Tune and Chromaticity Tracking in the Tevatron

C.Y. Tan

09 May 2006

Overview

What is the Tevatron tune tracker?

Selected results from some stores. (There are many stores with TT running).

Planned chromaticity tracker.

Tune Tracker

There are at least two ways to track the tune

Tune fitting which is available for 21.4MHz and 1.7 Ghz Schottky. Slow (~1 Hz)

Actively exciting the beam and tracking the zero phase response of the beam. This is the PLL method. Fast (100 Khz limited by ethernet bw)

Tune Tracker Principle (Thought Experiment)

Young Fermi wants to find the peak a pendulum resonance (SHO).

He is armed with

Sine wave generator to excite the pendulum

Phase detector

Detect phase of pendulum w.r.t. phase of sine wave

How to find the peak from phase

What I need

A detector

21. 4MHz Schottky at A1

A kicker

1 foot long stripline BPMs converted to kickers at A1

Can I measure a frequency response?

Can we measure a frequency response?

In Principle

The Hardware

Tune Tracker Status

Tune Tracker (TT) at Tevatron declared commissioned on 13 Sep 2005 with first successful tracking of both horz and vert tunes in store 4386.

As of this date, TT has been on for all stores.

The data that has been collected was extremely useful for analysis when things went wrong. E.g. Skew quads not ramping, separator polarity did not switch.

Normal Store

Thru' the squeeze

When Skew Quads Don't Turn On

Flying tunes

Tune Tracker Performance

TT has performed very well over the past 5-6 months.

Very little human intervention. It just works!

No retuning required even after 2 unscheduled shutdowns of approximately 1 week each.

However, the beam frequency response changed suddenly in the first week of Feb. Cause is still unknown. Also noticed phase drift over a few weeks.

Plans

No tune feedback plans.

RHIC has successfully done “feedback” but not during stores.

Important for LHC!

TT will be used as the basis for the chromaticity tracker (CT) using the McGinnis method.

LHC evaluating different ways to do chromaticity tracking. SPS experiment planned.

The McGinnis Method

Classical method: measures change in RF frequency (i.e. p/p) to betatron frequency change.

McGinnis method: Phase modulate the RF frequency. This couples to transverse plane because of chromaticity.

Novelty: Phase demodulate the betatron frequency to obtain chromaticity

The McGinnis Method (cont'd)

The McGinnis formula for calculating chromaticity when using the VSA as a phase demodulator:

McGinnis Method (cont'd)

McGinnis method requires that:

RF be phase modulated.

TT supplies the carrier frequency (i.e. Betatron tune) for phase demodulation.

Phase demodulated height is related to the chromaticity.

Why we need the TT

TT will lock at 0. If modulation is outside the bw of the TT, it will not follow the modulation and thus,  can be measured w.r.t. the locked frequency.

Requirements

Modulation frequency must be outside the closed loop bw of the TT. For Tevatron TT, closed loop bw is about 5Hz.

Modulation frequency must less than ½ the synchrotron frequency to prevent emittance growth. For Tevatron at 150GeV approx 20Hz.

mod is approx. 5 degrees or less to prevent beam loss and tune change < 0.0001. This corresponds to dp/p ~ 1e-5 if chromaticity is 5 to 20 in the Tev.

Limitations

If only using VSA to demodulate, can only get magnitude of chromaticity. Cannot get sign without adding more hardware. Coming real soon now ...

Will not work at transition because  = 0.

Not a problem at Tevatron or LHC.

Longitudinal dampers which work on coupled bunch mode zero will damp this.

Should not be a problem at the Tevatron because current dampers do not work on mode 0.

Set up

Demodulated Signal

Mod freq = 23Hz

mod= 10 deg.

Tracking with Phase Modulation ON

No beam loss during modulation of mod= 5 deg.

Tune moved when chrom knob changed.

(Checked that tune change not synch. freq or 23 Hz. However, I need to be sure)

Results for Tevatron

Note that calibration of phase shifter needs to be checked.

Using 3D-BBQ

TT still at 21.4MHz and locked.

(a) phase mod off.

(b) phase mod on.

mod= 40 deg.

BBQ Phase Demodulated

mod= 5deg.

Chrom Measured using BBQ

Planned Implementation (Phase II)

Conclusion

TT is now routinely used for stores.

People are using it in a “feedforward” sense to tune the Tevatron between stores.

Quite robust and not much attention needed for it to work.

Future plan is to work on chromaticity tracking.

Preliminary work shows that the McGinnis method is promising. However, I need to prove that it will work up the ramp and thru the squeeze.

Note that LHC synchrotron frequency is 64 Hz at injection (450GeV) and 23Hz at flattop (7TeV). TT for CT must have closed loop bw of ~5Hz if McGinnis method is used.

Acknowledgements

The Tevatron group for supporting this project.

US-LARP

Operated by Universities Research Association Inc. under Contract No. DE-AC02-76CH03000 with the United States Department of Energy.

Cheng-Yang Tan

2006-03-01T10:00:24