Bunch Flattening withRF Phase Modulation

T. Argyropoulos, C. Bhat, A. Burov, J. F. Esteban Müller,S. Jakobsen, G. Papotti, T. Pieloni, T. Mastoridis, E. Metral,

N. Mounet, B. Salvant, E. Shaposhnikova, H. Timko, +…

Thanks to the OP crew!

LSWG2

Motivation

15/02/2013

Heating depends on the beam spectrum and therefore bunch distribution Modifying the bunch distribution can

help to reduce the heating in some devices

The bunch distribution in the LHC can be changed by: Installing a higher harmonic RF system

(LMC open action) Flattening the bunches by applying a

sinusoidal RF phase modulation at a frequency slightly below the synchrotron frequency (~ 97% fs) Already done in SPS and in Tevatron and

checked in simulations

LSWG3

MD plan

15/02/2013

Flat bottom Single bunches

Synchrotron frequency distribution with PD Schottky Synchrotron frequency shift by scanning the modulation

frequency Check parameters for RF phase modulation

Nominal beam Test RF phase modulation

4 TeV Ramp with nominal beam and controlled

emittance blow-up Voltage variation for transverse instability studies RF phase modulation

LSWG4

(I) Phase Modulation Test at 450 GeV

15/02/2013

I.a) Test with 8 bunches with intensities in range (0.65-2.3) x 1011 scanning from above the modulation frequency

Bunch phase was monitored to see effect on each bunch Difference in synchrotron frequency between low and high

intensity bunches with length ~1.3 ns is ~ 0.2 Hz/1011 For impedance Im(Z/n) = 0.1 Ω and parabolic amplitude distribution dfs~0.27 Hz/1011 is expected

I.b) Test with nominal Beams 1 and 2: RF modulation at frev ± 0.97 fs with PL ON, led to a weak

bunch length variation at frev

Visible effect on beam spectrum and bunch profiles Beneficial effect on ALFA heating observed online

LSWG5

(II) 4 TeV : Phase Modulation

15/02/2013

Beam 2: RF phase modulation at frev ± 0.97 fs with PL ON Modulation in bunch lengths around the ring

Beam 1: RF phase modulation at 0.97 fs with PL OFF, also some increase in the BQM BL (~ 50 ps) due to PL OFF

Significant change in spectrum as expected for flat bunches (not only f ~1/): Lower power spectrum in the low frequency range Higher power spectrum in the range 1.2-1.6 GHz

LSWG6

(II) 4TeV:Beam Induced Heating

15/02/2013

Voltage variation in 0.5 MV steps (10-6-10 MV) to study transverse stability: No improvement observed Reduction in heating as expected

With flat bunches - heating reduced in many devices: Immediate reduction in temperature of TCTVB and ALFA Roman Pots Reduction in the BSRT, but indirect temperature monitoring For the MKI the test was not long enough compared to the characteristic

thermal time No temperature monitoring for the TDI

Tem

pera

ture

[ºC

]

UTC Time

Voltage

variation

RF phasemodulatio

n

Bunch lengths

ALFA

TCTVB

LSWG7

Peak-Detected Schottky

15/02/2013

Two bunches of B1 with similar length and intensities of 0.65 x 1011 and 1.65 x 1011 were monitored at 450 GeV during ~20 min (following the BL increase)

Advanced analysis needed to estimate accurately tiny frequency shift (~1Hz at 2fs)

LSWG8

Conclusions

15/02/2013

Flat bunches with RF phase modulation: Very promising method to control heating below 1.2 GHz (no

show-stopper found), but it needs more checks after LS1 with better diagnostics for heating in the 1.2-1.6 GHz range

If OK above 1.2 GHz, can be used in operation after usual controlled emittance blow-up (with smaller target BL) with PL off (during this manipulation); at the end of the ramp or beginning of the flat top

Inductive impedance estimation Data from PD Schottky and excitation fs shift is 100 times less than in SPS Needs advanced

analysis No improvement for transverse instability with 0.2 ns

longer bunches