Electron Cloud Studies made at CERN in the SPS

Transcription

1 Electron Cloud Studies made at CERN in the SPS J.M. Jimenez On behalf of the Electron Cloud Study Team, a Collaboration between AT and AB Departments

2 Main Topics Introduction LHC Injectors SPS Running with LHC-type beams Electron Cloud Signatures Main Results in the SPS Electron Cloud Build up Vacuum Cleaning and Beam Conditioning Role played by the Physisorbed Gasses Scrubbing Runs Scenarios Conclusions

3 Introduction CERN Facilities LHC Injectors (1)

4 Introduction CERN Facilities LHC Injectors (2)

5 Introduction SPS Running with LHC-type beams

6 Introduction Electron Cloud Signature in the SPS Pressure Pressure rise rise trigger trigger beam beam dump dump Emittance Emittanceblow-up (preserved (preserved in in the the LHC) LHC) Bending areas Straight areas Bending areas Red = P >10-4 Pa, Green = P < 10-4 Pa

7 Introduction Pressure readings give indications 10 8 P/P 6 4 Duty cycle ~ 45 % after 17h integrated time of LHC beam E E E E E E+11 Protons bunch intensity Different behavior between DF and FF 14 Beam conditioning and bunch dependence observed but results not easily understandable 12 P/P Duty cycle ~ 45 % after 17h integrated time of LHC beam E+00 1.E+10 2.E+10 3.E+10 4.E+10 5.E+10 6.E+10 7.E+10 8.E+10 9.E+10 1.E+11 Proton bunch intensity

8 Main Topics Introduction LHC Injectors SPS Running with LHC-type beams Electron Cloud Signatures Main Results in the SPS Electron Cloud Build up Vacuum Cleaning and Beam Conditioning Role played by the Physisorbed Gasses Scrubbing Runs Scenarios Conclusions

9 Main Experimental Set-ups SPS Electron Cloud Test Bench Cold Baffle CSD Cold Baffle MBA Pick-ups QSD Strip Detector in a Quadrupole RFD Strip2002 VASD Variable Aperture Strip Detector CSD Cold Strip Detector MBB Pick-ups

10 Main Results in the SPS (1) Electron Cloud Build-up is a threshold phenomenon: p/bunch in Dipole Field regions p/bunch in Field Free regions Intensity varies linearly with the bunch intensity Non-homogeneous spatial distributions in dipole and quadrupole fields Intensity varies linearly with the filling pattern No extinction during the 225 ns batch spacing evidence of surviving electrons NEG coatings will decrease the electron cloud activity by their intrinsic low SEY: 1.1 after activation and 1.3 if saturated by water Baseline for the LSS RT parts

11 Main Results in the SPS (2) Electron Cloud Build-up BNC 50 Ω Surviving e - e - cloud signal Simulations LHC beam signal 550 ns spacing

12 Main Results in the SPS (3) SEY Decrease measured in situ in the SPS 2.5 Evolution of the SEY=f(E) with the conditioning Total number of electrons contributing to the build up SEY Energy (ev) 4 h 10 h 21 h 63 h 90 h Courtesy of B. Henrist and N. Hilleret Relative decrease of the surface above Relative decrease of the SEY Number of secondary electrons generated by a primary electron

13 1.2E-09 Main Results in the SPS (4) Typical Signal from the RFD Detector Maximum of energy distribution around 100 ev Electrons with energies as high as 600 ev still visible SPS Scrubbing run Electron energy distribution 4 batches, four sweeps up and down Data#16 on Fri 11th June :21 Sweep 1 Sweep 2 Sweep 3 Sweep 4 8.0E E-10 Fraction of electrons with E > 300 ev is not negligible! 0.0E Electron energy (ev)

14 Main Results in the SPS (5) Spatial distribution of e - in Dipole and Field free regions In field free conditions In dipole field conditions Ramp Ramp 4 th batch 4 th batch 3 rd batch 3 rd batch Ne- (a.u.) 1 st batch 2 nd batch Time (2 s / Div) Time (2 s / Div) 26 mm 2 nd batch Ne- (a.u.) 1 st batch Lateral position (mm)\ Lateral position (mm)

15 Main Results in the SPS (6) Spatial and Energy Distributions in Dipoles Field free - Strip pick-ups Dipole field Strip pick-up Distribution (a.u.) dn/de (a.u.) Electron Energy (ev) 80 ev 180 ev Field free - Retarding Field Detector (fit) Energy (ev) Field free Dipole field Strip pick-up (multicycle measurement) dd2n/dxde 2 Strip-detector (single-cycle measurement-fit) Strip pick-up (single-cycle measurement) RFD Distribution (a.u.) Ne- (a.u.) Lateral position (mm) V V (exp. Decrease) Heat load efficiency = HLE e - HLE(DF)= 1.7 e - HLE(FF) -180 V -115 V -70 V Filtering potential E e ->180 ev located in the centre faster beam conditioning observed -30 V -50 V

16 Main Results in the SPS (7) Spatial Distribution in Quadrupoles a.u Time (2 s / Div) Detection area Lateral position (a.u.) , 2, 3 and 4 circulating batches

17 Main Results in the SPS (8) Spatial Distribution in Quadrupoles Results compared to Simulations Intensity 1, 2, 3 and 4 batches circulating 3D simulations made by L. Wang, 13 th ICFA Mini-Workshop, BNL 12/2003

18 Main Results in the SPS (9) Spatial Distribution in Quadrupoles Effect of the Field Strength Lateral positon (mm) 0.3 T/m 0.2 T/m 0.1 T/m 0.05 T/m 0 T/m Intensity (a.u.)

19 Main Results in the SPS (10) Energy Ramp / Orbit Displacement Appearance of the 3 rd central strip lateral strips (A/m 2 ) 400 GeV 4.5E-3 4.0E-3 3.5E-3 Ramp Ne- (A/m 2 ) Σ N e - (A/m) 26 GeV central strip (A/m 2 ) Ramp to 450 GeV 3.0E-3 2.5E-3 2.0E-3 1.5E-3 1.0E-3 5.0E-4 Ne- (A/m) N e - (A/m 2 ) 4.5E-2 4.0E-2 3.5E-2 3.0E-2 2.5E-2 2.0E-2 1.5E-2 1.0E-2 5.0E-3 0.0E Lateral position (mm) 1 s/div Pressures (mbar) E Time (ms) 1.0E-6 1.0E-7 1.0E-8 Ramp to 55 GeV 1.0E-9 1.0E E E E E E E E+23 Cumulated dose (protons) Dipole field (HP406) Field free (HP415)

20 Main Results in the SPS (11) Bunch Shortening enhances ECloud A 33 rd rd strip strip appears appears in in the the center center by by decreasing decreasing the the bunch bunch length length (-30%) (-30%) F increase increase of of the the beam beam potential potential N electrons (a.u.) Lateral position (mm) S15 S8 S1 S43 S36 S29 S22 S99 S92 S85 S78 S71 S64 S57 S50 Dipole field region 1.1x10 11 p/bunch 1 batch Time (25 ms/div)

21 Main Results in the SPS (12) Role played by the Physisorbed Gasses Physisorbed water identified as a potential problem: Conditioning has been observed in the SPS if the cold detector is protected against water back streaming from the unbaked parts In the LHC, low water coverage is expected: Pumping down to 10-6 torr of the cold parts prior to the cooling Controlled cool down sequence where the cold bore is cooled while the beam screen is kept as warm as possible Results obtained with the CSD in the SPS can not be extrapolated to the LHC since the SPS detector do not have the Beam screen/cold bore arrangement: The molecules desorbed by the electrons will stay in the beam pipe as they will migrate to the cold bore in the LHC case Thick coverage (>5 monolayer) are not expected in the LHC due to the continuous bombardment by the electrons and to the pumping speed through the beam screen pumping slots. Courtesy of V. Baglin (CERN) LHC cold surfaces should behave like bare copper surfaces

22 Main Results in the SPS (13) Conditioning of Cold Surfaces 1.0E-02 Dipole field (30-50 K), 25ns bunch spacing Flux to the wall (A/m) 1.0E-03 I (ma/m) CSD in T of the beam screen (K) E Cycle number

23 Main Results in the SPS (14) Vacuum Cleaning and Beam Conditioning Vacuum Cleaning observed in the SPS during the 25 ns / 75 ns Periods Factor 10 during the 75 ns period, 100 during the 25 ns period for both FF and DF Beam Conditioning on the cold surfaces has been observed in the SPS, probably a different physical process than at RT Beam 75 ns Dipole field at Cold Factor 100 after 7 hours Beam 25 ns Dipole field at Cold Factor 10 after 1½ day Quadrupole field at RT Factor 2.5 after 2 days Gasses physisorbed on the cryogenic surfaces will play a predominant role (see next slide) Effect of a Cycling in Temperature As expected from Lab measurements, a water condensation will reset the beam conditioning. A temperature cycling did not helped to recover the initial value. Electron cloud intensity back to the initial value before the conditioning

24 Main Results in the SPS (15) Role played by the Physisorbed Gasses Reduction of the SEY by the physisorbed CO Starting point baked sample Starting CO injection Courtesy of B. Henrist and N. Hilleret (CERN) 4-7 monolayers of CO CO 2 and H 2 O could have a detrimental effect as the others like H 2 and CO will decrease the SEY

25 Main Topics Introduction LHC Injectors SPS Running with LHC-type beams Electron Cloud Signatures Main Results in the SPS Electron Cloud Build up Vacuum Cleaning and Beam Conditioning Role played by the Physisorbed Gasses Scrubbing Runs Scenarios Conclusions

26 Scrubbing Runs Scenarios (1) Maximizing the Efficiency Beam related issues To maximize the scrubbing efficiency, the beam emittance, beam losses and beam instabilities has to remain under control F Not an issue in the SPS since a new beam is injected every 21 s Bunch Spacing, Filling Pattern and Beam energy Bunch Spacing No limitation expected with the 75 ns bunch spacing up to nominal No limitation expected with the 25 ns bunch spacing if < p/bunch > p/bunch F will depend on the induced heat load provided that beam instabilities and emittance growths are kept under control ~ p/bunch is the expected limit prior to the beam conditioning Filling pattern A modification of the filling pattern by increasing the gaps (RHIC case) between the batches shall be preferred to the reduction of the bunch intensity: Displacement of the lateral strips in a dipole field with the bunch intensity Decreases the average energy of the electrons from the cloud reduction of the conditioning efficiency

27 Scrubbing Runs Scenarios (2) Maximizing the Efficiency Scrubbing runs at injection energy Will increase the cooling budget available for the electron cloud-induced heat load, Will only work if not limited by other effects like beam instabilities or emittance growths, Will require a short scrubbing run top energy in case of a small orbit displacement during the ramp in energy De-conditioning effect Has been observed both in the Lab and in accelerators (EPA and SPS) when the surface is no longer bombarded F Subsequent conditioning is 10 times faster Is expected as a consequences of a partial warming up of the cold parts during the shutdown physisorbed gasses will go back to the gas phase and be recondensed during the following cooling down F Part of the molecules in the gas phase will be pumped by the mobile pumping stations

28 Main Topics Introduction LHC Injectors SPS Running with LHC-type beams Electron Cloud Signatures Main Results in the SPS Electron Cloud Build up Vacuum Cleaning and Beam Conditioning Role played by the Physisorbed Gasses Scrubbing Runs Scenarios Conclusions

29 Conclusions (1) SPS as the LHC Injector SPS shall be available for LHC injection but also for the CNGS After shutdowns, 5 days of beam conditioning are required No signal in the field-free regions (long straight sections) In the arcs (dipole field), electron cloud still visible Strong vacuum cleaning (factor 100 in 4 days) Beam conditioning limited to the parameters used during the scrubbing will not be effective if running conditions become more favourable F SPS Electron Cloud Test Bench will be kept for electron cloud build up and induced instabilities studies and benchmarking of the simulations Electron Cloud Build up Comparison 25 / 75 ns and 225 ns Batch Spacing In Dipoles: Factor ns compared to 25 ns In Quadrupoles: Factor 2 75 ns compared to 25 ns Batch Spacing Spacing > 1000 ns are useless if balanced with luminosity reduction Potential gain if 25ns / 75ns bunch spacing is adopted Measurement of Spatial and Energy Distribution consolidated

30 Conclusions (1) LHC Issues There is conditioning on the cold surfaces, probably a different physical process than at room temperature Water identified as the culprit in SPS No reason to have condensed water in the LHC arcs Physisorption of CO on beam screen will help conditioning But not yet clear where it will start after a warm-up! No limitation if running with 75 ns bunch spacing Beam 75 ns Dipole field at Cold Factor 100 after 7 hours Beam 25 ns Dipole field at Cold Factor 10 after 1½ day Quadrupole field at RT Factor 2.5 after 2 days Vacuum Scrubbing in the SPS during the 25 ns / 75 ns Periods Factor 10 during the 75 ns period, 100 during the 25 ns period for both FF and DF Effect of a Cycling in Temperature A cycling in temperature aimed to condensate water on the CSD. As expected from Lab measurements, it reset the beam conditioning Electron cloud intensity back to the initial value before the conditioning

31 Acknowledgements Many thanks to G. Arduini, V. Baglin, P. Collier, O. Gröbner, G. Ferioli, J. Hansen, B. Henrist, N. Hilleret, L. Jensen, D. Schulte, P. Strubin, A. Rossi, F. Ruggiero, J. Wenninger and F. Zimmermann for their help To the operation crew of both PS and SPS To the AT/VAC-SL Section collaborators for the installation of the detectors To the TS Department collaborators for the design and manufacturing of the detectors