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

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 high energies: ~ GeV ERL: high current operated linacs: 100 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

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

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

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, rms Relative energy spread, % rms~ … 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

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.

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 - … (?)

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)

UBW2012, A. Matveenko 9 DARK 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)

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

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

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

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   E = 50 MeV 0.2 … 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

UBW2012, A. Matveenko 14 … Machine Protection assumption on permanent (s, min, h ?) losses so far: 25  A / low energy: 5 … 10 MeV  250 W / section 5  A / 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 …

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

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

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

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

UBW2012, A. Matveenko MEASURES AT DESIGN STAGE TO MINIMIZE EFFECT OF DARK 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…