1 / 42

Recap of NGLS ElECTRON Collimation Design Studies

Recap of NGLS ElECTRON Collimation Design Studies. Christoph Steier SLAC – LCLS-2 accelerator physics meeting Oct . 17, 2013. NGLS Electron Collimation. Motivation (Gun) Dark Current Collimation System Layout Injector Kicker Energy Collimation Betatron Collimation

eldora
Download Presentation

Recap of NGLS ElECTRON Collimation Design Studies

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Recap of NGLSElECTRONCollimation Design Studies Christoph Steier SLAC – LCLS-2 accelerator physics meeting Oct. 17, 2013

  2. NGLS Electron Collimation Motivation (Gun) Dark Current Collimation System Layout Injector Kicker Energy Collimation Betatron Collimation Simulation of Collimation Effectiveness Dark Current Touschek Scattering Gas Scattering Collimator Hardware Development Plan Summary

  3. Motivation • High duty factor accelerators have main beams with considerable power (MW in our case) • Even small fractional losses can have substantial effects • Demagnetization of permanent magnet undulators • Quenches of s/c cavities and s/c undulators • Heat damage to vacuum envelope • Activation • Collimation system is essential • Needs to localize ‘routine’ losses away from sensitive areas • Needs to prevent equipment damage in case of malfunction until MPS stops beam • For routine losses, experience elsewhere has shown gun dark current to be dominant source – our calculations so far confirm this

  4. NGLS Layout e- diagnostics compressors e- diagnostics collimation FELs (1-9) injector linac spreader APEX-based e- injector (1 MHz,  = 0.6 m) 300 pC/bunch (0.3 mA max. current) 1.3-GHz CW SRF @ 16 MV/m (27 CM’s) Two bunch compressors + heater (500 A) Beam spreader using RF deflectors (9 FELs) Three (initial) very diverse FEL designs Diagnostics and collimation sections 720-kW main beam stops (3) beam stops exp. halls

  5. APEX: Injector Dark current status Resulting current profile of dark current Longitudinal Phase Space at Injector exit Fernando's Dark Current measurements Use integrated Fowler -Nordheim formula to fit with instantaneous formula (E=E0*cos(ωt))

  6. Transverse Distribution of Dark Current in APEX <750 keV, Identical to NGLS >750 keV, Similaro to NGLS, Not superconducting Solenoid magnet Buncher Single-cell RF cavity Gun Multi-cell RF cavity Laser pulse FLASH gun for comparison Dark current “Hotspots” Butterfly shape due to large energy spread

  7. Dark current losses in injector Simulated different initial distributions (spots or uniform) After transport in the injector, about 10% (spots) to 15% (uniform) of the dark current survives. C. Papadopulos

  8. Electron Collimation L1, Lh Ipk = 47 A L0 Ipk = 47 A L2 j = -23.2° Ipk = 90 A L3 j = +34.8° Ipk = 500 A • Assumed apertures for machine • +/- 18 mm radius pipe almost everywhere • No restriction (except collimator) in LH, BC1/2, FEL chicanes • Undulator chamber +/- 15 mm (x), +/- 3 mm (y) Lh j = 180° V0 = 0 MV L1 j = -20.0° Ipk = 47 A sz = 0.85 mm L0 j 0 Ipk = 47 Asz = 0.85 mm L2 j = -23.2° Ipk = 90 A sz = 0.44 mm L3 j = +34.8° Ipk = 500 A sz 0.08 mm Heater 94 MeV sd = 0.02% BC1 215 MeV sd = 0.44% BC2 720 MeV sd = 0.48% GUN 0.75 MeV SPRDR 2.4 GeV sd 0.04% CM01 CM2,3 CM09 CM27 CM04 CM10 CM01 CM2,3 CM09 CM27 3.9 CM04 CM10 3.9 BC1 215 MeV R56 = -94 mm sd = 0.44% BC2 720 MeV R56 = -76 mm sd = 0.48% SPRDR 2.4 GeV R56 = 0 sd 0.04% GUN 0.75 MeV Heater 94 MeV R56 = -5 mm sd = 0.02% Dark Current Kicker  10 4 -tron Coll.’s 10 mm (x) 2 mm (y) Energy Coll. 15 mm Energy Coll. 8 mm Energy Collimator 1.5 mm Energy Coll. 2.5 mm 300 pC; 2012-04-18 & 2012-07-02

  9. Motivation for Dark Current Kicker • First dispersive place to collimate - laser heater chicane (100 MeV) • 15% of 8 mA at 100 MeV corresponds to 120 W • Anything not captured there quickly gains more energy towards bunch compressor • FLASH stays below 100 W losses in bunch compressor due to radiological concerns • Coordinating with EHS – cost implications for shielding • Some of the other 85% is lost in injector cryo-module • XFEL guidance is <0.1W/m to avoid cavity quenches, simulation shows about 1 W/m for uniform emission case – <0.1 W/m for other more realistic distributions • Gaining factor 10 safety margin necessary – Dark Current Kicker

  10. FLASH: F. Obier Dark Current Deflector • Dark current produced in every injector RF bucket (186 MHz) – useful beam only 1 MHz • FLASH kicker reduces dark current intensity by factor of >3 • NGLS: • kick main bunches and compensate with DC magnet • high repetition rate (1 MHz) and fast rise and fall times • Just after the gun (0.75 MeV). • Reference: ALS camshaft kicker (1.5 MHz, rise/fall times of 20 ns, >70 mrad@1.9 GeV) • Simulations: scaled version of ALS kicker could reduce by factor of >10 ALS: S. Kwiatkowski

  11. Dark Current Kicker Simulation >750 keV, Cold <750 keV, Warm kicker collimator Buncher Cryomodule Gun Need to collimate kicked beam without scraping main beam:Collimator R=10 mm Plan to collimate in this region ALS kicker kicks >70 mrad at 1.9 GeV - 180 mrad at 750 keV Factor 2 shorter, factor 3 larger opening, about 30 mrad possible

  12. Dark Current Deflector H. Qian, S. de Santis, S. Kwiatkowski • Dark current produced in every injector RF bucket (186 MHz) – useful beam only 1 MHz • FLASH kicker reduces dark current intensity by factor of >3 • NGLS: • kick main bunches and compensate with DC magnet • high repetition rate (1 MHz) and fast rise and fall times • Just after the gun (0.75 MeV). • Reference: ALS camshaft kicker (1.5 MHz, rise/fall times of 20 ns, >70 mrad@1.9 GeV) • Simulations: scaled version of ALS kicker could reduce by factor of >10 New shape 54 mm 64 mm 21 mm 153 mm

  13. Electron Collimation L1, Lh Ipk = 47 A L0 Ipk = 47 A L2 j = -23.2° Ipk = 90 A L3 j = +34.8° Ipk = 500 A • Assumed apertures for machine • +/- 18 mm radius pipe almost everywhere • No restriction (except collimator) in LH, BC1/2, FEL chicanes • Undulator chamber +/- 15 mm (x), +/- 3 mm (y) Lh j = 180° V0 = 0 MV L1 j = -20.0° Ipk = 47 A sz = 0.85 mm L0 j 0 Ipk = 47 Asz = 0.85 mm L2 j = -23.2° Ipk = 90 A sz = 0.44 mm L3 j = +34.8° Ipk = 500 A sz 0.08 mm Heater 94 MeV sd = 0.02% BC1 215 MeV sd = 0.44% BC2 720 MeV sd = 0.48% GUN 0.75 MeV SPRDR 2.4 GeV sd 0.04% CM01 CM2,3 CM09 CM27 CM04 CM10 CM01 CM2,3 CM09 CM27 3.9 CM04 CM10 3.9 BC1 215 MeV R56 = -94 mm sd = 0.44% BC2 720 MeV R56 = -76 mm sd = 0.48% SPRDR 2.4 GeV R56 = 0 sd 0.04% GUN 0.75 MeV Heater 94 MeV R56 = -5 mm sd = 0.02% Dark Current Kicker  10 4 -tron Coll.’s 10 mm (x) 2 mm (y) Energy Coll. 15 mm Energy Coll. 8 mm Energy Collimator 1.5 mm Energy Coll. 2.5 mm 300 pC; 2012-04-18 & 2012-07-02

  14. Location of Collimators (LHS, BC1) Based on low impedance version of ALS collimators (as well as other places) 50 cm is reasonable length for collimators With safety margin for finalized mechanical design (impedance calculation) – desirable to reserve 1 m Enough space available in BC1, working to increase space in LH, downstream of undulator

  15. Location of Collimators (BC2, MCS) Enough space seems available in BC2 Generous space available in FODO section after main LINAC (MCS, which is just after L3S) MCS, SLS collimation section of NGLS design much more compact than XFEL • No requirement to transport energy chirped bunchtrains • No need for very high beta functions (bunchtrain power) • No separate need for R56 variability, … • Spreader angle and achromats in SLS provide natural place for energy collimation with secondary showers kept away from undulators

  16. Location of Collimators (SLSx) Current simulations are based on spreader lattice from October • Baseline change to RF spreader since then • General achromat layout and space similar – current collimator layout should work – will verify Space at first collimator OK, at second one a little tight. Beta functions at second collimator very small – better spaces later in arc (need trade-off analysis of required MPS speed vs. secondaries escape rate)

  17. Technical Details of Tracking • Started from CDR MAD file (sharepoint) • Translate (automated) with mad2elegant (does not accept matching routines, but bare lattice) • Needed to remove all CSR (just turning switch off is not enough) – otherwise dark current gets lost in first CSR element • Translate (automated) with mad2at • Added beamline apertures (see before) and collimators to resulting files • Will slowly add all apertures/collimators to baseline MAD files • Imported ASTRA distributions (astra2elegant, Matlab) • Need to carefully consider phase matching between different distributions, energy scaling, … • Important to use elegant fiducialization correctly • In elegant always need to track two bunches (fiducialization reference + dark current) • Tracked CDR beam (and gaussian approximation of it) to determine collimator settings • No loss of nominal beam (or 6 sigma particles) + 10-20%

  18. Collimator Location + Setting • LHEATCOL • |x|<1.5 mm • BC1COL • |x|<15 mm • BC2COL • |x|<8 mm • CXL3ED_1 • |x|<10 mm • CXL3ED_2 • |x|<10 mm • CYL3ED_1 • |y|<2 mm • CYL3ED_2 • |y|<2 mm • SPREADCOL1 • |x|<2.5 mm • SPREADCOL2 • |x|<5 mm

  19. Tracking Gun Dark Current Dark Current losses well controlled Most losses on Laser Heater Collimator Followed by BC1 and BC2 Remaining losses in warm section around laser heater Losses in Linac 1 below XFEL quench criterium of 0.1 W/m • Dark current kicker will help No losses beyond BC2 (and in undulator)

  20. Trajectories, Loss Power Power densities [W/m] on right are for 8 mA dark current from gun: 10-100 W on collimators • Likely need for reduction (deflector) Up to 1 W/m around laser heater • Would like to reduce 10s mW/m in Linac1 • Tesla used threshold 10 mJ/cm3 over 20 ms for 25 MeV/m – extrapolating their shower calculations this is safe by factor of >10

  21. Removing collimators (start to end) When removing collimators earlier in accelerator, undulators remain protected from dark current (until very last energy collimator is pulled) Of course, Linac does not and loss power gets much higher (because collimation does not occur at lowest possible energy) Encouraging with regards to protection from Touschek+Gas Scattering in Linac+Spreader

  22. Post Linac Collimation (Gas Scattering) • Test of post linac collimation by artificially increasing (20-50x) divergence of beam at points along the linac • In vertical plane, combination of two (90 degree apart) collimators and energy collimators protects undulators • Rough estimate of pressure requirements on next slide, plan to quantify further with montecarlo and tracking of scattered particles

  23. Estimate of gas scattering loss rates • For electrons one can simplify the formulas for gas Bremsstrahlung lifetime (in the approximation of <Z2> ~ 50): • In the same approximation, the elastic gas scattering lifetime becomes: For NGLS: • Assume 1% energy acceptance (logarithmic dependence)  relative losses of 10-9 for 100 nTorr due to inelastic scattering over full length • Assuming 7mm ID vacuum chamberrelative losses of 10-8 for 100 nTorr due to inelastic scattering • 1-10 mW for nominal beam power (ALS total beamloss power about 30 mW) – No concern

  24. Post Linac Collimation (Gas, Touschek Scattering) • Test of post linac collimation by artificially increasing (20x) energy spread of beam at points along the linac • For energy error originating within LINAC (inelastic gas or Touschek scattering), very small betatron amplitudes • First momentum collimator in spreader effectively removes scattered beam – very small amplitudes in undulator

  25. Touschek losses • In Rings- Bruck’s formula for Touschek lifetime – valid for flat beam • Only complicated part is to calculate momentum aperture/acceptance • For NGLS with its round beams and changing energy not sufficient • Multiple approaches: Monte-Carlo, … • We are using approach used by Xiao/Borland for APS-ERL studies: Based on analytic Piwinski formula: • Still needs calculation of momentum acceptance – because of tight collimator settings (dark current), acceptance is pretty small in parts of line. Above: APS-ERL example – dependence of Touschek loss-rate in full energy arcs on Momentum Aperture Below: Momentum Aperture of NGLS with baseline collimation.

  26. Touschek losses (2) • Scattering rate based on analytic Piwinski formula: • Scattering rates with NGLS momentum acceptance + design beam parameters: • Integrating local scattering probability leading to loss on a collimator of up to few 10-6 (<10 W on spreader collimator) – Acceptable • Verified calculation on ALS example – agree well with measured lifetimes

  27. Collimator Design • Main issues that determine space requirements for each collimator (necessary for CDR): • Heat load / beam power / power density • <=1 ms MPS -> similar to 3rd generation light sources (kJ) – consistent with XFEL scaling • Impedance heating -> similar to rings • Wake fields, effect on beam: • Need to not spoil beam quality • Radiation showers, secondary particle transport, activation: • Use of collimator pairs where possible • Considered for local shielding and tunnel wall thickness

  28. XFEL collimator damage • In XFEL design collimator damage sets requirements for large beta functions, one driver for length of collimation section (energy acceptance, R56 tunability, fixed (set of) collimator apertures …)

  29. Scaling of XFEL considerations to NGLS • Our assumption is 1 ms MPS, i.e. 1000 bunches • XFEL was 80 – 90 bunches • We assume 0.3 nC, XFEL is 1 nC • Gun (750 keV) • No concern, low power, very large beam • LH, BC • Beam is enlarged a lot due to dispersion • Post LINAC • 2.4 GeV vs. 20 GeV – total deposited energy is factor 2.2 higher in XFEL – but shower is deeper • Normalized emittance (0.6 vs 1.4 mm mrad) – absolute emittance is factor 3.6 larger in NGLS • NGLS beta functions at collimators factor 10 below XFEL • Potentially worse in spreader • Overall seems similar -> Need detailed quantitative analysis • But faster MPS response possible (desirable?), i.e. current solution is feasible

  30. Protector absorbers between cryomodules • At CD-0 design had distributed collimators along length of LINAC and large beta functions to make them effective • Based on tracking of gun dark current and gas/Touschek scattering estimates we do not believe we need those • It was proposed (by reviewers) that local fixed absorbers might be a good idea to localize most of losses (for fault conditions like quadrupole PS trip, …) away from cavities • Also provides well defined spots for where to place discrete, fast loss monitors for MPS • Marco incorporated those in new layout • However, looking at geometry in more detail, they naturally appear just downstream of cryomodule (70->35 mm) • Still need to verify that location is appropriate and consider potential impact for designing transition

  31. Smaller Magnetic Gap and Impact on Undulator Length (Emma) Note that 10-mm gap (XFEL) is only ~20 m longer! 7.5 mm Self-seeded undulator with breaks, etc 6.0 mm chamber gap is 2 mm less than magnetic gap 500 A, 0.6 um, 150 keV, 10 m beta, 2.4 GeV, 3.3 m segment, 4.4 m break, self-seeded (Lux1.5), 25% safety factor on length (Lux1.5x1.25), 60-um Nb3Sn SCU insulator at 80% (0.48 mm diam.)

  32. Estimate of gas scattering loss rates • For electrons one can simplify the formulas for gas Bremsstrahlung lifetime (in the approximation of <Z2> ~ 50): • In the same approximation, the elastic gas scattering lifetime becomes: For NGLS: • Assume 1% energy acceptance (logarithmic dependence)  relative losses of 10-8 for 100 nTorr due to inelastic scattering over full length • Assuming 4mm ID vacuum chamberrelative losses of 10-8 for 100 nTorr due to inelastic scattering • <20 mW for nominal beam power (ALS total beamloss power about 30 mW) – Still no concern

  33. Effect of smaller undulator gap on darkcurrent collimation • Smaller undulator gap means vertical collimation is necessary in addition to energy collimamation • Reducing YCOL from +/-2 mm to +/- 1 mm is sufficient • Losses on YCOL get much bigger – too high ? • Also tighter tolerances on orbit, collimator position, … - probably OK

  34. To do list + work in progress • Further characterize transverse dark current distribution from APEX. Refine models. Study how to reduce dark current and what final level might be achievable. • Study secondary particles, escaped particles after the collimators. Continue study of sensitivity to lattice errors, changes in initial distribution, collimator misplacements, … • Do trade-off study between cost for shielding/mitigation of activation and complexity and operational impact of collimation system • Carry out tracking of scattered particles (Monte Carlo of gas/Touschek). Potentially benchmark calculations with FLASH measurements. • Finish Collimator hardware reference design • Shower simulations, detailed thermal simulations. • calculate short and long range wakefields.

  35. Differences NGLS vs. LCLS-2 L1, Lh Ipk = 47 A L0 Ipk = 47 A L2 j = -23.2° Ipk = 90 A L3 j = +34.8° Ipk = 500 A Heater 94 MeV sd = 0.02% BC1 215 MeV sd = 0.44% BC2 720 MeV sd = 0.48% GUN 0.75 MeV SPRDR 2.4 GeV sd 0.04% Lh j = 180° V0 = 0 MV L1 j = -20.0° Ipk = 47 A sz = 0.85 mm L0 j 0 Ipk = 47 Asz = 0.85 mm L2 j = -23.2° Ipk = 90 A sz = 0.44 mm L3 j = +34.8° Ipk = 500 A sz 0.08 mm CM01 CM2,3 CM09 CM27 CM04 CM10 3.9 CM01 CM2,3 CM09 CM27 CM04 CM10 3.9 BC1 215 MeV R56 = -94 mm sd = 0.44% BC2 720 MeV R56 = -76 mm sd = 0.48% SPRDR 2.4 GeV R56 = 0 sd 0.04% GUN 0.75 MeV Heater 94 MeV R56 = -5 mm sd = 0.02% Dark Current Kicker  10 4 -tron Coll.’s 10 mm (x) 2 mm (y) Energy Coll. 15 mm Energy Coll. 8 mm Energy Collimator 1.5 mm Energy Coll. 2.5 mm 300 pC; 2012-04-18 & 2012-07-02 L2 j = -21° V0=1447 MV Ipk = 50 A Lb = 0.56 mm L3 j = 0 V0=2409 MV Ipk = 1.0 kA Lb = 0.024 mm L0 j= * V0=94 MV Ipk = 12 A Lb = 2.0 mm L1 j =-21° V0=223 MV Ipk = 12 A Lb =2.0 mm HL j =-165° V0 =55 MV CM01 CM2,3 CM15 CM35 CM04 CM16 3.9GHz LTU E = 4.0 GeV R56 = 0 sd 0.016% 2-km LH E = 95 MeV R56 = -14.5 mm sd = 0.05 % BC1 E = 250 MeV R56 = -55 mm sd = 1.4 % BC2 E = 1600 MeV R56 = -60 mm sd = 0.46 % GUN 0.75 MeV 100-pC machine layout: Oct. 8, 2013; v21 ASTRA run; Bunch length Lb is FWHM

  36. Summary • Have completed tracking with energy + betatron collimators in CDR lattice • Energy collimators sufficient to protect superconducting cavities + undulators from gun dark current • Dark current kicker appears necessary to minimize activation of collimators and protect injector s/c cavities • Betatron collimation (and post linac energy collimation) effective in stopping Touschek+Gas scattered particles before undulators • Space requirements for collimators are workable within current layout • No apparent show-stoppers remain for CD-1, will finish work in progress – main area is detailed collimator hardware design Thanks to Hiroshi Nishimura, Christos Papadopoulos, Fernando Sannibale, et al.

  37. Backup Slides

  38. State of the art 25 MV/m BCP cavities State of the art 35 MV/m EP cavities ΔE = 13.5 MV/m

  39. Elegant Tracking

  40. AT / Elegant comparison At first, results did not agree at all … Doubted my AT modifications However, reason turned out to be intricacies of how elegant tracks (fiducialization, no reference particle) and how data from astra was transferred Now good agreement – small remaining discrepancies are different modeling of apertures, small differences in import of large energy offset coordinates from ASTRA

  41. ALS Routine Stored Beam Losses • New scrapers localize losses away from beamline source points and undulators • Installed+work very well JH Scrapers Sector 1 JH Scrapers Sector 3

  42. DESY – FLASH / XFEL

More Related