charge X Offset 0.02e-3 0.075 0.025 1% 0.025 T degrees MV/m mm Arrival Time Jitter (standard deviation, fs) Phase (P) Bunch Compressor (B) Voltage (V)

Gun (G) P+V+B P+V+B+G 0.01 degrees 5e-5 fractional 1-e4 fractional 8 17 27 56 32 61 Requirements on the gun is even more stringent than in the 200 pc case 500pC Beam profiles with jitter FEL Scheme (N. Thompson) Modulator 1 HHG 75-100eV w = 44 mm e- @ 2.25 GeV Modulator 1 HHG 75-100eV w = 44 mm e- @ 2.25 GeV HHG 50-100eV

e- @ 2.25 GeV Modulator w = 49 mm Modulator 2 w = 44 mm Modulator 2 w = 44 mm APPLE-II Radiator w = 56.2 mm APPLE-II Radiator w = 32.2 mm APPLE-II Radiator w = 38.6 mm FEL3 430 - 1000eV FEL2 250-850eV FEL1 50-300eV - common electron energy for all 3 FELs, allows simultaneous operation - seeded operation for longitudinally coherent output

- HHG seeding with realistic laser parameters, up to 100 eV (100kW at 100 eV is realistic) (100 kW at 200 eV seeding level proved insufficient) - harmonic cascade FEL scheme to reach up to 1 keV in the fundamental

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