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Experiments and simulations with electron clouds in magnets – application to CESR

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-227649. Experiments and simulations

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Experiments and simulations with electron clouds in magnets – application to CESR

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  1. 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-227649

  2. Experiments and simulations with electron clouds in magnets– application to CESR Outline Introduction and e-cloud diagnostics on HCX Possibilities for CESR Clearing electrode experiments Application to CESR Summary

  3. HIFS-VNL has unique tools to study ECE Who we are – The Heavy Ion (Inertial) Fusion Science Virtual National Laboratory (HIFS-VNL) has participants from LLNL, LBNL, and PPPL. • 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)

  4. HCX used for gas/electron effects studies (at LBNL) (b) (c) (a) Q1 Q2 Q3 Q4 Retarding Field Analyser (RFA) Clearing electrodes Capacitive array Suppressor GESD K+ e- End plate Location of Current Gas/Electron Experiments MATCHING SECTION ELECTROSTATIC QUADRUPOLES INJECTOR 2 m long MAGNETIC QUADRUPOLES 1 MeV, 0.18 A, t ≈ 5 s, 6x1012 K+/pulse, 2 kV beam potential, tune depression to 0.10 2 m Short experiment => need to deliberately amplify electron effects: let beam hit end-plate to generate copious electrons which propagate upstream.

  5. MA3 Diagnostics in two magnetic quadrupole bores, & what they measure. Not in service MA4 FLS(2) GIC (2) BPM (3) 8 “paired” Long flush collectors (FLL): measures capacitive signal + collected or emitted electrons from halo scraping in each quadrant. GEC GEC BPM FLS FLS GIC 3 capacitive probes (BPM); beam capacitive pickup ((nb- ne)/ nb). 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

  6. Beam Potential +2 kV +1 kV Expelled ions  Pbackground Expelled ions Expelled ion current [mA] Beam n(Ar) [1010 cm-3] We measure electron sources – ionization • Ionization of gas by beam (ne/nb ≤ 3%) • Beam current known; from expelled ion current infer • - Ionization rate • - Also, gas density in beam Expelled ions

  7. (b) ( c) (a) 140 mA For low ion scrape-off, expect current near-zero B electrons -50 V +50 V 40 mA -10 mA 4 t(µs) 10 10 Clearing current (mA) (a) 0 (b) Internal diag (c) K+ beam Clearing electrode-c bias -30 0 2 6 kV 10 Suppressor Clearing electrodes a, b, c We measure electron sources – walls • Electron emission – beam tube (ne/nb ≤ 7%) 3. Electron emission – end wall (ne/nb, 0, 100%) Quadrupole magnets e- from end

  8. Point source of electrons to simulate synchrotron radiation photoelectrons Electron gun enables quantitatively controlled injection of electrons Electron gun operates over range ~10 eV to 2000 eV (cathode & grid indep.) <1 mA to 1000 mA

  9. 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, 054801 (2006); Instrument details to be pub. NIM-A

  10. /RFA – A first –time-dependent measurement of absolute electron cloud density* • Retarding field analyzer (RFA) measures potential on axis = ion-repeller potential • + beam/e- distr. => f=Ne/Nbeam Clearing electrode measures e- current + e- velocity drift => f=Ne/Nbeam Absolute electron fraction can be inferred from RFA and clearing electrodes *Michel Kireeff Covo, et al, Phys. Rev. Lett. 97, 054801 (2006).

  11. Outline Introduction & e -cloud diagnostics on HCX Possibilities for CESR Clearing electrode experiments Application to CESR Summary

  12. CESR – RFA measures energy distribution of expelled ions? • Bunch 1.6x1010 e+, 14 ns separation • For b ~ 50 V, expulsion time of H2 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, 054801 (2006).

  13. CESR – RFA to measure e-cloud density (in drift or wiggler)? • Bunch 1.6x1010 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 rb/rw ~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 H2+ ion: 0.3 µs, 0.6 µs (0.12-0.24 of revolution time) • Expelled ion current extremely small: • HCX: 180 mA, ~1E-15 cm2, P~3E-7 torr,  100 nA/cm2 • CESR: ~1 mA, Cross section much smaller, P ~1E-9,  fA/cm2? • CESR-100%ionized gas: 3.3E9cm-3(0.0012cm3)q/[(1µs)(12cm)~0.5nA/cm2 • Conclusion: difficult measurement – what other possibilities?

  14. 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?

  15. Outline Introduction & e -cloud diagnostics on HCX Possibilities for CESR Clearing electrode experiments Application to CESR Summary

  16. Trapping depth of electrons depends upon their source, in a quadrupole magnet (without multipactor) E-cloud in a quadrupole magnet [Electron mover also speeds simulation in wiggler fields] Electrons desorbed from beam pipe in quad upon ion impact Electrons from ionization of gas Electrons ejected from end wall beam pipe Deeply trapped electrons Weakly trapped electrons

  17. Gridded Electron Collectors (GEC) current measures electron depth of trapping Deeply-trapped electrons Weakly-trapped electrons Beam-tail scrapes wall GEC collects e- along B-field lines, detrapped at end of pulse HCX current flattops for ~4 µs (like super-bunch) Beam potential contours In drift region

  18. 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.

  19. Clearing electrode removes all electrons from a drift region Suppressor bias = 0 V, electrons can leak back into quads along beam. • Clearing electrode ring (C) – blocks electrons from (B) when biased more negatively than -3 kV • Clearing electrode ring (B) [with Vc= 0] blocks electrons from (A) when biased more negatively than -3 kV

  20. Vc = +2 kV b = +2 kV Vc = 0 kV b = +2 kV Vc = +10 kV b = +2 kV Clearing electrode fields, above 2 kV bias, dominate over beam space-charge field Beam space-charge potential b = +2 kV For CESR-TA, clearing field should dominate over beam space charge averaged over a few bunches (Vc > 50 V), or remove electrons in period between bunches (Vc ≥ 100 V).

  21. + new e-/gas modules quad beam 1 2 Key: operational; partially implemented (4/28/06) + New e- mover + Adaptive Mesh Refinement Allows large time step greater than cyclotron period with smooth transition from magnetized to non-magnetized regions concentrates resolution only where it is needed R Speed-up x10-104 e- motion in a quad Speed-up x10-100 3 4 Z WARP-POSINST code suite is unique in four ways merge of WARP & POSINST

  22. (a) (b) (c) 0V 0V 0V/+9kV 0V e- 200mA K+ Q1 Q2 Q3 Q4 Electrons Simulation 200mA K+ Beam ions hit end plate Electrons bunching Oscillations I (mA) 0. -20. -40. Simulation Experiment 0. 2. time (s) 6. Electron oscillations – simulation & experiment agree Current to clearing electrode(c) agrees in frequency ~ 6 MHz WARP-3D T = 4.65s And also Currents to capacitive electrode array agree in wavelength ~5 cm, and amplitude (below) Experiment Simulation

  23. Outline Introduction & e -cloud diagnostics on HCX Possibilities for CESR Clearing electrode experiments Application to CESR Summary

  24. 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

  25. Outline Introduction & e -cloud diagnostics on HCX Possibilities for CESR Clearing electrode experiments Application to CESR Summary

  26. Summary • • 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

  27. Backup slides

  28. Simulations with beam reconstructed from slit scans – improved agreement Low e- High e- • 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 X' High e- No e- X' Semi-gaussian load => New load from reconstructed data => High e- No e- X' X X Reconstructed X-Y distributionfrom slit-plate measurements

  29. Heavy Ion Inertial Fusion or “HIF” goal is to develop an accelerator that can deliver beams to ignite an inertial fusion target ~1km 4.0GeV; 94.A/beam; 0.2ms 1.6MeV; 0.63A/beam; 30ms 4.0GeV; 1.9kA/beam; 10ns DT Target Requirements: 3 - 7 MJ x ~ 10 ns  ~ 500 Terawatts Ion Range: 0.02 - 0.2 g/cm2 1- 10 GeV For A ~ 200  ~ 10 16 ions ~ 100 beams 1-4 kA / beam Near term goal: High Energy Density Physics (HEDP)

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