1. UBW2012, A. Matveenko Michael Abo-Bakr (presented by Alexander Matveenko) Unwanted Beam Workshop (UBW 2012) 17.-18.12 2012 Dark Current Issues for Energy Recovery Linacs

2. UBW2012, A. Matveenko ERL (REMINDER) 2 Storage Rings: beam dimensions result from an equilibrium state of excitation and damping, high energy (GeV) & current (~10 2 mA)  virtual beam power ~ 10 2 MW – GW Linacs: break equilibrium state limit from Storage Ring  benefit from present high brightness sources  single path device: power limited to ~ MW  only low currents @ high energies: ~ mA @ GeV ERL: high current operated linacs: 100 mA @ GeV  100 MW??? - recovery of the energy invested for acceleration  second passage trough rf structure with  = 180° - lowest bunch charge at a given current  cw operation

3. UBW2012, A. Matveenko ERL: BERLINPRO 3 BERLinPro: Berlin Energy Recovery Project funding decision late 2010, start of project 2011 extensive gun development: Gun0.1 (2011), Gun0.2 (2012) and Gunlab 2015: first electrons from Gun2 & Booster  6 MeV 2017: merger 2018: 50 MeV recirculation

4. UBW2012, A. Matveenko BERLINPRO: SRF R&D & CAVITY DESIGN 4 CornellJLAB BERLinPro: SRF componentsBoosterLinac Cavity type3 X 2 cell3 X 7 cell Acc. gradient (MV/m)1219 avail. RF power per cavity (kW)15 / 270 / 2703 X 15 Design (cavity / module)Cornell / Cornell mod.under dev. / under dev. Booster cavity:Linac cavity: Adapt KEK style high power coupler to Cornell cavity design Strongly HOM damped cavity by waveguides Calculate HOM spectrum for beam dynamics court.: A. Neumann

5. UBW2012, A. Matveenko BERLINPRO: BEAM OPTICS 5 Basic ModeShort Bunch Mode Bunch charge, pC77~ 10 Bunch repetition rate, GHz1.3variable Max average current, mA100≤ 1 Transv. emitt., norm., mm mrad~ 11 … 5 Bunch length, ps, rms2.00.1 Relative energy spread, % rms~ 0.50.1 … 1.0 a) b) c) Recirculator: a) linear beam optics (R 56 =0), comparision of basic and short bunch mode: b) evolution of R 56 (m), c) evolution of bunch length (m). Emittance is mostly conserved (thus not drawn). BM SBM BM SBM 2 ps 140 fs Injector: comparision between basic and short bunch mode, a) evolution of the normalized emittance (mm mrad), b) bunch length (mm). a)b) BM SBM  BM,x  BM,y  SBM,x  SBM,y

6. UBW2012, A. Matveenko BERLINPRO: HIGH CURRENT EFFECTS 6 μ x, rad μ y, rad I th, A Beam halo modeling: particle distribution from ASTRA. Red – active beam particles, blue – passive halo particles, green – particles lost in collimators. Initial distribution on the cathode in a) – x-y plane, b) – x-t plane. Particle distribution after the merger section in c) – x-z plane, d) – p z -z plane. a) b) c) d) Beam Break-up instability: Betatron phase scan of BBU threshold currents, using (reasonably optimistic) cavity parameters of CEBAF 5-cell cavities. Horizontal axes - betatron phase advances, vertical – threshold current.

7. UBW2012, A. Matveenko TERMINOLOGY 7 Accelerated Beam Unwanted Beam Wanted Beam I ~ 10 2 mA E ~ 10 2 – 10 3 MeV  n ~ 1.0 mm mrad low  E and short  s Halo generated by / together with the wanted beam - scattered particles (residual gas, IBS) - laser stray light - … (?) Dark Current generated indepen- dently of wanted beam (laser off) - field emission in rf cavities - … (?)

8. UBW2012, A. Matveenko DARK CURRENT 8 large amounts of dark current have to be expected in machines with: long rf sections – high energy: large number of potential emitters high accelerating gradients in the rf structures – compact machine layout: field emission current (density) scales according to Fowler- Nordheim with: E – electric field,  – work function high duty cycle – high current: emission only when rf is on  worst case: cw  one of the most affected kind of machine: Energy Recover Linac’s (ERL)

9. UBW2012, A. Matveenko 9 DARK CURRENT @ BERLINPRO LINAC Booster Gun Dark Current affects: energy recovery efficiency (available linac rf power) machine protection (affects aperture definition and choice of vacuum chamber material) collimation system requirements radiation safety (during and after a machine run)

10. UBW2012, A. Matveenko 10 BERLINPRO: IMPACT POSITIONS OF LOSS CURRENT Dark current from gun & booster:

11. UBW2012, A. Matveenko 11 Dark current from linac: BERLINPRO: IMPACT POSITIONS OF LOSS CURRENT

12. UBW2012, A. Matveenko 12 Optical functions in recirculator: BERLINPRO: IMPACT POSITIONS OF LOSS CURRENT

13. UBW2012, A. Matveenko CURRENT LOSSES AND … 13 … Linac RF Power available Linac RF power limits losses and dark current due to beam loading: P rf = 10 … 30 kW  I loss = 200 … 600  A @  E = 50 MeV 0.2 … 0.6 % @ 100 mA  T = 99.8 %  3.1  @ Gaussian Beam Sources of potentially lost beam: 1.Dark current (laser = OFF) source: field emission & multi-packting in rf cavities theory: hardly predictable in advance (neither amount nor spatial, temporal and energy distribution) field emission current = heat load for sc cavity  cryo budget sets intra cavity dark current limits; escape ratio defines current leaving cavities 100 nA (1  A ?) / Cavity  1  A for BERLinPro counteracts: -limit RF field amplitude -collimate beam -estimate more probable positions of impact

14. UBW2012, A. Matveenko 14 … Machine Protection assumption on permanent (s, min, h ?) losses so far: 25  A / section@ low energy: 5 … 10 MeV  250 W / section 5  A / section@ high energy: 50 MeV  250 W / section section = 2…3 m = one radiation control segment  100 W / m uniform distributed power  water cooling might be required, but no problem spot like distribution  may not occur, must be detected by transmission measurements and must lead to immediate machine switch off … Activation & Personal Interlock decide current limits on acceptable radiation limits: e.g. 100  Sv after 1 h w/o beam derive according loss currents (depending on chamber material, and other possibly activated surrounding hardware) CURRENT LOSSES AND …

15. UBW2012, A. Matveenko 15 BERLINPRO: INJECTOR ACCEPTANCE Wanted Beam + Fowler Nordheim DC  track & mark particles lost injector studies ongoing

16. UBW2012, A. Matveenko 16 BERLINPRO: RECIRCULATOR ACCEPTANCE Fill 2D phase space (uniform)  track & mark particles lost hor. & ver. phase space

17. UBW2012, A. Matveenko 17 BERLINPRO: RECIRCULATOR ACCEPTANCE Fill 2D phase space (uniform)  track & mark particles lost horizontal & longitudinal phase space

18. UBW2012, A. Matveenko injector linac merger booster recirculator (1 st half) 1 st 180° recirculator arc splitter chicane fractional losses / %: transv. phase space filled long. phase space filled aperture Loss positions strongly depend on assumed 6D dark current distribution !!! BERLINPRO: POTENTIAL LOSS POSITIONS

19. UBW2012, A. Matveenko MEASURES AT DESIGN STAGE TO MINIMIZE EFFECT OF DARK CURRENT @ BERLINPRO 19 Do not plan to operate at ultimate gradients Collimation Protect potentially weak places (water cooling of vacuum chambers) Diagnostics to control the unwanted beam losses Do the modeling … but do not believe it…