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Secondary Emission Monitor for very high radiation areas of LHC

Secondary Emission Monitor for very high radiation areas of LHC. Daniel Kramer for the BLM team. LHC Beam Loss Monitoring system. ~ 3700 BLMI chambers installed along LHC ~ 280 SEM chambers installed in high radiation areas: Collimation Injection points IPs Beam Dumps Aperture limits

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Secondary Emission Monitor for very high radiation areas of LHC

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  1. Secondary Emission Monitorfor very high radiation areas of LHC Daniel Kramer for the BLM team

  2. LHC Beam Loss Monitoring system • ~ 3700 BLMI chambers installed along LHC • ~ 280 SEM chambers installed in high radiation areas: • Collimation • Injection points • IPs • Beam Dumps • Aperture limits • Main SEM requirements • 20 years lifetime (up to 70MGray/year) • Sensitivity ~7E4 lower than BLMI D.Kramer BLM Audit

  3. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Ti HV electrodes Steel vessel (mass) < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) D.Kramer BLM Audit

  4. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Incoming particle Ti HV electrodes Steel vessel (mass) < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) D.Kramer BLM Audit

  5. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Incoming particle Ti HV electrodes Steel vessel (mass) < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) D.Kramer BLM Audit

  6. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Incoming particle Ti HV electrodes Steel vessel (mass) < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) D.Kramer BLM Audit

  7. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) Transit time 500ps • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Incoming particle Ti HV electrodes Steel vessel (mass) Incoming particle < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible and avoid capture of electrons • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) D.Kramer BLM Audit

  8. SEM production assembly • All components chosen according to UHV standards • Steel/Ti parts vacuum fired • Detector contains 170 cm2 of NEG St707 to keep the vacuum < 10-4 mbar during 20 years • Pinch off after vacuum bakeout and NEG activation (p<10-10mbar) • Ti electrodes partially activated (slow pumping observed during outgassing tests) • NEG St707 composed of Zr, Vn, Fe • Zr flamable -> insertion after the bottom is welded • Very high adsorbtion capacity of H2, CO, N2, O2 • Not pumping CH4, Ar, He D.Kramer BLM Audit

  9. Vacuum bakeout and activation cycle for SEM and BLMI • NEG inside the SEM needs additional activation at 350°C • Activation means releasing adsorbed gases on the surface which have to be pumped • Pinchoff done during the cool down of the chamber • Resulting pressure below measurement threshold (<10-10mbar) • Manifold stays colder to limit the load to the pumping system • Activation temperature limited by the feedthroughs NEG activation Vacuum bakeout He leak tests pinchoff Vacuum bakeout Ion pump started He leak tests D.Kramer BLM Audit

  10. Geant4 simulations of the SEM • Secondary Emission Yield is proportional to electronic dE/dx in the surface layer • LS = (0.23 Ng)-1 g = 1.6 Z1/310-16cm2 • “TrueSEY” of each particle crossing the surface boundary calculated and SE recorded with this probability • Correction for impact angle included in simulation • QGSP_BERT_HP as main physics model Model calibration factor Electronic energy loss Penetration distance of SE 0° impact angle Geant4 SEM Response function Comparison to literature values => CF = 0.8 D.Kramer BLM Audit

  11. SEM Calibration experiment in a mixed radiation field (CERF++ test) • Response of the SEM measured with 300GeV/c beam hitting 20cm copper target • Setup simulated in Geant4 • Response of SEM filled by AIR measured and simulated as well • SEM Response expressed in absolute comparison to Air filled SEM • Response = Dose in AIR SEM / output charge of SEM • 0.259 +/- 0.016 Gy/count H4 Calibration setup with Cu target and a box with 16 SEMs on a movable table D.Kramer BLM Audit

  12. Calibration results Only 2 chambers out of 250 had higher offset current Not corrected for systematic position errors Upper Limit on the SEM pressure: (equivalent to 3 of the histogram) 1bar(0.6 sigSEM / sigSEM AIR) = 0.26 mbar Pressure inside SEMs smaller than this Offset current without beam D.Kramer BLM Audit

  13. Table of SEM measurements and corresponding simulations D.Kramer BLM Audit

  14. Thanks D.Kramer BLM Audit

  15. Backup slides • Vacuum stand in IHEP for IC production • 36 ICs in parallel baked out and filled by N2 • For SEMs only 18 chambers in parallel • No N2 injection :o) • He leak detection done before and after bakeout (and after NEG activation for SEMs) D.Kramer BLM Audit

  16. Beam dumped on a Closed Jawof LHC collimator in LSS5. SEM to BLMI comparison 1.3 1013p+ BLMI A SEM Black line – signal not clipped 5*τ_filter = 350ms D.Kramer BLM Audit

  17. Cable crosstalks study – important crosstalks caused by long cables in the LSS • Ch 6..8 unconnected • Xtalk clearly depends on the derivation • Signal peak ratio 5e-2 (26dB) (worst case) • Integral ratio 4.4e-3 (47dB) • Similar behavior for system A • X-talks limited to 1 CFC card only! D.Kramer BLM Audit

  18. Standard BLMI ARC installation Small low pass filter in the CFC input stage HV ground cut here BLMI HV Power Supply CFC is always close to the quadrupole Up to 8 BLMs connected in parallel D.Kramer BLM Audit

  19. BLMI / SEM installation for collimation areas 8 chambers in 1 NG18 cable (up to 700m) HV capacitor removed 6 HV capacitors in parallel 150k for current limitation 280pF = chamber’s capacity ~25pF = SEM’s capacity SEM has not 150k protection! D.Kramer BLM Audit

  20. 150kOhm Rp resistor for BLMI i/o current limitation between HV capacitor & IC) • Limits the peak current on the chamber input to 1500 / 150k = 10mA • Fast loss has only the Chamber charge available 280pF * 1500V = 0.4 uC • Corresponds to ~ 7 mGy total loss • Corresponds to ~ 180 Gy/s (PM limit = 22 Gy/s) • Slows down the signal collection • DC current limited to 1500V / 1Mohm = 1.5 mA • Corresponds to ~ 26 Gy/s (total in max 8 chambers) D.Kramer BLM Audit

  21. BLMI and SEM in the dump line IR6 on the MKB D.Kramer BLM Audit

  22. 400 GeV Beam scan in TT20 SPS line • Longitudinal impact of proton beam • r = 2mm • Chamber tilted by ~1° • Simulation sensitive to beam angle and divergence • Negative signal due to low energy e- from secondary shower in the wall Integral of Simulation = 0.608 e-mm Integral of Scan2 = 0.476 e-mm Relative difference 22% D.Kramer BLM Audit

  23. Prototype tests with 63MeV cyclotron beam in Paul Scherer Institute PSI proton beam 62.9MeV BLMS prototypes F & C Type HV dependence of SEY • Prototype C -> more ceramics inside (no guard ring) • Prototype F -> close to production version • Current measured with electrometer Keithley 6517A • HV power supply FUG HLC14 • Pattern not yet fully understood • Not reproduced by simulation • High SE response if U_bias > 2V • Geant4.9.0 simulated SEY = 25.50.8% D.Kramer BLM Audit

  24. Measurements in PS Booster Dump line with 1.4 GeV proton bunches • Older prototype measured - Type C {Type F simulated} • Profiles integrated with digital oscilloscope • 1.5kV bias voltage • 80m cable length • 50  termination • Single bunch passage • SEY measurement • 4.9  0.2% • Geant4.9.0 simulation • 4.2  0.5% Normalized response D.Kramer BLM Audit

  25. Example loss induced by the fast moving SPS scraper. Measured in the collimation area by the LHC BLM system 4 different monitors (2006-old electronics) D.Kramer BLM Audit

  26. Example of beam losses in the SPS collimation area during a collimator movement of 10um (2006) Coasting beam FFT spectrum 2006 data CWG 19/3/07 D.Kramer BLM Audit

  27. SPS Coasting beam 270GeV 200um Left jaw move and FFT spectra D.Kramer BLM Audit

  28. Complete FFT from the previous plot Horizontal Tune calculation from the BLM measurement-> oscillations in the beam not in the BLM system D.Kramer BLM Audit

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