Introduction to Beam Instrumentation Hermann Schmickler (CERN Beam Instrumentation Group) Hermann Schmickler – CERN Beam Instrumentation Group
Introduction What do we mean by beam instrumentation? ● The “eyes” of the machine operators ● i.e. the instruments that observe beam behaviour ● ● An accelerator can never be better than the instruments measuring its performance! ● What does work in beam instrumentation entail? ● Design, construction & operation of instruments to observe particle beams R&D to find new or improve existing techniques to fulfill new requirements ● ● A combination of the following disciplines Applied & Accelerator Physics; Mechanical, Electronic & Software Engineering ● ● A fascinating field of work! What beam parameters do we measure? ● ● Beam Position ● Horizontal and vertical throughout the accelerator ● Beam Intensity (& lifetime measurement for a storage ring/collider) Bunch-by-bunch charge and total circulating current ● ● Beam Loss ● Especially important for superconducting machines Beam profiles ● ● Transverse and longitudinal distribution Collision rate / Luminosity (for colliders) ● ● Measure of how well the beams are overlapped at the collision point Hermann Schmickler – CERN Beam Instrumentation Group
More Measurements ● Machine Tune Characteristic Frequency QF QD QD QF QF of the Magnet Lattice Given by the strength of the Quadrupole magnets SF SD SD SF SF ● Machine Chromaticity Spread in the Machine Lens Optics Analogy: Tune due to Particle [Quadrupole] Energy Spread Controlled by Sextupole magnets Focal length is Achromatic incident light energy dependent [Spread in particle energy] Hermann Schmickler – CERN Beam Instrumentation Group
The Typical Instruments ● Beam Position ● electrostatic or electromagnetic pick-ups and related electronics ● Beam Intensity ● beam current transformers ● Beam Profile ● secondary emission grids and screens ● wire scanners ● synchrotron light monitors ● ionisation and luminescence monitors ● femtosecond diagnostics for ultra short bunches ● Beam Loss ● ionisation chambers or pin diodes ● Machine Tune and Chromaticity ● in diagnostics section of tomorrow ● Luminosity ● in diagnostics section of tomorrow Hermann Schmickler – CERN Beam Instrumentation Group
Measuring Beam Position – The Principle + + + - + + + - + + - - - - - - - - - - - - - - - - + + - + + - + + + + - + - + - - + - - - - + + + + + + + + + - - + - - + - + - + + + + + + + + + + + + - - - - - - - - - - - - Hermann Schmickler – CERN Beam Instrumentation Group
Wall Current Monitor – The Principle V + + + + - + - - - + - - - - - - - - - + + - + + + + - - + - - - + + + + + + - - + - - - + + + Ceramic Insert + + + + + + - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Hermann Schmickler – CERN Beam Instrumentation Group
Wall Current Monitor – Beam Response V 1 f H 2 R C Response R R f L 2 L C 0 0 I B Frequency L I B Hermann Schmickler – CERN Beam Instrumentation Group
Electrostatic Monitor – The Principle V + + + + - + - - - + - - - - - - - - - + + - + + + + - - + - - - + + - - - - - - - + + - - + + + + + + - - + - + + - + - - - + - - - + + + - + - + + - + + + + + - + + - - - - - - - - - - Hermann Schmickler – CERN Beam Instrumentation Group
Electrostatic Monitor – Beam Response Response (V) C 1 f L 2 R C 0 0 V B Frequency (Hz) V R + + + + + + + + + + + + + + + + + + - = + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + d + + + + + + + + + + + + + + + + Hermann Schmickler – CERN Beam Instrumentation Group
Electrostatic Pick-up – Button Area A Low cost most popular × Non-linear • requires correction algorithm when beam is off-centre r For Button with Capacitance C e & Characteristic Impedance R 0 Transfer Impedance: 20 20 20 16 16 16 A 12 12 12 Z 8 8 8 T ( f f ) 2 r c C 4 4 4 c Y [mm] Y [mm] Y [mm] e 0 0 0 -4 -4 -4 Lower Corner Frequency: -8 -8 -8 -12 -12 -12 1 -16 -16 -16 f -20 -20 -20 L -20 -20 -16 -16 -12 -12 -8 -8 -4 -4 0 0 4 4 8 8 12 12 16 16 20 20 -20 -16 -12 -8 -4 0 4 8 12 16 20 2 R C X [mm] X [mm] X [mm] 0 e 5 5 5 3 6 3 2 5 4 X 2 . 30 10 X 3 . 70 10 X 1 . 035 X 7 . 53 10 X Y 1 . 53 10 X Y 1 1 1 1 1 1 1 Hermann Schmickler – CERN Beam Instrumentation Group
A Real Example – The LHC Button Response (V) V I Z f f B T ( f f ) C C f C 0 1 1 Frequency (Hz) 0 f L 400 MHz 2 R C 2 50 8 pF 2 A 12 mm Z 1 . 2 T 2 r c C 2 24 . 5 mm c 8 pF e 9 19 N e 5 10 1 . 6 10 pilot I 0 . 8 A V 0 . 8 1 . 2 1 V B peak f peak 9 1 10 t 11 19 N e 1 10 1 . 6 10 nom 16 A V 16 1 . 2 20 V peak f peak 9 1 10 t Hermann Schmickler – CERN Beam Instrumentation Group
Improving the Precision for Next Generation Accelerators ● Standard BPMs give intensity signals which need to be subtracted to obtain a difference which is then proportional to position Difficult to do electronically without some of the intensity information leaking through ● ● When looking for small differences this leakage can dominate the measurement Typically 40-80dB (100 to 10000 in V) rejection tens micron resolution for typical apertures ● Solution – cavity BPMs allowing sub micron resolution ● ● Design the detector to collect only the difference signal Dipole Mode TM 11 proportional to position & shifted in frequency with respect to monopole mode ● Frequency Domain TM 01 TM 01 U / V TM 02 TM 02 TM 11 TM 11 f / GHz Courtesy of D. Lipka, U~Q U~Qr U~Q DESY, Hamburg Hermann Schmickler – CERN Beam Instrumentation Group
Today’s State of the Art BPMs Obtain signal using waveguides that only couple to dipole mode ● Further suppression of monopole mode ● Monopole Mode Dipole Mode Courtesy of D. Lipka, DESY, Hamburg Prototype BPM for ILC Final Focus ● ● Required resolution of 2nm (yes nano!) in a 6×12mm diameter beam pipe ● Achieved World Record (so far!) resolution of 8.7nm at ATF2 (KEK, Japan) Courtesy of D. Lipka & Y. Honda Hermann Schmickler – CERN Beam Instrumentation Group
Criteria for Electronics Choice - so called “Processor Electronics” ● Accuracy ● mechanical and electromagnetic errors ● electronic components ● Resolution ● Stability over time ● Sensitivity and Dynamic Range ● Acquisition Time ● measurement time ● repetition time ● Linearity ● aperture & intensity ● Radiation tolerance Hermann Schmickler – CERN Beam Instrumentation Group
Processing System Families AGC on S Synchronous Heterodyne MPX POS = (A-B) Detection Hybrid Homodyne Heterodyne POS = D / S D / S Detection Electrodes Direct POS = D / S A, B Digitisation Individual Logarithm. Differential POS = [log(A/B)] Legend: Treatment Amplifiers Amplifier = [log(A)-log(B)] / Single channel Limiter, Passive Amplitude Wide Band POS = [A/B] D t to Ampl . Normaliz . to Time Narrow band Limiter, Amplitude . POS = [ATN(A/B)] f to Ampl . to Phase Normalizer Processor Active Circuitry Hermann Schmickler – CERN Beam Instrumentation Group
LINEARITY Comparison 1 Transfer Function 0.5 D/S Atn(a/b) Normalized loga-logb Position (U) 0 -1 -0.5 0 0.5 1 -0.5 Computed Position (U) -1 Hermann Schmickler – CERN Beam Instrumentation Group
Amplitude to Time Normalisation 1.5 3.0 1.0 2.5 Amplitude A A 0.5 2.0 0.0 1.5 -0.5 1.0 Amplitude B B -1.0 0.5 -1.5 0.0 1.5ns B + 1.5ns -2.0 -0.5 -2.5 -1.0 Time [ns] A Combiner Splitter Delay lines Beam B Pick-up Hermann Schmickler – CERN Beam Instrumentation Group
Amplitude to Time Normalisation 1.5 3.0 A + (B + 1.5ns) 1.0 2.5 Amplitude A A 0.5 2.0 0.0 1.5 -0.5 1.0 Amplitude B B -1.0 0.5 -1.5 0.0 -2.0 -0.5 -2.5 -1.0 Time [ns] D t depends on position 1.5 3.0 B + (A + 1.5ns) 1.0 2.5 Amplitude A A 0.5 2.0 0.0 1.5 -0.5 1.0 Amplitude B B -1.0 0.5 -1.5 0.0 -2.0 -0.5 -2.5 -1.0 Time [ns] Hermann Schmickler – CERN Beam Instrumentation Group
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