Front-end amplifiers for the beam phase loops in the CERN PS Alessandro Meoli (CERN BE/RF/FB) Supervised by Heiko Damerau 21 April CERN

Beam current structure and wall image current DC beam current: Beam image current: At relativistic velocities, the wall current has the same time structure as the beam current, so it is a mirror (except the DC component) of the beam current How can we measure it? [1] Denard, J.C. “CERN Accelerator School on Beam Diagnostics”, 28 May -6 June 2008, Dourdan [1] 1

Wall current monitor The H field outside the beam pipe is 0, according to the Ampere’s law: We can measure the wall current, cutting the beam pipe, and inserting a ceramic break, to force the current through an impedance. [2] [2] D’Elia, A “Status of the Wall Current Monitor Design for EUROTeV”, CERN 2

Actual design Very high dynamic range (mV to hundreds V) Mechanical relay, switches every low intensity beam cycle! We need different levels of attenuation Bad reliability over time Bad reliability over time Difficult to chose the correct intensity range Difficult to chose the correct intensity range WCM PS SS95 LHCPROBE Beam TOF Beam More than 3 orders of magnitude! Injection Extraction 3

New design 0 dB 5 dB 10 dB 20 dB 30 dB WCM 40 dB 50 dB 60 dB 50 Ω Multiplexer The idea is to design a circuit similar to the front end of an oscilloscope: Large bandwidth Large bandwidth High input impedance High input impedance Protection against overvoltage Protection against overvoltage High dynamic range mV up to 800V Small dynamic range (+- 5V) Design specification: Bandwidth: 100KHz-100MHz (Highest frequency of interest for the phase loop is 80MHz) High input impedance, to perturb the signal as less as possible High sensitivity but at same time protection against overvoltage Signal distributor Future option 4

New design High input impedance attenuator Input stage Bandwidth limiter Output buffers 50 Ω 5

High impedance input attenuator stage Finite input capacitance of the amplification stage Resistive divider becomes a low-pass filter Need to compensate: Fine tuning in lab, to compensate the parasitic capacitance 6 Low frequency: resistive divider High frequency: capacitive divider

Prototype 7

Prototype: frequency response of the input stage Network analyser 50 Ω -20dB attenuator Expected: Measured: 150 MHz 100 KHz 1 GHz 100 KHz Resonances in the input stage. Out of our band of interest, so it will be removed in the band limiter stage Highest frequency of ⱷ loop presently at 80 MHz (h=169): response in band of interest almost constant This test shows the response of the -20dB version, other amplifiers have very similar response 8

Complete prototype with bandwidth limiter and output buffers Network analyser 20dB attenuator Band limiter 50 Ω 1 GHz 100 KHz 100 MHz 150 MHz Frequency response in band of interest quasi constant Components at higher frequency attenuated 9

Tests with beam signal Clipping under high intensity beam Injection Extraction Transition Low intensity beam 1,48 s Very high intensity beam High sensitive pickup High input impedance amplifier Bunch shape 10 LHCPROBE TOF Not saturated Saturated and correctly clipped

Beam phase loop closed with the prototype amplifier 11 ΔKΔⱷΔKΔⱷ Injection Extraction Transition Injection Extraction Transition Acceleration a LHCPROBE with High sensitive pickup Acceleration a LHCPROBE with prototype ΔKΔⱷΔKΔⱷ 50 mV/deg Phase loop closed on LHCPROBE beam Delay not compensated yet This Δⱷ depends only by the different delay ΔⱷΔⱷ ΔⱷΔⱷ ΔⱷΔⱷ WCM RF cavity 200 mV/div 500 mV/div Phase loop closed all along the cycle

Outlook and conclusion Assembly of complete setup of multiple amplifiers, in a 19" crate and installation in the PS RF Control Room Final tests with the amplifiers connected directly to the WCM and validation of prototypes foreseen in June Prototype successfully tested with beam signal (connected after the first passive splitter) Beam phase loop closed using the prototype amplifier connected to the WCM95 Board layout being finalized in electronics design office (Thanks to Jean Marc Combe) Present status: Next steps: 12

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