Beam Instrumentation Introduction Beam Charge and Current Summary
Beam Instrumentation Introduction Beam Charge and Current Summary Instrumentation I Introduction Beam Position Summary Instrumentation II Introduction Instrumentation for Transverse Beam Parameters Summary Instrumentation III Instrumentation for Longitudinal Beam Parameters Selected Developments Summary Instrumentation IV
Beam Instrumentation IV Instrumentation for Longitudinal Beam Parameters Beam Energy Spectrometer Time-of-flight Laser-Based Monitor Beam Energy Spread Beam Phase and Arrival Time Monitors Beam Phase and Bunch Length Wall Current Monitors Streak Cameras Transverse Deflecting Cavities Selected Developments Electron Back-Scattering Detector, Electron Wire Vertex Detector Beam Halo Monitors Summary
Monitors for Longitudinal Beam Parameters Beam Energy and Energy Spread
Beam Energy: Spectrometer principle: dipole magnet field, B length, L detector beam q deflection angle x with L the magnet length and r the radius of curvature of the beam in the field B where the numerator gives the integrated field seen by the beam and Br is the magnetic rigidity with Br =p/c, with p the momentum of the beam The deflection angle is measured independently (tan -1 x/d) with d the distance between the magnet center and the detector. Knowing the field B produced by the current I (by field mapping and power supply calibration) allows to solve for the momentum and beam energy. issue: as shown, the assumption is that the incoming beam is point-like (zero transverse emittance). To obtain higher accuracy, the transverse angular spread of the incoming particle trajectories should be small. This can be achieved (at the expense of intensity at the detector) by one or more vertical slits upstream of the dipole magnet. issue: reproducibility (hysteretic effects due to residual magnetization)
Beam Energy: Spectrometer “Energy Spectrometer. Test at End Station A”, M. Hildreth, SLAC ESA Testbeam Wksp. 2011 “Performance of BPM Electronics for the LEP Spectrometer”, E. Barbero et al, BIW (2000) motivated by limited accuracy of NMR probes Features: special cooling stations (to prevent thermal expansion due to synchrotron radiation) stretched-wire positioning system to correct remaining thermally-induced motion target accuracy: d. E/E < 1 E-4 or BPM error of 1 mm Issues: effect of “signal frequency changes on the BPM cards” effect of beam current changes (peak signal power in cables) stability of the beam position measurement (thermal effects)
Beam Energy: Spectrometer (+ synchrotron radiation) From “Energy Spectrometer Tests at End Station A”, SLAC ESA Testbeam Workshop (2011)
Beam Energy Spread: Spectrometer principle: dipole magnet field, B length, L beam q detector x The same topology can be used to measure the beam energy spread since lower / higher energy particles are deflected more / less provided the detector can measure a transverse profile (screen, wire scanner, etc. ). In practice, the dispersion is used with x = , where = (E-E 0)/E 0. ; i. e. the fitted rms beam size on the detector (where dispersion is large) is directly proportional to the rms energy spread. issue: the finite transverse emittance will also contribute to the spot size measured at the detector. If it’s contribution is not small (so that it’s contribution when subtracted in quadrature is negligible) higher accuracy can be obtained, as before using slits upstream of The dipole or by an independent measurement of the beam size (then transported to the location of detector).
Beam Energy Spread In linear accelerators, the energy spread is determined to a large extent by the length of the bunch and its overlap with the sinusoidal accelerating voltage effective energy gain (left) and energy spread (right) for low (a) and high (b) current bunches illustrating optimum phasing of the rf structures for minimum energy spread principle: the beam size as measured, e. g. with a screen or wire, is the convolution of the natural beam size , x, y and the energy spread = E/E: = sqrt ( 2 + [ ]2) where is the dispersion function By proper selection of location (for a screen /wire), where is large, the beam energy spread can be directly measured
sketch of the layout of TTF I Single-bunch OTR images from TTF I obtained in a region of high dispersion (courtesy F. Stulle, 2003)
Energy and Energy Spread in the LCLS injector beamline “Commissioning of the LCLS injector”, R. Akre et al, SLAC-PUB-13014 (2007) issues: - deconvolving gun phase and voltage errors - space charge effects “Diagnostics for the LCLS injector beamline”, C. Limborg-Deprey et al, SLAC-PUB-11775 / PAC 05 (2005)
Beam Energy: Time of Flight principle (for low energy beams): beam detector d time of flight t = d / v, where d = separation length between detectors, v = velocity of beam in practice: beam detector t 1 detector t = t 1 – t 2 detector options: - scintillators (e. g. nuclear physics application conventionally using so-called constant fraction discriminators to resolve time-of-peak amplitude using phase information), time difference scales ~ 1 ns - conventional BPMs (capacitive, stripline, button, etc. ) with peak-sensing timing - conventional BPMs with beam phase detectors - rf cavity BPMs with beam phase detectors issues (all practical): - accuracy of timing reference or zero-crossing detection method - cable effects (dispersion, thermal, etc. )
Homework For the low-energy electron cooling project at RHIC (FY 18 -FY 20), the relative energy (specifically velocity) difference between the to-be-cooled ion beams and electron beams is ~ 1 E-4. This is very challenging. For the electron beam (only) calculate the time difference to be resolved for a time-of-flight based energy measurement assuming 1 e-3 energy resolution, measured over 10 m with (a) = 4 (b) = 10 In practice, a distance between detectors of 10 m is impractical since the bunch length changes over this distance. Calculate the time difference assuming a separation between detectors of 2 ms with (c) = 4, (d) = 10 (e) Comment on the feasibility of this approach for determining the absolute electron beam energy.
Beam Energy: Laser-Based Monitor principle: accelerate emitted electrons by application of HV to “lollipop”, measure current versus retarding voltage applied to downstream grid. Since liberated electrons have same energy of beam protons, the proton energy E p is equal to the measured electron energy Ee times the mass ratio: Ep = Ee * mp/me red crosses – measurements red curve – fit to data using Gaussian error function blue curve – time derivative of blue curve “A detector to measure transverse profiles and energy of an H- beam using gas stripping and laser photoneutralization”, R. Connolly et al, JINST 7, P 02001 (2011).
Monitors for Longitudinal Beam Parameters Beam Phase and Arrival Time Monitors Bunch Length Monitors
Beam Phase Monitors (in linear transport systems) In linear systems, the terminology “beam phase” is often used to represent the time a bunch passes a particular point relative to an rf acceleration system located either upstream or downstream of the observation point. principle: measure arrival time using BPMs (peak of profile) relative to rf An example where this was important: The beam, in the center of the rf bucket in the damping ring, is extracted and must pass the compressor cavity at it’s zero crossing. The beam must also be appropriately placed for acceleration in the downstream linac.
Arrival Time / Synchronization Monitors (linear transport systems) Relative time-of-flight monitors are finding widespread use for monitoring path length variations (aka time-of-arrival monitors) in applications where this parameter is critical such as in FELs and ERLs. “Electron beam diagnostic system for the J-Parc XFEL, SACLA”, H. Maesaka et al, IBIC 2012 “Development of the rf cavity BPM of CXFEL/SPRING-8”, H. Maesaka et al, DIPAC 09 SACLA XFEL using C-Band Cavity BPMs: 4. 6 E-2 (rms) deg at 4760 MHz or <1 E-5 deg/MHz
SACLA arrival time monitors, continued measurements: drift in arrival timing (attributed to rf phase drift in the injector section) time difference between entrance and exit of undulator beamline (attributed to thermal effects in reference timing signals) ESRF (relative phase between booster and main storage ring) using existing striplines and buttons: 4. 4 E-3 (rms) deg at 350 MHz or 1. 6 E-5 deg/MHz example measurement of cable temperature coefficient “Upgrade of beam phase monitors for the ESRF injector and storage ring”, B. K. Scheidt, B. Joly, IBIC 2013
Beam Phase and Bunch Length (in circular accelerators) In a circular accelerator the wall current monitor is widely used. Recall:
This beautiful plot (showing very bad beam conditions) illustrates the measurements of beam phase and bunch length using a wall current monitor (in RHIC). We will see many more examples of beam phase and bunch length monitors later in the lectures on longitudinal dynamics.
Bunch Length – Streak Cameras Principle: photons (generated e. g. by SR, OTR, or from an FEL) are converted to e-, which are accelerated and deflected using a timesynchronized, ramped HV electric field; e- signal is amplified with an MCP, converted to s (via a phosphor screen) and detected using an imager (e. g. CCD array), which converts the light into a voltage Principle of a streak camera (from M. Geitz, “Bunch Length Measurements”, DIPAC 99) Issues: energy spread of e- from the photocathode (time dispersion) chromatic effects (dt/d. E ( )) in windows space charge effects following the photocathode
500 ns ( ~1/2 turn) FS 25 µs FS (every 4 th turn) Streak camera images from the Pohang light source evidencing beam oscillations arising from the fast-ion instability (courtesy M. Kwon, 2000)
Bunch Length – Transverse Mode Cavities (1965: Miller, Tang, Koontz) Principle: use transverse mode deflecting cavity to “sweep”/kick the beam, which is then detected using standard profile monitors Principle of the TM 11 transverse mode deflecting cavity introduce x-z correlation oriented to displace beam vertically while a horizontal bending magnet deflects the beam onto the screen y 2 = A(Vrf-Vrf, min)2 = y 02 z = A 1/2 E 0 rf/2 R 34 z expressed in terms of fit parameter, A (figures courtesy R. Akre, 2003)
Select developments: Electron back-scattering detector for beam profile and phase/time
Electron backscattered detector The backscattered electron intensity s is related to the quality of The proton-electron beam alignment (overlap monitor) P. Thieberger et al, “The electron backscattering detector, a new tool for the precise mutual alignment of the electron and ion beams in electron lenses”, IBIC 2014 (Sept, 2014)
Electron backscattered detector cross section for Coulomb scattering: Small deflections in the ion frame leads to large deflections in the lab. The classical Rutherford scattering equation with quantum and recoil corrections is used to calculate the cross sections s in the ion frame of reference. Transformation to the lab. frame yields the results shown next. Radiative corrections have not been included but may be small – to be verified. P. Thieberger
5 ke. V electrons in axial fields, back-scattered by high energy protons. The curves shown were calculated using 250 Ge. V protons but the results are almost independent of this energy. P. Thieberger
Electron backscattered detector Proof-of-principle electron-gold e. BSD “luminosity” scans Date: 4/15/2014 Ion Beam: Gold Beam energy: 100 Ge. V/u Bunch intensity: 7*108 # of bunches: 2 Solenoid Field: 2 T Electron energy: 6 ke. V e-beam current: 0. 565 A P. Thieberger et al, “The electron backscattering detector, a new tool for the precise mutual alignment of the electron and ion beams in electron lenses”, IBIC 2014 (Sept, 2014)
Electron backscattered detector: time-of-flight e. BSD time-resolved counting rates without (top) and with (bottom) electron beam. The small peaks to the left of the large ones may be due to misalignment. Residual gas electrons produce these signals. electron beam OFF The electron beam pulse overlaps two bunches. The residual gas background is still visible but small. electron beam ON Electron time-of-flight spectra may help with the angular alignment P. Thieberger
Select developments: Electron beam wires
Idea for a possible Coulomb Scattering Electron Wire (CSEW) beam profile monitor Ø A ribbon shaped magnetized electron beam intersects the ion beam. Ø Some of the scattered electrons trajectories are intercepted by the detector. Ø The ribbon is steered to map the ion to measure the profile of the ion beam, perhaps including the halo Ø The three sets of solenoids form a closed ion orbit bump. “Scattered electrons as possible probes for beam halo diagnostics”, P. Thieberger et al, Workshop on Beam Halo Monitoring, SLAC (2014)
Select developments: Vertex detector
CERN beam vertex detector Novel non-invasive beam-gas vertex monitor. Based on concepts used by the LHCb vertex detector, this detector will allow measurement of the absolute transverse profiles using reconstruction of the location of inelastic beam-gas interactions based on particle tracking with coincidence detectors. “A beam gas vertex detector for beam size measurement in the LHC”, P. Hopchev et al, IPAC 2014
Select developments: Beam halo monitors
Prelude - topic not covered: beam loss monitors using conventional technologies ionization chambers photomultiplier tube assembly pin diodes long ionization chambers, here heliax-style LHC ionization chambers halo scraper Yet uncontrolled beam loss and beam halo are of increasing importance and interest in accelerator community as designs aim toward higher increased total intensity and/or energy: cw linacs SASE FELs (XFEL, LCLSII, …) ERLs (BNL for LERe. C, Ce. C, e. RHIC; JLAB MEIC, CERN LHe. C, …) intense proton beams (FNAL LBNE, Sweden’s ESS, China’s CADs) isotope production (ion beams at FRIB, …) Reference: https: //uspas. fnal. gov/programs/JAS 14. shtml Joint International Accelerator School on “Beam Loss and Accelerator Protection”, Newport Beach, CA (2014)
In this last section, we cover recent developments in monitoring of beam halo. Here beam halo refers to particles that do not follow the design trajectory and/or do not have the design beam focusing properties. Categories: (1) Halo with unavoidable physical origins (for example due to intrabeam scattering, space charge, beam gas scattering in an imperfect vacuum, etc. ) (2) Halo of practical origins resulting for example from - small deviations in design parameters (such as errors in magnet alignment and/or magnet fields, parameter mismatches, both transverse and longitudinal, between linear and circular accelerators, for example) - external perturbations including noise in power supply currents, rf cavity voltages, environmental factors leading to vibrations in component position, etc. Goal: often cited as a dynamic range ~ 1 E-6 BUT, it is the absolute number of particles (not relative) striking accelerator components that leads to activation and damage
K. Yamamoto ”Beam instrumentation at the 1 MW proton beam of J-PARC RCS” wire scraper and scintillators with different sensitivities for simultaneous (? ) measurements of both the beam core and halo (vibration wire monitor) (next slide/talk) HB 2014, East Lansing, Nov. 10 -14, 2014 WG-F Summary – Part 1: M. Minty
New beam diagnostics: 2 D core and halo monitor Y. Hashimoto Beam Halo Measure Fluorescence From Chromium Doped alumina Screen Beam Core Measure OTR From 10 micro Titanium foil OTR/FLUORESCENCE BEAM PROFILE MONITOR HB 2014, East Lansing, Nov. 10 -14, 2014 WG-F Summary – Part 1: M. Minty
measured profile of beam core and halo with > 1 E 6 dynamic range Y. Hashimoto HB 2014, East Lansing, Nov. 10 -14, 2014 WG-F Summary – Part 1: M. Minty
phase space painting: two-dimensional imaging reveals halo rotation Y. Hashimoto HB 2014, East Lansing, Nov. 10 -14, 2014 WG-F Summary – Part 1: M. Minty
Giulio Stancari HB 2014, East Lansing, Nov. 10 -14, 2014 WG-F Summary – Part 1: M. Minty
Beam Halo Monitors: Hollow electron beams as possible halo probes “Scattered electrons as possible probes for beam halo diagnostics”, P. Thieberger et al, Workshop on Beam Halo Monitoring, SLAC (2014) A hollow electron beam seems ideal as a halo probe. But: Some residual gas electrons backscattered by the intense ion beam core will be counted too. Countermeasures: Ø Improve the vacuum. The best way to improve the vacuum would be to use a cold-bore solenoid. Ø Pulse the electron beam Developments in using hollow beams as collimators underway at FNAL and CERN
Summary Methods for measuring the longitudinal beam parameters were reviewed Including those for beam energy and energy spread spectrometer time-of-flight laser monitor beam phase and arrival time beam phase and bunch length wall current monitors Remark: recent great technological streak cameras advances from SASE FELs accelerators not covered transverse deflecting cavities Selected developments included electron back-scattering detector vertex detector beam halo monitors http: //www. nature. com/news/china-plans-super-collider-1. 15603 The energy frontier electron-positron (ILC) proton-proton / proton-ion / ion-ion (FCC, CEPC/Spp. S) require high accuracy beam energy measurements with methods yet to be developed. Polarization-based energy measurements will be covered later in this course.
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