CLARA (Compact Linear Advanced Research Accelerator)at Daresbury Laboratory
is proposed to be the UK's national FEL test facility. The accelerator will be
a ~250 MeV electron linac capable of producing short, high brightness electron
bunches. The machine comprises a 2.5cell RF photocathode gun, one 2 m and three
5 m normal conducting S-band (2998MHz) accelerating structures and a variable
magnetic compression chicane. CLARA will be used as a test bed for novel FEL
configurations. We present a comparison of acceleration and compression schemes
for the candidate machine layout.Comment: 3 pages, 5 figures, IPAC 201

Jones, James K.

McKenzie, Julian W.

Williams, Peter H.

English

arXiv

Williams PH

Jones JK

McKenzie JW

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COMPARISON OF COMPRESSION SCHEMES FOR CLARAP. H. Williams∗, J. K. Jones & J. W. McKenzieSTFC Daresbury Laboratory, ASTeC & Cockcroft Institute, UKAbstractCLARA (Compact Linear Advanced Research Acceler-ator) at Daresbury Laboratory is proposed to be the UK’snational FEL test facility. The accelerator will be a ∼250 MeV electron linac capable of producing short, highbrightness electron bunches. The machine comprises a 2.5cell RF photocathode gun, one 2 m and three 5 m normalconducting S-band (2998 MHz) accelerating structures anda variable magnetic compression chicane. CLARA will beused as a test bed for novel FEL configurations. We presenta comparison of acceleration and compression schemes forthe candidate machine layout.INTRODUCTIONThe design approach adopted for CLARA is to build inflexibility of operation and layout, enabling as wide an ex-ploration of FEL schemes as possible. For a full overviewof the aims of the project and details of FEL schemes underconsideration see [1]. To this end a range of possible accel-erator configurations have been considered, a selection ofthis work is presented here.A major aim is to be able to test seeded FEL schemes.This places a stringent requirement on the longitudinalproperties of the electron bunches, namely that the sliceparameters should be nearly constant for a large proportionof the full-width bunch length. In addition, the intentionis that CLARA has the ability to deliver high peak cur-rent bunches for SASE operation and ultra-short pulse gen-eration schemes, and low-emittance velocity compressedbunches. This flexibility of delivering tailored pulse pro-files will allow a direct comparison of FEL schemes in onefacility.ENERGY AT MAGNETIC COMPRESSORA large proportion of the FEL schemes under consider-ation require small correlated energy spread at the undu-lators, therefore when magnetic compression is to be usedthe compressor must be situated at substantially less thanfull energy. This ensures that the chirp needed at com-pression is able to be adiabatically damped or suppressedthrough running subsequent accelerating structures beyondcrest. This requirement must be balanced against the factthat compressing at low energy exacerbates space-chargeeffects. To quantify this we use the laminarity parameterρL ≡(I/(2IA)ǫthγγ′√1/4 + Ω2)2,∗ peter.williams@stfc.ac.ukwhere I is the current in the slice under consideration,IA is the Alfven current, ǫth is the thermal emittance,γ′≡ dγ/ds and Ω is a solenoidal focusing field (zero inour case). When this parameter is greater than 1, we shouldconsider space-charge effects in the bunch evolution. To in-form this we select two candidate configurations, one withmagnetic compression at 70 MeV and one at 130 MeV. Wetrack a candidate 200 pC bunch through both configura-tions, setting the machine parameters attempt to produce azero chirp bunch of peak current 350 A at 250 MeV. Track-ing is carried out with ASTRA [2, 3] to the exit of the firstlinac section to include space-charge, followed by ELE-GANT [4] taking into account the effect of cavity wake-fields, longitudinal space-charge and coherent synchrotronradiation emittance dilution. Fig. 1 shows the resultantlaminarities and final bunch longitudinal phase spaces. WeFigure 1: (Upper) Laminarity (black / blue) and peak cur-rent (red / green) in the 10% of charge slice containing thepeak current with compression at 70 MeV / 130 MeV (thegun and first 2 m linac module are not shown). (Lower left)Long. phase space with compression at 130 MeV. (Lowerright) Long. phase space with compression at 70 MeV.see that in both cases space-charge should be considered,this will be achieved by re-tracking the magnetically com-pressed bunches in ASTRA to elucidate any deviations dueto 3-d effects as compared to the purely 1-d effects includedin ELEGANT. However compression at 130 MeV does notallow us to subsequently de-chirp the bunch, note that wego no further than 30◦ beyond crest in the final acceleratingstructures in order to avoid large jitter effects. As we wishthe facility to be flexible we select a nominal compressorenergy of 70 MeV, but we achieve this by reducing the gra-Table 1: Specification of Variable Bunch CompressorValue UnitEnergy at compressor 70 - 150 MeVMin. : Max. bend angle 0 : 200 mradBend magnetic length 200 mmMax. bend field 0.5 TMin. : Max. transverse offset 0 : 300 mmZ-distance DIP-01/04 - DIP-02/03 1500 mmZ-distance DIP-02 - DIP-03 1000 mmMax. bellows extension 260 mmMin. : Max. R56 0 : -72 mmMax. σx from δE (±3σ) 0 : 10 mmMax. σx from βx (±3σ) 1.5 mmdient in the accelerating structure before the compressor.This gives us the option of compressing at higher energy inregimes where a de-chirped bunch is not required. With theabove considerations in mind we define engineering spec-ifications for the CLARA variable bunch compressor asshown in Table 1. For flexibility, the compressor has a con-tinuously variable R56 and is rated for maximum energy of150 MeV. The ability to set a straight through path also al-lows investigation of purely velocity compressed bunches.BUNCH FOR SEEDING SCHEMESA seeded FEL scheme requires constant bunch parame-ters over a large proportion of the bunch. This reduces thesensitivity to timing jitter between the seed laser and elec-tron bunch. Specifically, we require a constant peak currentof 350 A over 300 fs of the bunch, with zero chirp, constantemittances and zero transverse offset. In order to achievethis we must cancel the curvature that originates from RFacceleration. It is possible to do this purely magnetically,although typically it is done with higher harmonic RF. Asharmonic RF entails additional expense we compare twoschemes, a bunch compressor with non-linear elements in-serted, and a fourth harmonic X-band cavity.Linearisation via Nonlinear ChicaneThe lower right plot of Fig. 1 shows residual curvaturein the longitudinal phase space. This can be flattened bychanging the sign of the natural T566 term in the bunchcompressor chicane. To achieve this sextupoles were addedto the chicane. The number, positions and strengths ofthese were parameters of an optimisation. We impose theconstraints that the T566 < 20 cm, the derivative of dis-persion with respect to energy and it’s derivative with re-spect to s should be zero on exit of the chicane, the pro-jected emittances should not exceed 1 mm mrad and thesextupoles k2 < 2000 m−2. This constraint set was chosenby trial and error. Figure 2 shows the optimisation results.Flattening the longitudinal curvature is relatively straight-forward however the chromatic properties are easy to spoil,0 1 2 3 4 5 6-200020040060080010001200s @mDWx,y@mD,dΗxd∆@cmD0 1 2 3 4 5 6-20-15-10-50510s @mDR56@cmD,T566@cmD0 1 2 3 4 5 60123456s @mDΕ Nx,y@mmmradD-400 -200 0 200 40098.599.099.5100.0100.5101.0101.5t @fsDDpp@%D-400 -200 0 200 4000100200300400t @fsDI@AD-400 -200 0 200 400-1.0-0.50.00.51.01.5t @fsDx@mmD-400 -200 0 200 400 6000.51.01.52.0t @fsDΕ Nx,y@mmmradD-400 -200 0 200 400 6002468t @fsDΣ∆´104Figure 2: Results for an example optimised nonlinearbunch compressor. (1) Chromatic amplitude functions(black, red) & chromatic derivative of dispersion (blue). (2)R56 (black) & T566 (red). (3) εN(x,y) less dispersive con-tributions (black, red). (4) Longitudinal phase space (blue- optimised, red - without sextupoles). (5) Current profile(20 fs slices, blue - optimised, red - without sextupoles).(6) x − t phase space (blue - optimised, red - without sex-tupoles). (7) Normalised slice emittances (20 fs slices):horizontal (blue - optimised, red - without sextupoles) andvertical (green - optimised, orange - without sextupoles).(8) Slice energy spread (20 fs slices) (blue - optimised, red- without sextupoles)resulting in increased projected and slice emittance. Up tosix sextupoles were tried with similar results. These nonlin-ear compressors have also been studied under energy jitterand the bunch parameters found to vary substantially.Linearisation via Harmonic RFWe insert a fourth harmonic 0.7 m structure immediatelyprior to the magnetic compressor. An optimisation [5] wasthen performed with variables being the harmonic voltageand phase, the off crest phase of the preceding linac and theangle of the compressor dipoles. Results for two candidatetunings are shown in Fig. 3. The peak voltage on the lin-earising cavity is 7 MV/m. It can be seen that the additionalcomplication of a harmonic cavity is justified by ability topredictably tailor longitudinal phase space.0 5 10 15 20 25-1001020304050s @mDΒx,y@mD,Ηx@cmD0 5 10 15 20 25-0.50.00.51.0s @mDΕ Nx,y@mmmradD-400 -200 0 20099.9100.0100.1100.2100.3100.4t @fsDDpp@%D-400 -200 0 200 4000100200300400500t @fsDI@AD-400 -200 0 2000.30.40.50.60.7t @fsDΕ Nx,y@mmmradD-400 -200 0 200123456t @fsDΣ∆´104Figure 3: Two candidate optimisations linearising with har-monic cavity. (1) Optics: βx,y (black, red) & ηx. (2)εN(x,y) less dispersive contributions (black, red). (3) Lon-gitudinal phase space (blue - optimised for 200 fs flat top,red - optimised for 300 fs flat top). (4) Current profile (40 fsslices, optimised for 200 fs flat top, red - optimised for300 fs flat top). (5) Normalised slice emittances (40 fsslices): horizontal / vertical (blue / red - optimised for200 fs flat top, green / orange - optimised for 300 fs flat top)(6) Slice energy spread (20 fs slices) optimised for 200 fsflat top, red - optimised for 300 fs flat top.VELOCITY BUNCHINGAn alternative to magnetic compression is to use velocitybunching in the low energy section of the accelerator. Thefirst 2 m linac section is set to the zero crossing to impart atime-velocity chirp along the bunch. The bunch then com-presses in the following drift space. The second linac sec-tion is positioned at the waist of the bunch length evolutionafter 3 m of drift to rapidly accelerate the beam and capturethe short bunch length. Solenoids are required around thebunching section to control the transverse beam size andprevent emittance degradation. ASTRA was used to trackuntil the end of the second linac module followed by ELE-GANT. The quadrupoles between the first and second linacsections are switched off in order to keep the beam axiallysymmetric, and the bunch compressor set to zero angle. Anevolutionary algorithm was used to optimise the beamlinefor both bunch length and transverse emittance. We presenttwo tunings with 100 pC bunch charge. Fig. 4 shows abunch with similar peak current and current profile to thenon-linearised magnetically compressed bunch of Fig. 2-5.This is achieved at half the total bunch charge, with lowerslice energy spread, but higher slice emittance. In Fig. 4 weshow that a similar peak current to the non-linearised mag-netically compressed bunch of Fig. 2-5 is easily achievedFigure 4: Velocity bunched beam tuned for low emittance.Figure 5: Velocity bunched beam tuned for peak current.with smaller slice energy spread but higher slice emittance.Fig. 5 shows a beam tuned for peak current at the exit of thesecond linac module. The peak current then degrades alongthe accelerator. This bunch has the capabilities to providesingle-spike SASE FEL operation.CONCLUSIONSThis initial study has established an accelerator layoutfor CLARA that is inherently flexible in the pulse profilesit is capable of producing. We have shown this by simulat-ing bunches suitable for seeded and SASE FEL operation.Further work will entail jitter tolerance analysis of the pre-sented configurations.REFERENCES[1] J. A. Clarke et. al., TUPPP066, these proceedings.[2] K. Flöttmann, http://www.desy.de/~mpyflo[3] J. W. McKenzie & B. L .Militsyn, THPC132, IPAC 11.[4] M. Borland, Advanced Photon Source, LS-287 (2000).[5] R. Luus & T. H. I. Jaakola, AIChE Journal 19, 760 (1973).

Comparison of Compression Schemes for CLARA

Comparison of Compression Schemes for CLARA

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