The Heavy Ion Fusion Science Virtual National Laboratory Experiments and simulations with electron clouds in magnets – application to CESR Art Molvik Lawrence Livermore National Laboratory Heavy-Ion Fusion Science Virtual National Laboratory at Cornell University Jan. 31-Feb. 2, 2007 UCRL-PRES

The Heavy Ion Fusion Science Virtual National Laboratory 2 Molvik – CesrTA – 2/2/07 Outline 1.Introduction and e-cloud diagnostics on HCX 2.Possibilities for CESR 3.Clearing electrode experiments 4.Application to CESR 5.Summary Experiments and simulations with electron clouds in magnets – application to CESR

The Heavy Ion Fusion Science Virtual National Laboratory 3 Molvik – CesrTA – 2/2/07 HIFS-VNL has unique tools to study ECE WARP/POSINST code goes beyond previous state-of-the-art –Parallel 3-D PlC-adaptive-mesh-refinement code with accelerator lattice follows beam self-consistently with gas/electron generation and evolution HCX experiment addresses ECE fundamentals relevant to HEP (as well as WDM and HIF) –trapping potential ~2kV (~20% of ILC bunch potential?) with highly instrumented section dedicated to e-cloud studies Combination of models and experiment unique in the world –unmatched benchmarking capability provides credibility ‘Benchmarking’ can include: a. Code debug b. validation against analytic theory c. Comparison against codes d. Verification against experiments –enabled us to attract work on LHC, FNAL-Booster, and ILC (2007) Who we are – The Heavy Ion (Inertial) Fusion Science Virtual National Laboratory (HIFS-VNL) has participants from LLNL, LBNL, and PPPL.

The Heavy Ion Fusion Science Virtual National Laboratory 4 Molvik – CesrTA – 2/2/07 (a) (b) (c) Capacitive array Clearing electrodes Suppressor e-e- End plate Retarding Field Analyser (RFA) GESD Q1Q2Q3Q4 K+K+ Short experiment => need to deliberately amplify electron effects: let beam hit end-plate to generate copious electrons which propagate upstream. INJECTOR MATCHING SECTION ELECTROSTATIC QUADRUPOLES 2 m long MAGNETIC QUADRUPOLES 1 MeV, 0.18 A, t ≈ 5  s, 6x10 12 K + /pulse, 2 kV beam potential, tune depression to 0.10 HCX used for gas/electron effects studies (at LBNL) Location of Current Gas/Electron Experiments 2 m

The Heavy Ion Fusion Science Virtual National Laboratory 5 Molvik – CesrTA – 2/2/07 Diagnostics in two magnetic quadrupole bores, & what they measure. MA4 MA3 8 “paired” Long flush collectors (FLL): measures capacitive signal + collected or emitted electrons from halo scraping in each quadrant. 3 capacitive probes (BPM); beam capacitive pickup ((n b - n e )/ n b ). 2 Short flush collector (FLS); similar to FLL, electrons from wall. 2 Gridded e - collector (GEC); expelled e - after passage of beam 2 Gridded ion collector (GIC): ionized gas expelled from beam BPM (3) BPM FLS(2) FLS GIC (2) GIC Not in service FLS GEC

The Heavy Ion Fusion Science Virtual National Laboratory 6 Molvik – CesrTA – 2/2/07 Expelled ions Expelled ions  P background Expelled ion current [mA] n(Ar) [10 10 cm -3 ] We measure electron sources – ionization 1. Ionization of gas by beam (n e /n b ≤ 3%) Beam Expelled ions Beam Potential +2 kV +1 kV Beam current known; from expelled ion current infer -Ionization rate -Also, gas density in beam

The Heavy Ion Fusion Science Virtual National Laboratory 7 Molvik – CesrTA – 2/2/07 B -50 V+50 V We measure electron sources – walls 2. Electron emission – beam tube (n e /n b ≤ 7%) 3.Electron emission – end wall (n e /n b, 0, 100%) Quadrupole magnets (a) (b)( c) Internal diag Clearing electrodes a, b, c K + beam Suppressor e - from end For low ion scrape-off, expect current near-zero 4 t(µs) mA 40 mA 140 mA Clearing electrode-c bias (a) (b) (c) kV Clearing current (mA) 0

The Heavy Ion Fusion Science Virtual National Laboratory 8 Molvik – CesrTA – 2/2/07 Point source of electrons to simulate synchrotron radiation photoelectrons Electron gun operates over range ~10 eV to 2000 eV (cathode & grid indep.) <1 mA to 1000 mA Electron gun enables quantitatively controlled injection of electrons

The Heavy Ion Fusion Science Virtual National Laboratory 9 Molvik – CesrTA – 2/2/07 Retarding field analyzer (RFA) measures energy distribution of expelled ions RFA an extension of ANL design (Rosenberg and Harkay) Can measure either ion (shown) or electron distributions Potential of HCX beam edge ~1000 V, beam axis ~ 2000 V Ref: *Michel Kireeff Covo, et al, Phys. Rev. Lett. 97, (2006); Instrument details to be pub. NIM-A

The Heavy Ion Fusion Science Virtual National Laboratory 10 Molvik – CesrTA – 2/2/07 /RFA Retarding field analyzer (RFA) measures potential on axis = ion- repeller potential + beam/e- distr. => f=N e /N beam – A first – time-dependent measurement of absolute electron cloud density* Clearing electrode measures e - current + e - velocity drift => f=N e /N beam Absolute electron fraction can be inferred from RFA and clearing electrodes Beam neutralizationB, C, S onB, C off S on B, C, S off Clear. Electrode A~ 7%~ 25%~ 89% RFA(~ 7%)~ 27%~ 79% *Michel Kireeff Covo, et al, Phys. Rev. Lett. 97, (2006).

The Heavy Ion Fusion Science Virtual National Laboratory 11 Molvik – CesrTA – 2/2/07 Outline 1.Introduction & e -cloud diagnostics on HCX 2.Possibilities for CESR 3.Clearing electrode experiments 4.Application to CESR 5.Summary

The Heavy Ion Fusion Science Virtual National Laboratory 12 Molvik – CesrTA – 2/2/07 CESR – RFA measures energy distribution of expelled ions? Bunch 1.6x10 10 e+, 14 ns separation For  b  ~ 50 V, expulsion time of H 2 ion ~ 0.6 µs Need separate ion and electron repeller grids. Measure potential at which ion current  zero. Difficult measurement: low energy, low current with ultrahigh vacuum, need to shield against rf pickup, poor time dependence, detector in wiggler B-field? Ref: *Michel Kireeff Covo, et al, Phys. Rev. Lett. 97, (2006).

The Heavy Ion Fusion Science Virtual National Laboratory 13 Molvik – CesrTA – 2/2/07 CESR – RFA to measure e-cloud density (in drift or wiggler)? Bunch 1.6x10 10 e+, 14, 28, or 42 ns separation now (≥4 ns later) – what is the energy of expelled ions? Beam line charge: at 4 ns spacing  ~2 nC/m , at 14 ns  ~0.6 nC/m  Beam potential:  ~180 V ,  ~50 V  [Assuming r b /r w ~0.1, 0.01 better] My conclusion is that the peak ion energy is determined by the beam line charge, averaged over a bunch spacing. Time to expel H 2 + ion: 0.3 µs, 0.6 µs ( of revolution time) Expelled ion current extremely small: -HCX: 180 mA, ~1E-15 cm 2, P~3E-7 torr,  100 nA/cm 2 -CESR: ~1 mA, Cross section much smaller, P ~1E-9,  fA/cm 2 ? -CESR-100%ionized gas: 3.3E9cm-3(0.0012cm3)q/[(1µs)(12cm)~0.5nA/cm2 Conclusion: difficult measurement – what other possibilities?

The Heavy Ion Fusion Science Virtual National Laboratory 14 Molvik – CesrTA – 2/2/07 CESR – other diagnostics to measure e-cloud density? Capacitively coupled electrodes -Simple diagnostic, well developed for BPMs and other applications -But, also measures emission and collection of electrons, which can have a magnitude similar to capacitive. Perhaps adjacent biased electrodes, e.g., ±bias. + measures capac. - e- collection, and - measures capac. + e- emission Perhaps adjacent bare and gridded diagnostics: capacitive electrode and grid-shielded electrode (or RFA). The latter measures only e-, the former both. These ideas need more analysis – Is there a pony in this pile?

The Heavy Ion Fusion Science Virtual National Laboratory 15 Molvik – CesrTA – 2/2/07 Outline 1.Introduction & e -cloud diagnostics on HCX 2.Possibilities for CESR 3.Clearing electrode experiments 4.Application to CESR 5.Summary

The Heavy Ion Fusion Science Virtual National Laboratory 16 Molvik – CesrTA – 2/2/07 Trapping depth of electrons depends upon their source, in a quadrupole magnet (without multipactor) Electrons ejected from end wall Electrons from ionization of gas Electrons desorbed from beam pipe in quad upon ion impact E-cloud in a quadrupole magnet [Electron mover also speeds simulation in wiggler fields] beam pipe Deeply trapped electrons Weakly trapped electrons

The Heavy Ion Fusion Science Virtual National Laboratory 17 Molvik – CesrTA – 2/2/07 Gridded Electron Collectors (GEC) current measures electron depth of trapping GEC collects e- along B-field lines, detrapped at end of pulse Weakly-trapped electrons Deeply-trapped electrons Beam-tail scrapes wall Beam potential contours In drift region HCX current flattops for ~4 µs (like super-bunch)

The Heavy Ion Fusion Science Virtual National Laboratory 18 Molvik – CesrTA – 2/2/07 Weakly trapped electrons cleared with ~300 V bias, whereas deeply trapped require >1000 V Weakly trapped electrons originate on or near a wall (beam tube) – turning points near wall. Deeply trapped electrons originate from beam impact ionization of gas, or scattering of weakly trapped electrons – turning points within beam.

The Heavy Ion Fusion Science Virtual National Laboratory 19 Molvik – CesrTA – 2/2/07 Clearing electrode removes all electrons from a drift region Clearing electrode ring (C) – blocks electrons from (B) when biased more negatively than -3 kV Clearing electrode ring (B) [with V c = 0] blocks electrons from (A) when biased more negatively than - 3 kV Suppressor bias = 0 V, electrons can leak back into quads along beam.

The Heavy Ion Fusion Science Virtual National Laboratory 20 Molvik – CesrTA – 2/2/07 Clearing electrode fields, above 2 kV bias, dominate over beam space-charge field V c = 0 kV  b = +2 kV V c = +2 kV  b = +2 kV V c = +10 kV  b = +2 kV Beam space-charge potential  b = +2 kV For CESR-TA, clearing field should dominate over beam space charge averaged over a few bunches (V c > 50 V), or remove electrons in period between bunches (V c ≥ 100 V).

The Heavy Ion Fusion Science Virtual National Laboratory 21 Molvik – CesrTA – 2/2/07 1 WARP-POSINST code suite is unique in four ways merge of WARP & POSINST Key: operational; partially implemented (4/28/06) + new e - /gas modules 2 + Adaptive Mesh Refinement Z R concentrates resolution only where it is needed 3 Speed-up x beam quad e - motion in a quad + New e - mover Allows large time step greater than cyclotron period with smooth transition from magnetized to non- magnetized regions 4 Speed-up x10-100

The Heavy Ion Fusion Science Virtual National Laboratory 22 Molvik – CesrTA – 2/2/07 And also Electron oscillations – simulation & experiment agree WARP-3D T = 4.65  s time (  s) 6. Simulation Experiment I (mA) Current to clearing electrode (c) agrees in frequency ~ 6 MHz Currents to capacitive electrode array agree in wavelength ~5 cm, and amplitude (below) e-e- (a)(b)(c) 200mA K + OscillationsElectrons bunching Beam ions hit end plate 0V 0V 0V/+9kV 0V Q4Q3Q2Q1 200mA K + Electrons Simulation Experiment Simulation

The Heavy Ion Fusion Science Virtual National Laboratory 23 Molvik – CesrTA – 2/2/07 Outline 1.Introduction & e -cloud diagnostics on HCX 2.Possibilities for CESR 3.Clearing electrode experiments 4.Application to CESR 5.Summary

The Heavy Ion Fusion Science Virtual National Laboratory 24 Molvik – CesrTA – 2/2/07 Clearing electrodes for CESR Drift regions easiest: clearing electrode can clear entire region Quadrupole magnets next: Clearing electrodes clear upstream in each drift region Dipole and wigglers hardest: Clearing electrodes clear only along intercepted field lines

The Heavy Ion Fusion Science Virtual National Laboratory 25 Molvik – CesrTA – 2/2/07 Outline 1.Introduction & e -cloud diagnostics on HCX 2.Possibilities for CESR 3.Clearing electrode experiments 4.Application to CESR 5.Summary

The Heavy Ion Fusion Science Virtual National Laboratory 26 Molvik – CesrTA – 2/2/07 Extensive diagnostics on HCX, some may be useful on CESR Simulations benchmarked against experiment – accurately reproduce many details of experiment May measure e-cloud density on CESR, along with measuring effects on beam: need more analysis -RFA (more difficult than on HCX) -Multiple capacitive electrodes with varying bias -Capacitive and grid-shielded electrodes Clearing electrodes -Effective at clearing drift region -In magnets (dipole, wiggler) effective only on intercepted field lines, or (quad) nearby drift surfaces Summary

The Heavy Ion Fusion Science Virtual National Laboratory 27 Molvik – CesrTA – 2/2/07 Backup slides

The Heavy Ion Fusion Science Virtual National Laboratory 28 Molvik – CesrTA – 2/2/07 Simulations with beam reconstructed from slit scans – improved agreement Effects of electrons on beam – beam loading in simulation now uses reconstructed data from slit- plate measurements leads to improved agreement between simulation and experiment Semi-gaussian load => New load from reconstructed data => X' X X Reconstructed X-Y distribution from slit-plate measurements Low e- High e- No e- High e-

The Heavy Ion Fusion Science Virtual National Laboratory 29 Molvik – CesrTA – 2/2/07 1.6MeV; 0.63A/beam; 30  s ~1km 4.0GeV; 94.A/beam; 0.2  s 4.0GeV; 1.9kA/beam; 10ns Heavy Ion Inertial Fusion or “HIF” goal is to develop an accelerator that can deliver beams to ignite an inertial fusion target Target Requirements: MJ x ~ 10 ns  ~ 500 Terawatts Ion Range: g/cm 2  GeV Near term goal: High Energy Density Physics (HEDP) For A ~ 200  ~ ions ~ 100 beams 1-4 kA / beam DT