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

2. An Inductive Pick-Up (IPU) for Beam Position and Current Measurement
Marek GASIOR, CERN, AB/BDI
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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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
An Inductive Pick-up for Beam Position and Current Measurement

18. Thank you for your attention
An Inductive Pick-up for Beam Position and Current Measurement

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