Tune and Orbit Feedbacks perfomances:
A user perspective
L. Ponce
On behalf of the OP team
Special thanks for discussions to Joerg, Ralph, Marek and Wolfgang
Outline

Some statistics over the year (OP point of view)

Status and major issues:


QPS trip

Tune measurement

The OFB H plane
Feed-forward strategy
Evian 2011
2
Some statistics

Total 131 dumps when feedbacks are ON (ramp + squeeze):
– 33 dumps attributed to Feedbacks

RAMP
SQUEEZE
Total # of dumps
61
70
Assigned to
feedbacks
8
(6 in March)
25
%
13
37
Distribution by machine modes:
Machine mode
RAMP
SQUEEZE
MACHINE DEVELOPMENT
1
4
BEAM SETUP
3
6
PHYSICS
4
15
Evian 2011
3
Dump reasons

3 main causes:
– Triggering of trim quadrupoles (RQTF/D) QPS
– Wrong references sent to FB controllers
– Instabilities of the tune measurement:
• Tune FB driving tunes to third order resonance (beam
dumped by BLM) during the squeeze
• Orbit FB not correcting (dump by SIS) during the ramp

Large majority of the dumps due to QPS trips:
Number
% of the total
QPS triggering
23
69
Wrong references
5
14
Instabilities
5
(2 in MD, 2+1 in PHYSICS)
14
Evian 2011
4
Wrong references
Since beginning of the run, both orbit and tune
feedback are sequencer driven


Settings stored in LSA
Most of the errors came when cloning
hypercycles for MDs and in BEAM SETUP mode

=> need test, consistency checks, more
discipline
Evian 2011
5
QPS trips
QPS is triggered by the real time trims sent by the tune
feedback oscillating or too large

Above 50 A, QPS triggered if U_res is > 100 mV for more
than 190 ms

Typical case : signal
oscillating around the
threshold.

Evian 2011
6
1st solution : increase threshold?
New simulations of the protection system showed that
threshold could be relaxed (see R. Denz talk).

– Present strategy: bellow 50A, Ures threshold is at 2V
and decrease to 100 mV above 50 A
– Proposed change: keep the threshold at 2 V till 100 A,
providing current is limited to 200 A on the hardware
level

Should avoid problem for 2012

But still not enough for higher energy.
Evian 2011
7
2nd solution : reduce «noise»
Test during ions run: Tune FB response bandwidth has been
reduced by a factor 5

– To be seen if it is also possible with protons, especially
after FF?
– High gain was motivated by initial specification of keeping
dQ < 0.001 => could it be relaxed?
response bandwidth reduced by a factor 5:
Evian 2011
8
Tune measurement

2 different problems:
ADT on (max gain)
ADT off

S/N ratio = multiple peaks

Saturation = peak disappearing
– Tune peak damped
– High bunch intensity
– BBQ vs ADT settings
– Internal limitation of the system
Evian 2011
9
Improvements for 2012
Saturation effect appeared with the increase of bunch intensities
above nominal.

– For next year, dynamic range will be adapted for higher bunch
intensities.
– HW modification to allow remote settings (no more access needed)
– Could lead to less sensibility for the pilot
=> should not be a problem anymore for 2012 run

Problem of tune measurement : ADT vs BBQ war!
– CHIRP could be used to enhance the tune peak (MPP for high
intensity ?)
– More set-up time to study compatible settings for ADT and BBQ
– Other mean for tune measurement : ADT signal for Q measurement,
gating...
Evian 2011
10
ADT + BBQ Gating
At the end of 2011 run, bunch gating for ADT has been
successfully tested :

– Principle is to disable the ADT for defined bunch train
Problem: BBQ signal sensitive to bunch intensity, the more
intense bunches contribute more to tune signal


ADT gating tested on train with 20% more bunch intensity
– could use the first 12 bunches and guarantee increased bunch
intensity on these bunches?
To be used in nominal operation, ADT gating should be
combined with BBQ gating:

– Not fully ready yet, to be tested at the beginning of next run
Evian 2011
11
3rd solution : switch OFF QFB
Several fills (difficult to trace in the statistics) were put in physics with FB
OFF (~ 10 with ions, several fill when increasing bunch intensities)


To avoid big trims/oscillations when the tune signal is not stable/too low.
– Automatic with tune stability measurement : self-protection in the
QFB
– Operators decision
Evian 2011
12
Feed-forward vs feedback
The extreme solution to switch OFF tune feedback during
ramp or squeeze is OK provided that the bigger part of the Real
Time corrections have been incorporated in the functions

FF regularly done during commissioning of the beam
processes.

– Incorporation of the RT corrections into functions at the
matched points

For the orbit corrections, FF regularly done in V plane:
– Reduction of the strength of the RT trims from 10 to 3
µrad

more difficult in H (see next slide)
Evian 2011
13
FF during ALICE squeeze
Courtesy of N. Ryckx
Evian 2011
14
Problem with H plane
Orbit feedback is “pushing” up the orbit : H
tune is drifting.

Q feedback needed to compensate: QFB
cannot be switched OFF

– Origin not yet understood
– Cannot feedforward the corrections
– Cannot switch off the feedback
Evian 2011
15
High gain test of OFB

Detailed presentation by J. Wenninger at LBOC
Orbit excursion spikes are clearly visible, especially during the
squeeze:

– Becoming a problem (losses) when trying tight collimators settings
When investigating possible cures, test with higher gain of OFB
have been performed:

– Rather successful in the first part of the squeeze where FF was
performed
– But driving instabilities in the squeeze of IR2 to 1m (beam dumped)

As a result of the test, possible to envisage the following sequence:
– Perform a test squeeze with high gain (low intensity)
– Apply FF based on the high gain test
– If successful, operate the OFB with lower gain for high intensity
Evian 2011
16
Requests for 2012

From Feedback expert side:
– TIME to optimize settings at each significant step in the
commissioning
– MD time (see BI MD requests)

From OP side:
– More transparency on used settings
– Some GUI modifications to ease the operation
• Distinction between on-demand and continuous
• Direct access to signals seen by the FB

From both side:
– Software release and test procedure
– Test time allocated after new software version deployment
Evian 2011
17
Summary
Feedbacks saved more fills than they dumped: we cannot leave
without it

Most of the dumps we had in 2011 should be avoided next year
thanks to :

– Change of QPS thresholds to avoid RQTs trips
– HW modifications to avoid BBQ saturation
=> Should be left with 2-3 dumps! but what will we found if beams not
dumped?
Proposal to improve tune measurement to be tested at the
beginning of the run : ADT gating in combination with BBQ gating

Problem with real time trims in H plane to be sorted out to allow FF,
already exploited as much as we could in 2011.

Evian 2011
18