An Inductive Pick-Up (IPU) for Beam Position and Current Measurement Marek GASIOR, CERN, AB/BDI email: marek.gasior@cern.ch 6th European Workshop on Beam Diagnostics and Instrumentation for Particle Accelerators 5 – 7 May 2003, Mainz, Germany Contributed Talk #01

An Inductive Pick-Up (IPU) for Beam Position and Current Measurement Marek GASIOR, CERN, AB/BDI email: marek.gasior@cern.ch Third CLIC Test Facility Evolution from a WCM to an IPU IPU Design and Model Active Hybrid Circuit Results An Inductive Pick-up for Beam Position and Current Measurement

Third CLIC Test Facility (CTF3) Delay Loop, f’ = f  2, I’B = IB  2 1.5 s bunch train  5 pieces of 140 ns Drive Beam Linac (f =1.5 GHz, IB = 3.5 A) A 1.5 s bunch train, some 2300 pulses Drive Beam Decelerator Main Beam Accelerator Combiner Ring, f” = f’  5, I”B = I’B  5 5 pieces of 140 ns  1 train of 140 ns Requirements for a DBL Beam Position Monitor: Low cut-off frequency at least 10 kHz to limit a droop of the 1.5 s pulse to about 10 % High cut-off frequency at least 100 MHz to observe fast beam movements (rise time some 3 ns) The bandwidth 10 kHz – 100 MHz means 4 decades The pick-up structure must be as transparent as possible for the beam and corresponding longitudinal coupling impedance should be low in the GHz range An Inductive Pick-up for Beam Position and Current Measurement

Wall Current Monitor (WCM) principle The BEAM current is accompanied by its IMAGE A voltage proportional to the beam current develops on the RESISTORS in the beam pipe gap The gap must be closed by a box to avoid floating sections of the beam pipe The box is filled with the FERRITE to force the image current to go over the resistors The ferrite works up to a given frequency and lower frequency components flow over the box wall An Inductive Pick-up for Beam Position and Current Measurement

WCM as a Beam Position Monitor For a centered BEAM the IMAGE current is evenly distributed on the circumference The image current distribution on the circumference changes with the beam position Intensity signal () = resistor voltages summed Position dependent signal () = voltages from opposite resistors subtracted The  signal is also proportional to the intensity, so the position is calculated according to / Low cut-offs depend on the gap resistance and box wall (for ) and the pipe wall (for ) inductances An Inductive Pick-up for Beam Position and Current Measurement

A Beam Position Sensitive WCM A position sensitive WCM is still used in the CERN PS It contains 96 resistors of 10  in 32 groups of 3 in series), V/IB  1  Position measurement bandwidth is 9 MHz – 1.5 GHz (2.2 decade) Current measurement bandwidth is 3 MHz – 1.5 GHz (2.7 decade) G.C. Schneider, A 1.5 GHz Wide-Band Beam Position and Intensity Monitor for the Electron-Positron Accumulator (EPA), CERN/PS 87-9 (BT), 1987 An Inductive Pick-up for Beam Position and Current Measurement

A new design: Inductive Pick-Up (IPU) MORE INDUCTANCE LESS RESISTANCE An eight electrode “tight” design to avoid resonances in the GHz range The electrodes cover 75 % of the circumference The electrode internal diameter is only 9 mm larger then the vacuum chamber of 40 mm and it is occupied by the ceramic insertion (alumina) The transformers are as small as possible to gain high frequency cut-off with many turns The transformers are mounted on a PCB The connection between the electrodes and the cover is made by screws Electrode diameter step is occupied by the ceramic tube The tube is titanium coated on the inside An Inductive Pick-up for Beam Position and Current Measurement

Inductive Pick-Up – A Low Frequency Model Electrodes are combined in pairs so that each transformer sees half of the load Frequency low cut-offs are limited by connection parasitic resistances Each transformer has one calibration turn (not shown) n = 30, RS  7  giving RT  0.1  and RP  4 m fL  150 Hz (RP with L  5 H) fL  10 kHz (RP with L  70 nH) The electrode signal high cut-off frequency is beyond 300 MHz An Inductive Pick-up for Beam Position and Current Measurement

Inductive Pick-Up New Design The ceramic tube is coated with low resistance titanium layer, resistance: end-to-end 10 , i.e.  15 / Primary circuit has to have small parasitic resistances (Cu pieces, CuBe screws, gold plating) Tight design, potential cavities damped with the ferrite The transformers are mounted on a PCB and connected by pieces of microstrip lines (minimizing series inductances) An Inductive Pick-up for Beam Position and Current Measurement

Active Hybrid Circuit (AHC) More than four decades of bandwidth required High Common Mode Rejection Ratio needed, at least -40 dB at 100 MHz Active circuit with a differential amplifier AD8129 – “active feedback” architecture, i.e. one feedback network needed Datasheet CMRR is -42 dB at 100 MHz Bandwidth 200 MHz with a gain of 10 An Inductive Pick-up for Beam Position and Current Measurement

Active Hybrid Circuit – Performance The CMRR at 100 MHz is as high as 55 dB (datasheet 42 dB) The CMRR for frequencies below 10 MHz is limited by the measurement setup  signal high cut-off frequency about 200 MHz An Inductive Pick-up for Beam Position and Current Measurement

IPU and AHC – Frequency Characteristics A wire method with a 50  coaxial setup which the IPU is a part  signal – flat to 0.5 dB within 5 decades, almost 6 decades of 3 dB bandwidth (no compensation)  signal – 5 decades (four decades + one with an extra gain for low frequencies) BW: 300 Hz – 250 MHz ( 6 decades) BW: 1 kHz – 150 MHz (> 5 decades) An Inductive Pick-up for Beam Position and Current Measurement

IPU and AHC – Displacement Characteristics A thin wire forming a coaxial line was displaced diagonally across the pick-up aperture. The measurement was done with a network analyzer: signal was applied to the wire and hybrid signals were observed. An Inductive Pick-up for Beam Position and Current Measurement

IPU – Longitudinal Coupling Impedance reference The pick-up was inserted into a 50  coaxial line (again the wire method) The signal drop along the pick-up was evaluated by measuring the S21 scattering transmission coefficient As a reference was measured the same setup with the pick-up replaced by an equivalent length of a tube (to be independent of the setup) An Inductive Pick-up for Beam Position and Current Measurement

IPU – Time Domain Reflectometry Measurements The wire method with the 50  coaxial setup A fast step was applied to the coaxial line and the reflection was observed The electrode diameter step is visible only for components of lower frequency. Higher frequency components do not see the step since they flow over the titanium low resistance coating An Inductive Pick-up for Beam Position and Current Measurement

IPU and AHC – Beam tests in the CTF2  - CH2 H - CH3 V - CH4 Electron beam of one 1 nC , 5 psRMS bunch The signals have the rise time of about 2 ns (one division) An Inductive Pick-up for Beam Position and Current Measurement

An Inductive Pick-up for Beam Position and Current Measurement Conclusions An inductive pick-up and a dedicated active hybrid circuit were designed for the drive beam linac of the CTF3 They allow to measure beam position with a bandwidth of 5 decades and absolute beam current over 6 decades The chain IPU-AHC can be tested and calibrated in place with precise current pulses, applied to calibration turns of the IPU transformers Neither the pick-up nor the AHC contain adjustable elements The pick-up longitudinal coupling impedance is limited to about 10  in the GHz range Very many thanks to J. Belleman, J. Durand, J.L. Gonzalez, L. Søby, J.P. Potier, Y. Cuvet and J.L. Chouvet http://www.cern.ch/gasior/pap/dipac2003.ppt An Inductive Pick-up for Beam Position and Current Measurement

Thank you for your attention http://www.cern.ch/gasior/pap/dipac2003.ppt An Inductive Pick-up for Beam Position and Current Measurement

Emergency slide – the parameter table An Inductive Pick-up for Beam Position and Current Measurement