Introduction to Beam Diagnostics

Transcription

1 Introduction to Beam Diagnostics As part of the lecture on Ion Source at University Frankfurt Oliver Kester and Peter Forck, GSI Outline of Beam Diagnostics related topics relevant for ion sources: The role of Beam Diagnostics Measurement of beam current (today): Faraday Cup, transformer, particle detector Measurement of beam profiles (January 27, 2012): SEM-Grid, scanner, particle detector, laser scanner Methods for transverse emittance measurement (February 3, 2012): Slit-Grid scanner, Allison scanner, reconstruction using beam optics L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 1p-physics at the future GSI Beam facilities Current Measurement

2 Demands on Beam Diagnostics Diagnostics is the organ of sense for the beam. It deals with real beams in real technical installations including all imperfections. Three types of demands leads to different installations: Quick, non-destructive measurements leading to a single number or simple plots. Used as a check for online information. Reliable technologies have to be used. Example: Current measurement by transformers this lecture Instrumentation for daily check, malfunction diagnosis and parameter variation. Example: Profile measurement, in many cases intercepting i.e destructive to the beam 2 nd lecture Complex instrumentation used for hard malfunction, commissioning and accelerator development. The instrumentation might be destructive and complex. Example: Emittance determination 3 rd lecture Non-destructive ( non-intercepting ) methods are preferred: The beam is not influenced The instrument is not destroyed. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 2p-physics at the future GSI Beam facilities Current Measurement

3 The Role of Beam Diagnostics The cost of diagnostics is about 3 to 10 % of the total facility cost: 3 % for large accelerators or accelerators with standard technologies 10 % for versatile accelerators or novel accelerators and technologies. Cost Examples: The amount of man-power is about 10 to 20 %: very different physics and technologies are applied technologies have to be up-graded, e.g. data acquisition and analysis accelerator improvement calls for new diagnostic concepts. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 3p-physics at the future GSI Beam facilities Current Measurement

4 Relevant physical Processes for Beam Diagnostics Electro-magnetic influence by moving charges: classical electro-dynamics, voltage and current meas., low and high frequencies Examples: Faraday cups, beam transformers, pick-ups Coulomb interaction of charged particles with matter: atomic and solid state physics, current measurement, optics, particle detectors Examples: scintillators, viewing screens, ionization chambers, residual gas monitors Interaction of particles with photons: optics, lasers, optical techniques, particle detectors Examples: laser scanners, polarimeters, photo detachment for H - beams Emission of photon by accelerated charges: (mainly for electrons) optics, optical techniques (from visible to x-ray) Not important Example: Synchrotron radiation monitors for ion of Nuclear- or elementary particle physics interactions: E nuclear physics, particle detectors kin < 1MeV/u Examples: beam loss monitors, polarimeters, luminosity monitors And of cause accelerator physics for proper instrumentation layout. Beam diagnostics deals with the full spectrum of physics and technology, this calls for experts on all these fields and is a challenging task! L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 4p-physics at the future GSI Beam facilities Current Measurement

5 Beam Diagnostics for LEBT at GSI UNILAC The measurement at a LEBT (Low Energy Beam Transport) comprises of: Current, profile, emittance, charge composition Example: LEBT at GSI UNILAC Length: 10.8 m Emittance Cup L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 5p-physics at the future GSI Beam facilities Current Measurement

6 UNILAC at GSI: Overview RFQ, IH1, IH2 Alvarez DTL Single Gap Resonators Transfer to Synchrotron MEVVA MUCIS HLI: (ECR,RFQ,IH) All ions, high current, 5 ms@50 Hz, 36&108 MHz To SIS Foil Stripper Alvarez DTL PIG 2.2 kev/u β = RFQ IH1 IH2 120 kev/u β = U 4+ U 28+ Gas Stripper 11.4 MeV/u β = 0.16 Constructed in the 70th, Upgrade 1999, further upgrades in preparation 1.4 MeV/u β = Injector for FAIR ion operation L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 6p-physics at the future GSI Beam facilities Current Measurement

7 Measurement of Beam Current The beam current is the basic quantity of the beam. It this the first check of the accelerator functionality It has to be determined in an absolute manner Important for transmission measurement and to prevent for beam losses. Different devices are used: Faraday cups: Measurement of the beam s electrical charges They are destructive. For low energies only Low currents can be determined. Transformers: Measurement of the beam s magnetic field They are non-destructive. No dependence on beam energy They have lower detection threshold. Particle detectors: Measurement of the particle s energy loss in matter Examples are scintillators, ionization chambers, secondary e emission followed by e amplification, MCPs, channeltron. Used for low currents at high energies e.g. for slow extraction. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 7p-physics at the future GSI Beam facilities Current Measurement

8 Faraday Cups for Beam Charge Measurement The beam particles are collected inside a metal cup The beam s charge are recorded as a function of time. The cup is moved in the beam pass destructive device Currents down to 10 pa with bandwidth of 100 Hz! Magnetic field: To prevent for secondary electrons leaving the cup And/or Electric field: Potential barrier at the cup entrance. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 8p-physics at the future GSI Beam facilities Current Measurement

9 Realization of a Faraday Cup at GSI LINAC The Cup is moved into the beam pass. 150 mm L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 9p-physics at the future GSI Beam facilities Current Measurement

10 Secondary Electron Suppression: Magnetic Field Arrangement of Co-Sm permanent magnets within the yoke and the calculated magnetic field lines. The homogeneous field strength is B 0.1 T. 10 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 10p-physics at the future GSI Beam facilities Current Measurement

11 Secondary Electron Suppression: Electric Field A ring shaped electrode is used at the entrance of Faraday Cup: Typical voltage 100 to 500 V Field calculation and secondary electron trajectories J. Harasimowicz et al. BIW 2010 Secondary electrons Potential lines 11 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 11p-physics at the future GSI Beam facilities Current Measurement

12 Energy Loss of Ions in Copper de 2 Z Z p = 2m c γ β Bethe Bloch formula: t e - 4πN Are mec ρt ln β dx A 2 β t I E 1 max de Range: R = 0 dx de with approx. scaling R E max 1.75 Numerical calculation with semi-empirical model e.g. SRIM Main modification Z p Z eff p(e kin ) Cups only for E kin < 100 MeV/u due to R < 10 mm Source 2 LINAC, Cycl. 2 Synchrotron 12 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 12p-physics at the future GSI Beam facilities Current Measurement

13 Faraday Cups for high Intensity Ion Beam Surface Heating The heating of material has to be considered, given by the energy loss. The cooling is done by radiation due to Stefan-Boltzmann: P r = εσt 4 For a temperature close to meting: thermal e - emission: J = A r T 2 exp(-φ/k B T) Example: Beam current: 11.4 MeV/u Ar 10+ with 10 ma and 1 ms Beam size: 5 mm FWHM 40 kw/mm 2, 1 MW toal power Foil: 1 µm Tantalum, emissivity ε = 0.49 Temperature increase: T > C during beam delivery Even for low average power, the material should survive the peak power! 13 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 13p-physics at the future GSI Beam facilities Current Measurement

14 High Power Faraday Cups Cups designed for 1 MW, 1 ms pulse power cone of Tungsten-coated Copper Bismuth for high melting temperature and copper for large head conductivity. 60mm 60mm beam 14 Beam Current Measurement P. Forck, Lecture on ion sources, Uni Frankfurt, th 14 L. Groening, Sept. GSI-Palaver, Dec.15th, 10, , A dedicated proton accelerator for p-physics at the future GSI facilities

15 Signal to Noise Consideration The minimum noise is given thermal noise: U eff = (4k B T R f) 1/2 R is the input Ohmic resistor low noise amplifier at input stage (realistic 2-3 x U eff ) f the bandwidth of the signal chain noise only S/N=4 signal only sample filter S/N=4 signal only S/N=2 signal only sample Simulation: Broadband white noise For Signal-to-Noise = 2 peak just visible For Signal-to-Noise = 4 peak well visible Filtering improve S/N here: N=50 tap moving average bandwidth restriction = i j + N / 2 = y i y new j i= j N / 2 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 15p-physics at the future GSI Beam facilities Current Measurement 1 N

16 Operational Amplifier Principle Operational Amplifier: Chips used as a simple equivalent circuit, but has complex built-in electronics Without feedback: U a = g ol ( U + -U - ) Open loop gain: g ol 10 4 With feedback: compensation of both inputs Examples of applications: Inverting voltage amplifier: U a = gu e = -R 2 / R 1 U e U - U + U a Current-to-voltage converter: U a = - R I e i.e. feedback leads current sink at inverting input L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 16p-physics at the future GSI Beam facilities Current Measurement

17 Transimpedance Amplifier = I/U Converter Current-to-voltage converter: U a = - R I e Z t I e i.e. feedback leads current sink at inverting input Z t [V/A] Full scale Bandwidth [khz] Risetime Equivalent noise relative µa µs 10 na µa µs 300 pa na µs 10 pa pa µs 0.1 pa 10-4 Counteraction between Current resolution Time resolution Conclusion Faraday Cup: Charge collection sec. e - suppression Sensitive conversion to a voltage L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 17p-physics at the future GSI Beam facilities Current Measurement

18 Excurse: Time Domain Frequency Domain Time domain: Recording of a voltage as a function of time: I beam Instrument: Oscilloscope Fourier Transformation: ~ 1 iωt f ( ω ) = f ( t) e dt 2π Frequency domain: Displaying of a voltage as a function of frequency: Instrument: Spectrum Analyzer or Fourier Transformation of time domain data Care: Contains amplitude and phase L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 18p-physics at the future GSI Beam facilities Current Measurement

19 Measurement of Beam Current The beam current is the basic quantity of the beam. It this the first check of the accelerator functionality It has to be determined in an absolute manner Important for transmission measurement and to prevent for beam losses. Different devices are used: Faraday cups: Measurement of the beam s electrical charges They are destructive. For low energies only Low currents can be determined. Transformers: Measurement of the beam s magnetic field They are non-destructive. No dependence on beam energy They have lower detection threshold. Particle detectors: Measurement of the particle s energy loss in matter Examples are scintillators, ionization chambers, secondary e emission followed by e amplification, MCPs, channeltron. Used for low currents at high energies e.g. for slow extraction. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 19p-physics at the future GSI Beam facilities Current Measurement

20 Magnetic Field of the Beam and the ideal Transformer Beam current of N charges with velocity β N N I beam = qe = qe βc t l cylindrical symmetry only azimuthal component I B = µ 0 beam eϕ 2πr Example: 1 µa, r = 10cm 2 pt Idea: Beam as primary winding and sense by sec. winding. Loaded current transformer I 1 /I 2 = N 2 /N 1 I sec = 1/N I beam Inductance of a torus of µ r µ r 2 r L = 0µ ln ln out 2π rin Goal of Torus: Large inductance L and guiding of field lines. I beam Torus to guide the magnetic field R v out Definition: U = L di/dt L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 20p-physics at the future GSI Beam facilities Current Measurement

21 Passive Transformer (or Fast Current Transformer FCT) Simplified electrical circuit of a passively loaded transformer: A voltages is measured: U = R I sec = R /N I beam S I beam with S sensitivity [V/A], equivalent to transfer function or transfer impedance Z. Equivalent circuit for analysis of sensitivity and bandwidth (disregarding the loss resistivity R L ) L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 21p-physics at the future GSI Beam facilities Current Measurement

22 Bandwidth of a Passive Transformer Analysis of a simplified electrical circuit of a passively loaded transformer: For this parallel shunt: 1 Z 1 = + iωl 1 + iωc R S Low frequency ω << R/L : Z iωl i.e. no dc-transformation High frequency ω >> 1/RC S : Z 1/iωC S i.e. current flow through C S Z iωl = 1+ iωl / R + ωl / R ωrc Working region R/L < ω < 1/RC S : Z R i.e. voltage drop at R and sensitivity S=R/N. No oscillations due to over-damping by low R = 50 Ω to ground. S f low =R/L f high =1/RC S L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 22p-physics at the future GSI Beam facilities Current Measurement

23 Response of the Passive Transformer: Rise and Droop Time Time domain description: Droop time: t droop = 1/3f low = L/R Rise time: t rise = 1/3f high = 1/RC S (ideal without cables) Rise time: t rise = 1/3f high = L S C s (with cables) R L : loss resistivity, R: for measuring. For the working region the voltage output is R t / τ droop U( t) = e Ibeam N f low =R/L f high =1/RC S L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 23p-physics at the future GSI Beam facilities Current Measurement

24 Example for passive Transformer For bunch observation e.g. transfer between synchrotrons a bandwidth of 2 khz < f < 1 GHz 1 ns < t < 200 µs is well suited. Example GSI type: From Company Bergoz L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 24p-physics at the future GSI Beam facilities Current Measurement

25 Active Transformer with longer Droop Time Active Transformer or Alternating Current Transformer ACT: uses a trans-impedance amplifier (I/U converter) to R 0 Ω load impedance i.e. a current sink + compensation feedback longer droop time τ droop Application: measurement of longer t > 10 µs e.g. at pulsed LINACs The input resistor is for an op-amp: R f /A << R L τ droop = L/(R f /A+R L ) L/R L Droop time constant can be up to 1 s! The feedback resistor is also used for range switching. An additional active feedback loop is used to compensate the droop. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 25p-physics at the future GSI Beam facilities Current Measurement

26 Active Transformer Realization Active transformer for the measurement of long t > 10 µs pulses e.g. at pulsed LINACs Torus inner radius r i =30 mm Torus outer radius r o =45 mm Core thickness l=25 mm Core material Vitrovac 6025 (CoFe) 70% (MoSiB) 30% Core permeability u r =10 5 Number of windings 2x10 crossed Max. sensitivity 10 6 V/A Beam current range 10 µa to 100 ma Bandwidth 1 MHz Droop 0.5 % for 5 ms rms resolution 0.2 µa for full bw L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 26p-physics at the future GSI Beam facilities Current Measurement

27 Active Transformer Measurement Active transformer for the measurement of long t > 10 µs pulses e.g. at pulsed ion sources Example: Transmission and macro-pulse shape for Ni 2+ beam at GSI LINAC Torus inner radius r i =30 mm Torus outer radius r o =45 mm Core thickness l=25 mm Core material Vitrovac 6025 (CoFe) 70% (MoSiB) 30% Core permeability u r =10 5 Number of windings 2x10 crossed Max. sensitivity 10 6 V/A Beam current range 10 µa to 100 ma Bandwidth 1 MHz Droop 0.5 % for 5 ms rms resolution 0.2 µa for full bw L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 27p-physics at the future GSI Beam facilities Current Measurement

28 Charge State Spectrum Determination with Transformer The LEBT (Low Energy Beam Transport) is used for charge state selection analyzing dipole Example: LEBT at GSI UNILAC Beam Bi at 2.2 kev/u Variation of analyzing dipole Current determination with transformer Beam Current [ma] Bi 4+ Bi 5+ Bi 3+ B-Field Emittance Cup L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 28p-physics at the future GSI Beam facilities Current Measurement

29 Shielding of a Transformer The image current of the walls have to be bypassed by a gap and a metal housing. This housing uses µ-metal and acts as a shield of external B-fields as well. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 29p-physics at the future GSI Beam facilities Current Measurement

30 Design Criteria for a Current Transformer Criteria: 1. The output voltage is U 1/N low number of windings for large signal. 2. For a low droop, a large inductance L is required due to τ droop = L/R: L N 2 and L µ r (µ r 10 5 for amorphous alloy). 3. For a large bandwidth the integrating capacitance C s should be low τ rise = L s C s. Depending on applications the behavior is influenced by external elements: Passive transformer: R = 50 Ω, τ rise 1 ns for short pulses. Application: Transfer between synchrotrons : 100 ns < t pulse < 10 µs Active transformer: Current sink by I/U-converter, τ droop 1 s for long pulses. Application: macro-pulses at LINACs : 100 µs < t pulse < 10 ms General: The beam pipe has to be intersected to prevent the flow of the image current through the torus. The torus is made of 25 µm isolated flat ribbon spiraled to get a torus of 15 mm thickness, to have large electrical resistivity. Additional winding for calibration with current source. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 30p-physics at the future GSI Beam facilities Current Measurement

31 The Artist View of Transformers The active transformer ACCT The passive, fast transformer FCT Cartoons by Company Bergoz, Saint Genis 31 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 31p-physics at the future GSI Beam facilities Current Measurement

32 The dc Transformer How to measure the DC current? The current transformer discussed sees only B-flux changes. The DC Current Transformer (DCCT) look at the magnetic saturation of two torii. ¾Modulation of the primary windings forces both torii into saturation twice per cycle. ¾Sense windings measure the modulation signal and cancel each other. ¾But with the Ibeam, the saturation is shifted and Isense is not zero ¾Compensation current adjustable until Isense is zero once again. 32 Beam Current Measurement P. Forck, Lecture on ion sources, Uni Frankfurt, th 32 L. Groening, Sept. GSI-Palaver, Dec.15th, 10, , A dedicated proton accelerator for p-physics at the future GSI facilities

33 The dc Transformer ¾ Modulation without beam: typically about 1 khz to saturation no net flux ¾ Modulation with beam: saturation is reached at different times, net flux ¾ Net flux: double frequency than modulation, ¾ Feedback: Current fed to compensation winding for larger sensitivity ¾ Two magnetic cores: Must be very similar. 33 Beam Current Measurement P. Forck, Lecture on ion sources, Uni Frankfurt, th 33 L. Groening, Sept. GSI-Palaver, Dec.15th, 10, , A dedicated proton accelerator for p-physics at the future GSI facilities

34 The dc Transformer Realization Example: The DCCT at GSI synchrotron (designed 1990 at GSI): Recent commercial product specification (Bergoz NPCT): Most parameters Temperature coeff. Resolution comparable the GSI-model 0.5 µa/ o C several µa (i.e. not optimized) 34 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 34p-physics at the future GSI Beam facilities Current Measurement

35 Design Criteria and Limitations for a dc Transformer Careful shielding against external fields with µ-metal. High resistivity of the core material to prevent for eddy current thin, insulated strips of alloy. Barkhausen noise due to changes of Weiss domains unavoidable limit for DCCT. Core material with low changes of µ r due to temperature and stress low micro-phonic pick-up. Thermal noise voltage U eff = (4k B T R f) 1/2 only required bandwidth f, low input resistor R. Preventing for flow of secondary electrons through the core need for well controlled beam centering close to the transformer. The current limits are: 1 µa for DCCT 30 µa for FCT with 500 MHz bandwidth 0.3 µa for ACT with 1 MHz bandwidth. 35 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 35p-physics at the future GSI Beam facilities Current Measurement

36 The Artist View of Transformers The active transformer ACCT The passive, fast transformer FCT The dc transformer DCCT Company Bergoz 36 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 36p-physics at the future GSI Beam facilities Current Measurement

37 Measurement of Beam Current The beam current is the basic quantity of the beam. It this the first check of the accelerator functionality It has to be determined in an absolute manner Important for transmission measurement and to prevent for beam losses. Different devices are used: Faraday cups: Measurement of the beam s electrical charges They are destructive. For low energies only Low currents can be determined. Transformers: Measurement of the beam s magnetic field They are non-destructive. No dependence on beam energy They have lower detection threshold. Particle detectors: Measurement of the particle s energy loss in matter Examples are scintillators, ionization chambers, secondary e emission followed by e amplification, MCPs, channeltron. Used for low currents at high energies e.g. for slow extraction. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 37p-physics at the future GSI Beam facilities Current Measurement

38 Low Current Measurement: Particle Detectors Electronic amplifier have finite noise contribution Theoretical limit: U eff = 4k B R f T Signal-to-Noise ratio limits the minimal detectable current Idea: Amplification of single particles with secondary electron multiplier or MCPs and particle counting typically up to /s Scheme of a secondary electron multiplier: Voltage divider HV Beam hits first dynode beam dynodes R Secondary electrons are electrons acc. to next dynode U 100 V Readout Typ. 10 dynodes 10 6 fold amplification Advantage: no thermal noise due to electro static acceleration Typical 1 V voltage for further electronics processing 38 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 38p-physics at the future GSI Beam facilities Current Measurement

39 Multi Channel Plate MCP MCP are used as particle detectors with secondary electron amplification. A MCP is: 1 mm glass plate with 10 µm holes thin Cr-Ni layer on surface voltage 1 kv/plate across e amplification of 10 3 per plate. resolution 0.1 mm (2 MCPs) Anode technologies: SEM-grid, 0.5 mm spacing fast electronics readout phosphor screen + CCD high resolution, but slow timing fast readout by photo-multipliers single particle detection for low beam current. L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 39p-physics at the future GSI Beam facilities Current Measurement

40 Detector System for Ion Counting Particle counting: Foil on HV to create secondary electrons Acceleration toward detector Amplification with MCP Detection: counting or current measure Alternative: Phosphor profile measurement foil -10 kv Mash 0 V e - MCP beam Readout: pulses or current J. Harasimowicz et al. IPAC L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 40p-physics at the future GSI Beam facilities Current Measurement

41 Secondary Electron Emission by Ion Impact Energy loss of ions in metals close to a surface: Distant collisions slow e - with E kin 10 ev diffusion & scattering wit other e - : scattering length L s 1-10 nm at surface 90 % probability for escape Closed collision: slow e - with E kin >> 100 ev inelastic collision and thermalization Secondary electron yield and energy distribution comparable for all metals! Y = const. * de/dx (Sternglass formula) Different targets: beam e - e - L s 10 nm δ-ray Electrons per ion From E.J. Sternglass, Phys. Rev. 108, 1 (1957) 41 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 41p-physics at the future GSI Beam facilities Current Measurement

42 Secondary Electron Emission by Ion Impact Energy loss of ions in metals close to a surface: Distant collisions slow e - with E kin 10 ev diffusion & scattering wit other e - : scattering length L s 1-10 nm at surface 90 % probability for escape Closed collision: slow e - with E kin >> 100 ev inelastic collision and thermalization Secondary electron yield and energy distribution comparable for all metals! Y = const. * de/dx (Sternglass formular) beam e - e - δ-ray L s 10 nm From C.G. Drexler, R.D. DuBois, Phys. Rev. A 53, 1630 (1996) 42 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 42p-physics at the future GSI Beam facilities Current Measurement

43 Particle Counting by Solid State Detector Determination of energy loss of ion in matter: Solid state detector or surface barrier detector, inn most cases Si Creation of electron-hole pair along the track Separation by electric field Amplification and particle counting up to /s Modern type of diamond detectors: High electron and hole mobility Separation by electric field and counting up to /s E-field amplifier beam HV h + h + h + e - e - e - e µm ground 43 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 43p-physics at the future GSI Beam facilities Current Measurement

44 Summary for Current Measurement Current is the basic quantity for accelerators! Faraday Cup: measurement of beam s charge low threshold by I/U-converter: I beam > 10 pa totally destructive, used for low energy beams Transformer: measurement of the beam s magnetic field magnetic field is guided by a high µ toroid types: passive (large bandwidth), active (low droop) and dc (two toroids + modulation) lower threshold by magnetic noise: about I beam > 1 µa non-destructive Particle measurement of the particle s energy loss detector: particle counting (solid state detector, scintillator) secondary electrons: emission from foil & amplification with SEM or MCP no lower threshold due to single particle counting destructive 44 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 44p-physics at the future GSI Beam facilities Current Measurement

45 Literature on Beam Diagnostics Klaus Wille, The Physics of Particle Accelerators, Oxford Uni. Press (2000). V. Smaluk, Particle Beam Diagnostics for Accelerators: Instruments and Methods, VDM Verlag Dr. Müller, Saarbrücken D. Brandt (Ed.), Beam Diagnostics for Accelerators, Proc. CERN Accelerator School CAS, Dourdan, CERN (2009) see P. Strehl, Beam Instrumentation and Diagnostics, Springer-Verlag, Berlin H. Koziol, Beam Diagnostic for Accelerators, Proc. CERN Accelerator School CAS, University Jyväskylä, Finland, p. 565 CERN (1994), see J. Bosser (Ed.), Beam Instrumentation, CERN-PE-ED , Rev P. Forck, JUAS Lecture Notes on Beam Diagnostics, see www-bd.gsi.de/conf/juas/juas.html. 45 L. GSI-Palaver, P. Groening, Forck, Lecture Sept. Dec. 15th, on 10 ion, 2003, sources, A dedicated Uni Frankfurt, proton accelerator for 45p-physics at the future GSI Beam facilities Current Measurement