0 1 Update on Beam Tests with PS Finemet Cavity H. Damerau, L. - - PowerPoint PPT Presentation

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0 1 Update on Beam Tests with PS Finemet Cavity H. Damerau, L. - - PowerPoint PPT Presentation

Jun 04, 2023 208 likes •539 views

0 1 Update on Beam Tests with PS Finemet Cavity H. Damerau, L. Ventura LIU-PS Working Group Meeting June 9, 2015 Many thanks to Matthias Haase and Mauro Paoluzzi 2 Overview Introduction Coupled-bunch feedback Beam-loading

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SLIDE 1
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1

Update on Beam Tests with PS Finemet Cavity

  • H. Damerau, L. Ventura

LIU-PS Working Group Meeting

June 9, 2015

Many thanks to Matthias Haase and Mauro Paoluzzi

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SLIDE 3

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Overview

  • Introduction
  • Coupled-bunch feedback
  • Beam-loading compensation feedback
  • Low intensity and high intensity, triple splitting
  • Excitation of coupled-bunch oscillations
  • Mode scans
  • Excitation rates
  • Summary and outlook
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Introduction

  • LS1:
  • New Finemet cavity installed in SS02 (M. Paoluzzi et al.)
  • 2014:
  • First cavity gap available for test with LLRF system
  • Tests of beam-loading compensation feedback
  • 12 harmonics damped with nominal intensity LHC25ns
  • 2015:
  • 3 gaps available, 4 gaps after June technical stop
  • Excitation of coupled-bunch instabilities in open loop
  • Measurement of excitation amplitudes vs. cavity voltage
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4

Overview

  • Introduction
  • Coupled-bunch feedback
  • Beam-loading compensation feedback
  • Low intensity and high intensity, triple splitting
  • Excitation of coupled-bunch oscillations
  • Mode scans
  • Excitation rates
  • Summary and outlook
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Beam-loading compensation (1 harmonic)

Low-pass Low-pass ADC DAC

Cavity return Cavity drive

sin(hFB frevt + f) sin(hFB frevt) cos(hFB frevt + f) cos(hFB frevt)

fs side- band filter ADC

Wall current monitor

cos((hRF-hFB)frevt + f) sin((hRF-hFB)frevt + f)

fs side- band filter

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First measurements with beam in 2014

Open/closed loop transfer function 500 kHz 6 MHz

  • Moderate feedback gain

(very first test!)

  • Transfer function

measurement: ~ 10…12 dB

  • Spectrum of beam induced

voltage in Finemet cavity

  • 26GeV-test cycle, low

intensity single bunch accelerated on h = 8

 Impedance reduction

  • bserved with

beam as expected

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High-intensity, 6/7 filling, triple splitting

  • First 12 harmonics

simultaneously  1 bunch for nominal LHC25ns beam injected from PSB  6 bunch with nominal intensity for LHC25ns injected

 15…20 dB reduction of beam induced voltage, also during triple splitting

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SLIDE 9

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Overview

  • Introduction
  • Coupled-bunch feedback
  • Beam-loading compensation feedback
  • Low intensity and high intensity, triple splitting
  • Excitation of coupled-bunch oscillations
  • Mode scans
  • Excitation rates
  • Summary and outlook
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Coupled-bunch oscillations, freq. domain

 Synchrotron frequency sidebands of the frev harmonics:

  • F. Pedersen, F. Sacherer, PAC77, pp. 1397-1399

 In the case of LHC-type beams in the PS (h = 21)

upper lower

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Beam excitation with the Finemet cavity

Low-pass Low-pass ADC DAC

Cavity return Cavity drive

sin(hFB frevt + f) sin(hFB frevt) cos(hFB frevt + f) cos(hFB frevt) Amplitude

Low freq. DDS

Amplitude

sin cos

Side-band selection Excitation frequency, Df f

hFBfrev Df

 Excitation frequency ~ fs away from hfrev  ~ 400 Hz at 476 kHz

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11

First excitation test using Finemet cavity

  • Observe beam stability during acceleration after transition

No excitation, gaps open Excitation at exactly 20frev

 No effect with voltage from Finemet cavity at frev harmonic

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Excitation, Df = + 300 Hz Excitation, Df = - 300 Hz

 Strong excitation with frequency offset with respect to 20frev  Beam qualitatively behaves as expected

  • Frequency offset of ~ 300 Hz at start of excitation

First excitation test using Finemet cavity

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Overview

  • Introduction
  • Coupled-bunch feedback
  • Beam-loading compensation feedback
  • Low intensity and high intensity, triple splitting
  • Excitation of coupled-bunch oscillations
  • Mode scans
  • Excitation rates
  • Summary and outlook
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  • LHC25 ns beam with ~1.3  1011 ppb equivalent intensity
  • 4+2 and 4+3 bunches (full ring) injected from PSB
  • Two independent mode analysis techniques:

Mode scan measurements

gtr

Start excitation

  • L. Ventura
  • r
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Mode scan with 18 bunches in h = 21, cavity 11

Data with old coupled bunch feedback  Some modes can be excited very cleanly, others as a mixture; artefact?

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New coupled-bunch feedback LLRF, excitation of each mode  All 18 modes can be excited

Mode scan with 18 bunches in h = 21, Finemet

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SLIDE 18

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Excite each mode individually and measure mode spectrum  Clean observation of all possible modes

Mode scan with 21 bunches in h = 21, cavity 11

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Upper side-band: n = nexc New coupled-bunch feedback LLRF, excitation of each mode

Mode scan with 21 bunches in h = 21, Finemet

Lower side-band: n = 21 - nexc  Every oscillation mode from n = 1…21 can be excited on both side-bands

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SLIDE 20

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Overview

  • Introduction
  • Coupled-bunch feedback
  • Beam-loading compensation feedback
  • Low intensity and high intensity, triple splitting
  • Excitation of coupled-bunch oscillations
  • Mode scans
  • Excitation rates
  • Summary and outlook
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Excitation amplitude scan

Upper sideband of frev Lower sideband of frev Vary excitation amplitude and check mode spectrum ~20 ms after excitation starts:  Oscillation amplitude proportional to excitation  linear regime  Mode amplitudes comparable to excitation with spare cavity C10-11

  • Absolute excitation voltages to be analyzed
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Growth of coupled-bunch oscillation

Mode amplitude versus time: two measurement techniques

40 ms VRF Finemet cav. Signal around 20frev ~ 38 ms

Down-converted sidebands at 20frev Oscillation amplitude from mode analysis

  • L. Ventura

 Both measurement techniques give very similar results  Growth not exponential

  • How to derive growth times?
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Growth of coupled-bunch oscillation

Mode amplitude versus time: two measurement techniques

40 ms VRF Finemet cav. Signal around 20frev

Down-converted sidebands at 20frev

 Both measurement techniques give very similar results  Growth not exponential

  • How to derive growth times?
  • Driven harmonic oscillator model?

Driven harmonic oscillator model

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Overview

  • Introduction
  • Coupled-bunch feedback
  • Beam-loading compensation feedback
  • Low intensity and high intensity, triple splitting
  • Excitation of coupled-bunch oscillations
  • Mode scans
  • Excitation rates
  • Summary and outlook
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SLIDE 25

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Summary

  • Beam induced voltage reduction tests

 12 harmonic damped simultaneously with up to 20 dB gain  Nominal intensity of 25 ns beam, follows RF manipulations

  • First tests without and with beam successful

 Coupled-bunch oscillations excited as expected  Each mode can be excited individually  Confirms measurements with C10-11 in 2013

 Qualitatively: Finemet cavity touches beam as expected  Quantitatively: More measurements/analysis needed

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Outlook

  • Future MDs to complete excitation measurements

 Scan excitation amplitude and frequency offset  Derive growth/damping rates for given cavity voltage  Check with shorter bunches (50 ns-like beam)  Excite multiple modes simultaneously  Excite quadrupolar oscillation modes

  • Follow-up firmware development

 Complete filter design for synchrotron frequency side-bands  Close the loop on one harmonic

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SLIDE 27

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26

THANK YOU FOR YOUR ATTENTION!

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Operational PS coupled-bunch feedback

  • Analogue signal processing, two channels
  • Two accelerating cavities as feedback kickers  limits to modes h – 1 and h - 2

 New wide-band Finemet cavity as kicker  Digital feedback electronics

J.-L. Vallet, https://ab-div.web.cern.ch/ab- div/Meetings/APC/2005/apc050609/JL_Vallet_slides.pdf

Cover all modes

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New cavity (#25) in the PS ring

  • M. Paoluzzi
  • Wide-band (0.4 – >5.5 MHz, VRF = 5 kV) cavity based on Finemet material
  • No acceleration, but damping of coupled-bunch oscillations

SS02

6-cell cavity unit Accelerating gap Power amplifiers

(solid state)

F G

  • Cavity installed in SS02, amplifiers on 2 gaps

 First installation of transistor power amplifiers close to beam in PS

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Damping rate versus gain and intensity

Measured damping rate with feedback on Corrected for natural damping

Versus gain Versus intensity

 Zero damping at zero gain  Natural damping in- dependent from gain  Damping increases with intensity, more signal for given CB oscillation amplitude ‐ Saturation leads to non- zero damping with zero Np?

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Damping rate versus el and cycle time

Versus el (RMS) During cycle

 Uncorrected: damping efficiency increases for larger el  Reduced natural stability for smaller el  Corrected damping independent from el  Damping efficiency reduces at higher energy  To be checked with simulations

~9 GeV ~26 GeV ~9 GeV ~26 GeV

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Kick voltage versus oscillation amplitude

  • Excite a coupled-bunch oscillation and measure its amplitude
  • Observe maximum damping voltage required
  • Only order of magnitude for

kick voltage  Overestimate expected as feedback normally started before oscillations are well developed

Frequency range 0.4 to 5.5 MHz RF voltage per sideband, Vmode ~ 1 kV Maximum total RF voltage, Vmax ~ 5 kV Un-damped shunt impedance at n·frev < 200 W

  • M. Paoluzzi, H.D.,

CERN-ACC-NOTE-2013-0019

Basic specifications of kicker cavity: