Bingxin Yang 1/24/2008 Test and Calibration Plan for LCLS-BLM at the APS

2 Objectives and outline Objective for the BLM test / calibration at the APS 1. Validate high-energy shower simulation for relevant geometry. 2. Calibration of BLM? Outline 1.Test and calibration of LCLS-BLM with a single electron Single-electron calibration procedures Statistical analysis of BLM signal and pulse height spectrum 2.Reality check: experience with the APS Cherenkov detectors Control of APS beam loss rate Cherenkov detector measurements: pulse height, length, and charge 3.Preparation for test and calibration of LCLS-BLM at the APS Progress to date Other beam loss scenarios 4.Proposed calibration scheme for the LCLS-BLM Scattering foils and energy question

3 Testing LCLS-BLM with a single electron Simple procedures for testing LCLS-BLM using one electron Store beam current < 0.5 mA in APS storage ring. Count rate < 10 K (c/s). Measure the pulse height spectrum of the PMT signal Scan stored beam current / beam loss rate and record pulse height spectra. The peaks from n-shower-particle events are proportional to n-power of the loss rate. Identify peak for single-electron scattering event and calculate expection value: V 1. Calibration for the BLM: PMT charge for one APS-electron = C A V 1, where C A is inverse of the charge amplifier calibration factor. Exchange rate from a standard APS electron to LCLS electrons needs to be performed with computer simulation (Jeff Dooling et. al.). Under what conditions will these procedures work?

4 Statistics of BLM signal originated from a single electron A statistical model for shower detection process A Large number of shower particles are created (N S >> 1). Assume that the number of shower particles intercepted by BLM is given by Poisson distribution (right, n s0 = average number of shower particle intercepted). Each intercepted shower particle creates many Cherenkov photons, which in turn generates m 0 photoelectrons at the PMT cathode, on average. # of photoelectrons are given by Poisson distribution (right, m 0 = average # of photoelectrons generated by one shower particle). The distribution of total # of photoelectrons, n, and the PMT signal charge generated, nq 0 :

5 Impact of “collection efficiency” of the BLM Conclusion: High collection efficiency, n s0 >> 1, is highly desirable. For high collection efficiency: n s0 >> 1, the spectrum is peaked around V 0 = n so * m 0 * q 0 For low collection efficiency, n s0 <= 1, the spectrum is dominated by photoelectron distribution P 1

6 Reality check: Control beam loss rate in the APS-SR Extrapolate from operation experience of the APS storage ring: At 324-bunch user run, stored beam has 0.3 mA current per bunch, lifetime ~ 50 hours Assuming gas scattering dominates and lifetime = 50 hours with 1-bunch 0.1 mA. Tracking studies by M. Borland and L. Emery estimated that ~ 1% of total loss occur at each normal ID chamber (non-limiting aperture). Hence the single electron deposit rate at a normal ID chamber is ~ 128 hits/sec, comparable to the beam frequency of the LCLS. In fact, a higher loss rate is more desirable for better efficiency collecting data. The bottle neck is defined by the charge amplifier output pulse width of ~100  s.

7 Estimate of Cherenkov detector signal strength PMT pulse is generated by a single shower particle: Frank-Tamm formula for Cherenkov radiation can be used to estimate energy deposit of an electron traversing the entire thickness of the radiator: This yields 640 eV/cm for wavelength region 300 – 500 nm. For radiator thickness = 1.2 cm, we have ~ 240 photons, with 20% optical efficiency and ~ 15% quantum efficiency, we get 7 photoelectrons/shower-particle. For PMT-HV = 900 V, gain = 1.5 × 10 6, each photoelectron produces ~ 0.25 pC. Each shower particle produces ~ 1.7 pC, on average.

8 APS Cherenkov detector measurements Construction of the APS Cherenkov detector: 8 mm quartz radiator enclosed by 15 mm thick lead can. Located at 2.3 m downstream of chamber entrance, 0.1 radian off-axis. Detector has very low collection efficiency. Pulse height spectrum is dominated by photoelectron statistics. Estimate = 4 – 5 photoelectrons, or ~ 1 pC PMT change per shower particle. Measurements with the APD detector would help us get familiar with the PMT and compare its signal with the above estimates from statistical analysis.

9 Weakest PMT pulses: height, length and charge Pulses of lowest amplitude can be observed during user operation using a scope. Pulse width is about 2.5 – 3.5 ns FWHM. The pulse shown in the following example carries a charge of (V) / 50 (ohm) * 2.5 (ns) = 1.7 pC, consistent with an event for 7 photoelectrons. Typical pulse height ranges from 10 mV (2 photoelectron) to 100 mV (20 photoelectron). No detailed pulse height analysis was performed due to a lack of equipment. Conclusion: Signal estimate is OK.

10 Most intense PMT pulses: height, length and charge Pulses of highest amplitude can be observed when dumping a 19-mA single bunch beam. PMT-HV = 750 V. Gain reduced by a factor of four. Pulse train recorded with 5 GS/s scope. Height = 7 V. PMT is heavily saturated and maximum pulse width > 20 ns. Maximum charge per pulse is 6 nC! Conclusion: > 10 4 dynamic range for single pulse charge.

11 Preparation for testing the LCLS-BLM in the APS-SR Planning and discussion has many participants: Jim Bailey, Jeff Dooling, Marion White, Bill Berg, Glenn Decker, Liz Moog, Tony Pietryla, Eric Norum, Isaac Vasserman, … Status: Physics: Concept development still in progress and in flux. Program / script to be developed. Mechanical support: Version 0.0 made and tested. Approved by APS ID group with suggestions. Improvement expected: better protect ID chamber. Electronics: Charge amplifier work in progress (other talks). Spectroscopy amplifier: ANL or eBay ($200). Pulse height analyzer: to be specified Cables: to be specified and installed. BLM itself: Expected in March.

12 Other test / calibration scenarios 1.Beam dump Stored beam from 0.1 mA to 19.2 mA. FWHM of the lost charge pulse is 14 turn. 3 × 10 8 to 6 × electrons hit the wall in a single turn. Pulse spacing 3.6  s, not resolved by the charge amplifier. Distribution among sectors to be studied. 2.Kicker-induced beam loss Use controlled kick to perturb the stored beam. Motion-related loss lasts about 1 ms, or 200 – 300 turns. Loss can be controlled from 10 5 to 10 7 per turn. Pulse spacing 3.6  s, not resolved by the charge amplifier. Distribution of lost particles are to be studied. 3.Injected beam Injector sends 0.2 – 2 nC ( 10 9 – electrons) into the storage ring. A faction of them can be scraped on the ID chamber using steering. A systematic measurement technique needs to be developed.

13 Summary 1.Single-electron test  If simulation or experiment shows that the BLM intercept more than one shower particle per hit, the test will work, at least in principle.  The PMT signal will be in the range of 10 – 100 pC per pulse, as scaled from the APS Cherenkov detector measurements. 2.Other measurements  If we intercept less than one shower particle per 7-GeV electron, we will need to have additional measurements / knowledge about the lost beam.  We will continue to develope concept and plans to use three other beam loss scenarios:  Kicker-induced beam loss (10 5 – 10 7 e/turn).  Injection, where the storage ring is treated as a long transport line after the injectors  Beam dump (10 10 e/turn) 3.LCLS calibration foil  Proposal / request for simulation of the calibration foil was made last March. We hope to see some results soon.