Electron-Cloud Studies for the NLC and TESLA

Transcription

1 Electron-Cloud Studies for the NLC and TESLA Mauro Pivi NLC Division SLAC Jan 2004

2 this is a progress report builds on previous simulation experience with PEP-II, LHC and SPS, KEKB, APS, PSR, SNS and other machines where the electron cloud has been observed or expected builds on the experience of many people; I am particularly grateful to: M. Furman, O. Brüning, Y. Cai, A. Chao, F. Le Pimpec, O. Gröbner, K. Harkay, S. Heifets, N. Hilleret, B. Henrist, R. Kirby, R. Macek, K. Ohmi, J. Rogers, R. Rosenberg, G. Rumolo, G. Stupakov, T. Raubenheimer, F. Zimmermann, A. Wolski

3 Main sources of primary electrons Picture: The NLC positron MDR stores 3 trains, separated by 65 nsec with each train consisting of 192 bunches. Each bunch generates a small number of electrons by residual gas ionization which does not dissipate completely during the train-gap and may grow up to an equilibrium saturation limit due to space charge, secondary yield etc. Simulations starting with initial e - density near the saturation limit

4 Simulation code features POSINST 3D electron kinematics multi bunches on axis electron multiplication by SEY model generation of the cloud HEAD-TAIL / CLOUD_MAD 3D electrons and single-bunch particles dynamics constant number of electrons single-bunch dynamic and instability

5 Electron-cloud generation in NLC/TESLA DR Secondary electron yield SEY (δ) model δ max Secondary electron yield - SEY Primary electron energy (ev) Field-free regions: NLC NLC and and TESLA. Threshold for for the the development of of the the electron-cloud is is for for a SEY SEY δ max max ~1.5Ö1.6 and and 2Ö2.1 respectively.

6 Thresholds for the electron cloud in NLC MDR arc elements quadrupole wiggler dipole In In the the NLC NLC MDR MDR wigglers (3D (3D model) threshold occurs at at δ max max ~1.2Ö1.3. NLC NLC dipole δ max max ~1.3 ~1.3 Ö1.4 Ö1.4 and and quadrupole (3D (3D model + fringe fields included) δ max max ~1.2 ~1.2 Ö1.3. Note: Note: In In quadrupole field field regions electron trapping mechanism occurs occurs (mirror effect), electrons are are trapped for for long long time. time.

7 Electron distribution in the TESLA wiggler Snapshot of the transverse x-y phase space electron distribution in the TESLA wiggler Equilibrium density in in the the TESLA wiggler. Threshold occurs at at peak peak SEY~1.2Ö1.3

8 Single-bunch head-tail instability Main Damping Ring y ( x) osc 1 π Electrons are pulled into the bunch and oscillate in the beam well potential. Thus, electron oscillation frequency is important. If number of oscillation n osc ~ 0 no instability expected. For n osc ~1 instability tends to reach equilibrium. n ω = t 2 N ey ( x) bunch 2 π σ y( x) p ( σ σ r x z e + σ y ) Average_electron_cloud_density_along_the_ring_(1/m^3): 1.0e+11 Number_of_particles_per_bunch: 0.75e+10 Horizontal_beta_function_at_the_kick_sections_[m]: 10.0 Vertical_beta_function_at_the_kick_sections_[m]: 10.0 Bunch_length_(rms_value)_[m]: Horizontal, Verticla_beam_size_(rms_value)_[m]: 4.900E-05, 6.000E-06 Longitudinal_momentum_spread: Synchrotron_tune: Momentum_compaction_factor: 1.388e-3 Number_of_kick_sections: 1 Number_of_laps: 2024 Multiplication_factor_for_pipe_axes x, y 10, 10 Horizontal, Vertical _tune: , Horizontal, Vertical chromaticity: 0, 0 Flag_for_synchrotron_motion: 4 Switch_for_wake_fields: 0 Switch_for_pipe_geometry_(0->round_1->flat): 0 Res_frequency_of_broad_band_resonator_[GHz]:* 1.3 Transverse_quality_factor:* 1. Transverse_shunt_impedance_[MOhm/m]:* 10. Res_frequency_of_longitudinal_resonator_[MHz]:* 200 Longitudinal_quality_factor:* 140. Longitudinal_shunt_impedance_[MOhm]:* 0.0 Flag_for_the_tune_spread_(0->no_1->space_charge_2->random):* 0 Flag_for_the_e-field_calc_method_(0->no_1->soft_Gauss_2->PIC): 2 Magnetic_field_(0->no_1->dipole_2->solenoid_3->combined): 0 x-kick_amplitude_at_t=0_[m]: 0. y-kick_amplitude_at_t=0_[m]: 0. Flag_for_the_proton_space_charge: 0 Solenoid_field_[T]: 0.00 Switch_for_amplitude_detuning:* 0 Coherent_centroid_motion_(0->off_1->on): 1 El_distrib_(1->Rect_2->Ellip_3->[1_strp]_4->[2_strp]_5->Parab): 2 Linear_coupling_switch(1->on_0->off):* 0 Linear_coupling_coefficient_[1/m]:* Average_dispersion_function_in_the_ring_[m]:* 0. Position_of_the_stripes_[units_of_sigmax]: 3.0 Width_of_the_stripes_[units_of_sigmax]: 0.5 Kick_in_the_longitudinal_direction_[m]: 0. Number_of_turns_between_two_bunch_shape_acquisitions: 10 Cavity_voltage_[V]: 2.0e6 Cavity_harmonic_number: 714 Sextupolar_kick_switch(1->on_0->off):* 0 Sextupole_strength_[1/m^2]: Dispersion_at_the_sextupoles_[m]: 2.24 Switch_for_losses_(0->no_losses_1->losses): 1.0 Second_order_horizontal_chromaticity_(Qx''):* 0. Second_order_vertical_chromaticity_(Qy''):* 0. Switch_for_boundary_conditions(0->open_space_1->rect_box): 0 Switch_for random phase advance (0->no_1->yes): 0 synchrotron synchrotron oscillations oscillations à damping damping mechanism mechanism for for fast fast head-tail head-tail instability instability

9 Single-bunch instability É MDR NLC MDR: time evolution of the horizontal and vertical beam size at different electron-cloud density. Synchrotron tune of (Note: recent simulations with a round initial electron-cloud distribution and sinusoidal RF bucket).

10 Single-bunch É MDR <yz> correlations: signature of head-tail for for cloud cloud densities densities > 8e11 8e11 e/m e/m 3 3 à fast fast head-tail, head-tail, growth growth time time ~ 100µs* 100µs* *synchrotron tune tune =

11 Electron-cloud generation in NLC/TESLA DR Secondary electron yield SEY (δ) model Secondary electron yield - SEY δ max Single-bunch Single-bunch instability instability threshold threshold for for the the NLC NLC MDR MDR Primary electron energy (ev) Threshold for for the the development of of the the electron-cloud head-tail instability in in the the NLC NLC field field free free region region is is at at ~8E+05 e/cm e/cm 3 3 occuring for for a peak peak SEY SEY δ max max ~1.5Ö1.6.

12 Single-bunch instability studies: Beam Delivery System Used CLOUD_MAD code developed at SLAC to simulate the interaction between the electron-cloud and a single bunch propagating along the NLC beam delivery system, to estimate undesired additional focusing effects: Phase advance change in the Chromatic Correcting Section (CCS) leading to possible uncompensated geometric aberrations Direct focusing, estimated as 2.50E E+00 Near Threshold Near Threshold σx σx( ( y) y) φ(s) k 1 L 0 k1 4 0r e β π n / γ x( y) phase advance ds' sin 2 [ φ( s) φ( s') ] focusing gradient depending on e- cloud density and beam energy The The increase of of the the IP IP spot spot size size has has a threshold for for a uniformly uniformly distributed distributed electron electron cloud cloud density density of of ~ e/m e/m Threshold Threshold is is constant constant when when switching switching the the sextupoles sextupoles ON/OFF, ON/OFF, indicating indicating that that the the dominant dominant contribution contribution is is given given by by direct direct focusing rather rather than than breakdown breakdown of of the the CCS. CCS. Threshold Threshold scales scales inversely inversely with with beam beam energy energy è ~ 2e10 2e10 e/m e/m Sigma Growth Sigma Growth 2.00E E E E E E E E E E E E E E E E+12 Density (e/m-3) Density (e/m-3) S igma X/X0 S igma X/X0 Sigma Y/Y0 Sigma Y/Y0 Threshold in in the the 3 stages stages of of the the Bunch Bunch Compressor system system is is found found at at e/m e/m 3 3,, close close to to neutralization. neutralization. See D. Chen, A. Chang, D. Bates, M. P., T. Raubenheimer LCC-0126

13 Secondary electron yield: measurements and surface treatments at SLAC Unique Unique apparatus apparatus in in U.S. U.S. laboratories laboratories for for Secondary Secondary Electron Electron Yield Yield (SEY) (SEY) and and secondary secondary energy energy spectrum spectrum measurements. measurements. In In situ situ Auger, Auger, XPS XPS (surface (surface characterization). characterization). R. R. Kirby, Kirby, F. F. Le Le Pimpec, Pimpec, M. M. P. P. at at SLAC, SLAC, A. A. Wolski, Wolski, K. K. Kennedy, Kennedy, L. L. Koth Koth at at LBNL LBNL Test Test promising remedies remedies TiN TiN coating, coating, reproducible results. results. TiZrV TiZrV not not evaporable getter getter (NEG) (NEG) coatings. coatings. Results Results improve improve electron-cloud simulation model model

14 Secondary electron yield: TiN coating samples (from BNL Brookhaven) SEY measurements of TiN / Al as received - ok with literature, #4 Mg contamination

15 Secondary electron yield: TiZrV non-evaporable getter thin film coating TiZrV TiZrV TiZrV is is a a non-evaporable non-evaporable getter getter (NEG) (NEG) which which requires requires in in situ situ activation activation at at o C o to to provide provide pumping pumping and and low low SEY SEY Re-increase of the SEY under vacuum. Note: In laboratory measurements when the activated TiZrV sample is left under vacuum a high re-contamination rate may occur (due to the small pumping sample surface compared to the large external degassing vacuum chamber?). Need a SEY coating durability test in a TiZrV entirely coated chamber. SLAC measurements of the TiZrV non-evaporable getter (NEG) CERN measurements show δ max ~1.1 after activation Òas long as the initially activated TiZrV sample is not exposed to air, δ max never exceeds 1.3, regardless of the kind of gas (H2, CO, CO2, and H2O) to which the sample is exposed. Hence, the SEY of an activated TiZrV coating does in UHV not exceed the critical value of 1.35 and in an entirely NEG coated LHC beam pipe.. Ò Sheuerlein et al. CERN EST/ (SM) At At LBNL LBNL preparing preparing for for TiZrV TiZrV sample sample coating, coating, need need R&D R&D

16 next: electron conditioning effect electron conditioning of copper samples laboratory test V. Baglin, I. Collins, B. Henrist, N. Hilleret and G. Vorlaufer CERN LHC Project Report 472. electron bombardment may decrease the secondary electron yield to acceptable levels. We are ready to test the electron conditioning effect on TiZrV and TiN.

17 Benchmark sim. Simulations Lab measurements e - trapping mechanism in Quad e - detector meas. in PEPII beam dynamics e - cloud generation & equilibrium single and multi-bunch instability self-consistent 3D simulations SEY meas. coatings + treatments Coating durability under vacuum Requirements Path TiN TiZrV radius Other? Demonstration build vacuum chamber, install it in PEPII, meas. SEY in or ex situ

18 Summary Simulations and laboratory measurements are making progresses in the understanding of the requirements. Simulations of electron cloud generation set thresholds for the SEY at values ~1.2Ö1.3 in NLC and TESLA wigglers Single-bunch head-tail instability simulations: - Estimating emittance growth thresholds in the MDR, BDS at ordersof-magnitude lower than equilibrium electron density level and close to neutralization level in BCS Laboratory measurements of the SEY - testing electron conditioning on TiZrV and TiN, other possible remedies and treatments. - developing technology to coat TiZrV samples LBNL Benchmarking: electron trapping mechanism in quadrupole set-up measurements, installation of electron detectors in straight section of PEPII LER.

19 Road map for future work Simulations ü Single-bunch Head-Tail: Instability threshold studies for NLC MDR and PPDR. A more complete set of simulations with overall picture. February Analytic studies. Instability threshold for NLC Transport Lines. ü Coupled-bunch: Simulation for the NLC MDR and PPDR. January Simulations of the NLC BDS. ü Self consistent 3D simulations: starting collaboration with USC, 3D plasma simulation code. Measurements and Coatings ü Conditioning: Setting up electron gun in the SEY system, that allows faster conditioning. Starting conditioning the TiZrV CERN sample. Results Jan Conditioning TiN samples. Results February ü Continuing SEY measurements ü TiZrV coating of Al substrate samples LBNL coating from a TiZrV sinthered coupon. First TiZrV coated samples end of Jan Sending SAES Getter samples for TiZrV coating. ü Demonstration coating, full-test in accelerator (PEP-II). 1 _ years time scale Simulations Benchmark ü Install electron detectors in the PEP-II. Tune the winding solenoid fields and bunch pattern in the LER. Waiting for decision. ü Electron trapping in Quadrupole. under discussion, collaboration with LANL and PEPII

20 ÒPOSINSTÓ simulation codes features Started Ô96 by Miguel Furman at LBNL, implemented together with M. P. since 2000, single or multi-bunch passages, short or long bunches, and effective bunch profile the electron cloud is dynamically generated from: residual gas ionization secondary electron yield (SEY), detailed model included other possible sources: photoelectron emission LHC, proton losses in PSR and SNS field-free region, dipole sections, solenoid field, quadrupole, sextupole, wiggler, etc. bunch divided longitudinally into N k kicks (typ. 251 for NLC) 3D electron kinematics purely transverse electron space-charge effects purely transverse beam-electron forces round or elliptical vacuum chamber geometry, with a possible antechamber perfect-conductor BCs (surface charges included)

21 Single-bunch head-tail instability Main Damping Ring ÒHEAD-TAILÓ simulation codes features Started by G. Rumolo at CERN, single bunch interacting with an electron cloud. The electron-cloud density is given, the number of electrons is constant, the electron-cloud is re-initialized at each bunch passage. PIC calculation, FFT open space. bunch divided longitudinally into N k kicks (typ. 70), field free region (dipole, solenoid) The electrons and bunch macro-particles are mutually kicked at N-locations (typ. 1) in the ring One turn transfer map applied to the bunch macrop: damping not included 3D macro-particle kinematics typ macro-electrons and macrop. bunch Purely transverse electron space-charge effects Vacuum chamber impedance wake fields are turned off for these simulations