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Beam Instrumentation

Beam Instrumentation. Introduction Beam Charge and Current Summary Introduction Beam Position Summary Introduction Instrumentation for Transverse Beam Parameters Summary Instrumentation for Longitudinal Beam Parameters Selected Developments Summary.

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Beam Instrumentation

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  1. Beam Instrumentation Introduction Beam Charge and Current Summary Introduction Beam Position Summary Introduction Instrumentation for Transverse Beam Parameters Summary Instrumentation for Longitudinal Beam Parameters Selected Developments Summary Instrumentation I Instrumentation II Instrumentation III Instrumentation IV

  2. Beam Instrumentation I Introduction Beam Charge and Current Faraday Cups Current Transformers Wall Gap Monitors Summary

  3. Introduction Accelerator performance depends critically on the ability to carefully measure and control the properties of the accelerated particle beams In fact, it is not uncommon, that beam diagnostics are modified or added after an accelerator has been commissioned This reflects in part the increasingly difficult demands for high beam brightness (high currents and smaller emittances), and the tighter tolerances on these parameters (e.g. position stability) in modern accelerators A good understanding of diagnostics is therefore essential for achieving the required performance A beam diagnostic consists of the measurement device associated electronics and processing hardware high-level applications focus of today’s lectures not covered, usually very technical, often application specific topic of later lectures

  4. Fields of a relativistic particle induced wall current iw(t) has opposite sign of beam current ib(t): ib(t)=-iw(t) Lorentz-contracted “pancake” Detection of charged particle beams – beam detectors: most intensity monitors are designed to measure the wall current iw is a current source with infinite output impedance, iw will flow through any impedance placed in its path many “classical” beam detectors consist of a modification of the walls through which the currents will flow

  5. Sensitivity of beam detectors: beam charge: = ratio of signal size developed V() to the wall current Iw(), units:  beam position: = ratio of signal size developed /dipole mode of the distribution, given by D()=Iw() z, where z = x (horizontal) or z = y (vertical), units: /m

  6. Beam charge and current: definitions Charge, Q = total number of particles in the beam = n e V where n = number of charges per unit volume, V = volume e = electric charge = 1.6E-19 Coulombs examples: • a bunch with 100 pC charge (typical for coherent light sources, • ERLs) contains 6.25E8 particles; terminology: 100 pC/bunch • a bunch of 1E11 particles (typical for LHC, RHIC) contains 16 nC; • terminology 1E11 ppb – particles per bunch Current, I = charge passing a given point per unit time (t) = Q / t = Q f, f = bunch repetition frequency examples: • an electron gun operating producing 100 pCbunches with 700 MHz • repetition frequency yields a current of 70 mA (example: linear • accelerator for bunched beam electron cooling at BNL) • at RHIC, ~ 110 bunches each with 1E11 ppb circulate with a revolution • frequency of 78 kHz; the current of each bunch is 1.25 mA while the • total current is 110 times that (110 bunches) or ~ 135 mA

  7. Homework #1 In an early design of the SuperKEKB project (asymmetric collider) the design single bunch populations were 5.25E10 (electrons) and 12E10 (positrons). The ring circumferences are 3016 m each containing 5000 bunches. Determine (a) the bunch spacing, (b) the bunch revolution frequency, (c) the bunch repetition frequency, (d) the total circulating current in each of the two accelerators. The beams are injected at full energy and relativistic. What are the total currents for the present design with per bunch currents of 1.04 mA (HER, electrons), 1.44 mA (LER, positrons) and 2500 bunches per ring?

  8. Early design: https://indico.cern.ch/event/47961/session/9/contribution/25/material/slides/3.pdf

  9. Present design: http://www-superkekb.kek.jp/documents/MachineParameters150410.pdf

  10. Homework #2 This exercise is similar to the previous one, serves to illustrate different parameter ranges of circular lepton accelerators with different objectives. The new NSLS-II light source has a ring circumferences of 792 m and 1056 bunches. Determine (a) the bunch spacing, (b) the bunch revolution frequency, (c) the bunch repetition frequency, (d) the total circulating current. The beam energy is 3 GeV and is injected at full energy.

  11. Homework #3 The European Spallation Source, ESS, in Sweden, the Facility for Rare Isotope Beams, FRIB, in Michigan, and the China Accelerator Driven System, CADS, planned for waste transmutation in China, all involve very high power beams. This exercise involves computation of beam powers with realistic constraints using as an example the BNL energy recovery linac (ERL). electron gun laser 5-cell cavity

  12. The beam power depends on average current which depends on duty factors, Fi P P = beam power Qb = charge per bunch flaser = laser repetition frequency V = gun voltage P = Qb flaser Fi V i I = Qb flaser Note P <I> = Qb flaser Fi i Calculate the average beam current and the total beam power for the following different possible modes of operation. • high current scenarios (uses 700 MHz laser, duty factor=1) with Qb=0.5 nC and • (i) 5-cell cavity off, so the beam energy is given by the gun • voltage V = 2.5 MV • (ii) 5-cell cavity on providing a total voltage to the beam (gun + • cavity) of V = 25 MV • high charge scenarios (uses 9.383 MHz laser, duty factor=1) • (i) with 5-cell cavity off • (ii) with 5-cell cavity on

  13. another scenario in which the gun is used as the electron source for low energy • cooling (uses 700 MHz laser, duty factor=0.43) and with • (i) 5-cell cavity off • (ii) 5-cell cavity on • Initial commissioning (here finding bunch structures suitable for so-called fault studies). Assume a laser frequency of 9.383 MHz, 5-cell cavity off, a gun voltage of 1 MV operating for 1 second intervals, a gun focussing solenoid operable for 5 s once every 5 minutes, and a maximum duration of bunches of 7 ms as limited by the current transformer used to measure the beam current.

  14. Faraday Cup

  15. Instrumentation: Beam Charge – the Faraday Cup (1) thick (e.g. ~0.4 m copper for 1 GeV electrons) or series of thick (e.g. for cooling) charge collecting conducting receptacles Q, Principle: beam deposits (usually) all energy into the cup (invasive) charge converted to a corresponding current (I=dQ/dt) voltage across resistor proportional to instantaneous current absorbed In practice: termination usually into 50  cooling needed for high power beams bandwidth-limited (~1 GHz) due to capacitance to ground positive bias to cup (or to grid preceding cup) to retain e- produced by secondary emission (ionization products) backscattering (due to electromagnetic showers) Advantages / Disadvantages: invasive / serves as beam stop – beneficial for transport line tuning “Faraday-Cup Monitors for High-Energy Electron Beams”, K.L. Brown and G.W. Tautfest, Rev. of Sci. Instr. 27, 696 (1956)

  16. Beam Charge – the Faraday Cup (2) photo of FC used in the BNL tandem-to-downstream transfer lines “Beam instrumentation for the BNL Heavy Ion Transfer Line”, R.L. Witkover et al, 1987 PAC http://www.n-t-g.de/beam_diagnostic_Seite_2.htm

  17. Beam Charge – the Faraday Cup (3) cross-sectional view of the FC of the KEKB injector linac Features: cylindrically symmetric blocks of lead (~35 rad lengths) carbon and iron - for suppression of “backscattering” / “electro- magnetic showers”; i.e. particles liberated from nuclear reactions generated by the lead) bias voltage (~many 100 Volts) for suppression of “secondaries”; i.e. electrons generated by Compton scattering (courtesy T. Suwada, 2003)

  18. Homework #4 Estimate the charge intercepted by a Faraday Cup (terminated into 50 W) based on the voltage signal below. BNL ERL (11/17/14)

  19. Current Transformers

  20. Instrumentation: Beam Intensity – Current Transformers (1) I Consider a magnetic ring surrounding the beam, from Ampere’s law: if r0 (ring radius) >> thickness of the toroid, m = permeability of core = mr m0, where mr is magnetic permeability of medium and m0=4pe-7 [H/m] is the permeability of free space (integral over circumferential length of magnetic core) Add an N-turn coil – an emf is induced which acts to oppose B: A is the cross-sectional area of the magnetic ring Load the circuit with an impedance; from Lenz’s law, iR=ib/N: Principle: the combination of core, coil, and R produce a current transformer such that iR (the current through the resistor) is a scaled replica of ib. This can be viewed across R as a voltage. The N-turn coil serves as a primary/secondary winding of a current transformer while the intercepting beam acts as secondary/primary winding.

  21. Beam Intensity – Current Transformers (2) with Rh = reluctance of magnetic path sensitivity: A = core cross section l = 2pr0 (ideal parallel R-L circuit) cutoff frequency, L, is small if L~N2 is large trade-off between bandwidth and signal amplitude detected voltage across resistor: if N is large, the voltage detected is small

  22. Beam Intensity – Current Transformers (3) A iron B Mu-metal C copper D “Supermalloy” (distributed by BF1 Electronique, France) with ~ 8104 E electron shield F ceramic gap shielding schematic of the toroidal transformer for the TESLA Test facility (courtesy, M. Jablonka, 2003) (based on design of K. Unser for the LEP bunch-by-bunch monitor at CERN) linacs: resolution of 3106 storage rings: resolution of 10 nA rms details: www.bergoz.com (one of many) current trans- formers available from Bergoz Precision Instru- ments (courtesy J. Bergoz, 2003)

  23. Beam Intensity – Current Transformers (4) developments of toroids for TTF II (DESY) 2 iron halves 50 output impedance ferrite ring calibration windings (25 ns , 100 mV / dvsn) bronze pick-ups ferrite rings (for suppression of high frequency resonance) (courtesy D. Noelle, L. Schreiter, and M. Wendt, 2003)

  24. Beam Intensity – Current Transformers (5) From: Bergoz Instrumentation - Beam Charge Monitor, Integrate-Hold-Reset User’s Manual 1 3 1 2 2 2 3 3 1

  25. Beam Intensity – Current Transformers (6) Bergoz ICT (1 of 5 in transfer line between AGS and RHIC) after before Issue: heavy ions travel through transport light with different velocity than light ions, therefore time-of-flight is different so requiring timing adjustments for the integration windows.

  26. Beam Intensity – Current Transformers (7) http://www.agsrhichome.bnl.gov/RHIC /Instrumentation/Systems/DCCT “Overview of RHIC Beam Instrumentation and First Experience from Operations”, P. Cameron et al, DIPAC 2001

  27. Beam Intensity – Current Transformers (8) Bergoz tests of a their wide-band “in flange” current transformer measurement showing frequency-independent response up to very high frequency http://www.bergoz.com/products/In-Flange.CT/In-flange-downloads/files/TN_FCT_04-08r1.pdf output input Bench-test with 50 Ω source and output impedance Beam-test with infinite source and 50 Ω output impedance

  28. Wall Gap Monitors

  29. Beam Intensity – Wall Gap / Current Monitor (1) principle: remove a portion of the vacuum chamber and replace it with some resistive material of impedance Z detection of voltage across the impedance gives a direct measurement of beam current since V= iw(t) Z = -ib(t) Z (susceptible to em pickup and to ground loops) add high-inductance metal shield add ferrite to increase L add ceramic breaks add resistors (across which V is to be measured) alternate topology - one of the resistors has been replaced by the inner con- ductor of a coaxial line

  30. Beam Intensity – WCM (2) sensitivity: circuit model using parallel RLC circuit: high frequency response is determined by C: (C =1/RC) low frequency response determined by L: (L =R/L) intermediate regime:R/L <  < 1/RC – for high bandwidth, L should be large and C should be small remark: this simplified model does not take into account the fact that the shield may act as a resonant cavity

  31. Beam Intensity – WCM (3) RHIC design based on prototype WGM shown below by R.C. Webber. 3 WCM’s in RHIC: 2 at 2 o’clock, 1 at 4 o’clock. From “The RHIC [WCM] system”, P. Cameron, et al, PAC (1999) resistors ceramic gap ferrite “An Improved Resistive Wall Monitor”, B. Fellenz And Jim Crisp, BIW (1998) for the FNAL main injector “Longitudinal Emittance: An Introduction to the Concept and Survey of Measurement Techniques Including Design of a Wall Current Monitor”, R.C. Webber (FNAL, 1990) available at: http://www.agsrhichome.bnl.gov/RHIC/Instrumentation/Systems/WCM /WCM%20Shafer%20BIW%201989%2085_1.pdf

  32. Beam Intensity – WCM (4) RHIC WCM http://www.agsrhichome.bnl.gov/RHIC/ Instrumentation/Systems/WCM/WCM.html

  33. Beam Intensity – WCM (5)

  34. Beam Intensity – WCM (6)

  35. (Selected) classic references

  36. Summary Detection of the wall current Iw allows for measurements of the beam intensity and position. The detector sensitivities are given by for the beam charge and intensity for the horizontal position with for the vertical position We reviewed beam diagnostics for measuring: the beam charge – using Faraday cups the beam intensity – using toroidal transformers - using wall gap monitors - using BPM sum signals We note that the equivalent circuit models presented were often simplistic. In practice these may be tailored given direct measurement or using computer models. Impedances in the electronics used to process the signals must also be taken into account as they often limit the bandwidth of the measurement. Nonetheless, the fundamental design features of the detectors presented were discussed (including variations in the designs) highlighting the importance of detector geometries and impedance matching as required for high sensitivity. (If interest / time: view real-time data at RHIC)

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