Working Group 1Synchrotrons/LHC

Summary Report

Mike BrennanPhilippe Baudrenghien

Four Talks, and much discussion (LHC)

1. Development of LLRF control system for J-PARC RCS, by Dr.Tamura

Rapid cycling implies1. High gradient cavities2. Fast frequency sweep3. Finemet cavities are the

solution4.Two harmonics are imposed on the

cavities simultaneously (Q=2)Direct Digital synthesizers

are used to generate the waveforms

Feedforward beam loading compensation is used

DDS is used to extract the harmonic component of the beam current, up to h=6

2. Experiments of Low Level Control Based on Analog IQ Technique, by Mr. Zhao

1. At the Shanghai Synchrotron Radiation Facility both analog and digital I/Q processing are being considered

2. So far an analog prototype system has been built and tested

3. Some suggestions were proposed on how to extend the bench tests to better simulate a real cavity

3. An IQ-based low-level RF prototype for ALBA, by Mr. Hassanzadegan

ALBA is the light source being built in Spain (near Barcelona), 350 meter 3.5 GeV ring

Tests of the IQ hardware demonstrate 1% amplitude and 1 degree phase regulation

A high-Q pill box cavity has been built and will soon be added to the test setup

4. (Digital version of IQ control for ALBA), by Angela Sarasqueta

1. Rather similar to IQ board reported on by Tony Rohlev

2. Angela invited suggestions by the experts in the house

Issues of particular interest to LHC

1. RF noise and longitudinal emittance control

2. Klystron gain saturation and phase noise remedies

3. Beam Control topics (not presented in talks)

RF Noise

• Is there an optimal value for rf noise– Too much costs luminosity (bunch lengthening)– Some rf noise may balance synchrotron radiation

damping (too small emittance enhances IBS)– There were many opinions about if controlled noise

can be beneficial in emittance blow-up– Consensus (perhaps?) that the noise spectrum

should be shaped in a controlled way

RF Noise

• J. Tuckmantel reported on his study (via simulation/analytic calculation) of noise versus synchrotron radiation damping

RF Noise

• We discussed if narrowband noise and can cause de-bunching

• Lebedev, “In theory… it’s impossible”

• Brennan, “In practice…I’ve seen it”

• Linnear has seen something similar in the SPS (ICFA 2004)

• Probably some other mechanism is involved, such as IBS or instability

De-populated spot in momentum distribution

First synchrotron sideband at 4 GHz

Klystron Gain/Noise

• All agree that the klystron makes phase noise, and we must have feedback to reduce it

•J. Fox showed how to keep the gain high even when the klystron saturates (linearizer)

•Philippe B. showed results of tests of direct feedback in reducing phase ripple

• the “polar loop” has basically the same effect at the linearizer

Beam Control for LHC

• Loops (store with open or closed loop?)– Phase, programming the gain, opening the loop,…– Radius, used only in commissioning– Synchronization, conflict with phase loop? AC

coupled phase loop• Cogging

– Keeping it adiabatic – Operations interface– Direct measurement (WCMs) of vertex phasing

Beam Control for LHC• Technology

– Noise floor of phase detecor– Phase noise in VCXO vs state-of-the-art DDS

• Diagnostics– How do you reduce the flood of data on a fault (post

mortem)– Out-of-range trapping should be engineered into

firmware designs– Templates of good system states should be readily

available• Beam Inhibits (not a casual matter)

– A 100 Hz frequency jump would de-bunch the beam