Collimator Loss Map, with or without dp offset

There are now two ways of doing loss maps:

  • Using the Loss Map Control Application (blowup of a bunch by the ADT)
  • Traditional loss map (by moving the tunes or the RF frequency)

For  both methods, the masks, setup etc. have to be set as per the instructions for the Traditional method.

Loss maps using the ADT

After the ADT upgrade during the LS1 the cleaning and excitation function is completely decoupled from the transverse feedback. Four loops are implemented:

  • Loop A is the main transverse feedback loop for all bunches
  • Loop B is a reduced gain loop for witness bunches
  • Loop C is dedicated to injection and abort gap cleaning
  • Loop D is dedicated to noise excitation (blow-up) and ADT-AC dipole excitation

State of each loop can be seen on the Transverse damper control application. Grey colour means the feature is not enabled, orange the feature is enabled but not running, green the feature is enabled and active. 

The purpose of the blow-up is to excite a transverse movement. This movement is however seen by the ADT, and if not set-up correctly, the blowup excitation can be counter-acted (damped) by the feedback loop causing not sufficient losses produced. In order to make an efficient excitation for good loss-maps the following things need to be checked:

  • Pilot intensity bunches are typically not seen by the damper (when the machine is run in nominal intensity mode), therefore the excitation will not be stabilised by the feedback. Less strong excitation will produce large losses.
  • Response of the ADT blow-up channel had been optimized to produce sufficiently strong excitation in a presence of transverse feedback. This is standard mode of operation, which should normally be in place. The response can also be set to blow-up only a single bunch within a train. This is an on-demand expert mode of operation. 
  • In case the produced loss rate is not sufficient, the transverse feedback gain has to be reconsidered. If the beam is stable without feedback, the "stop dampers" timing event can be sent (from the Transverse damper control app). In case the feedback is necessary to keep the beam stable, one should consider lowering the LoopA normalised gain ("NORM_GAIN_MAIN" knob in LSA).
  • The excitation strength is a product of the "Exc. Amplitude" from the Loss map control application and the "NORM_GAIN_BLOWUP" gain in LSA. 
  • In order to produce sufficient excitation at 450 GeV the Exc. amplitude should be in a range 0.05 and 0.3 and the NORM_GAIN_BLOWUP in range 0.1 to 0.25. This translates into a LoopD gain, displayed on the application between -15 dB and 0 dB.
  • In order to produce sufficient excitation at 6.5 TeV the Excitation amplitude should be in a range 0.05 and 0.3 and the NORM_GAIN_BLOWUP in range 0.05 - 0.1. This translates into a LoopD gain, displayed on the application between -15 dB and 0 dB.

In case only a very low excitation, but of a correct bunch is observed the blow up channel response can be set to the precision single bunch mode. Please call experts to have it corrected.

In case all of the above was checked, and no excitation at all is visible, or even losses are visible on different bunch than the one being excited, please call the experts.

 

The "Loss map control" application allows to select the bunch to be blown-up, the strength and the excitation length. Recommended configuration is as per the picture below, where the strength is between 0.05 and 0.3. To configure the settings:

  • Select the right beam process (injection/flat-top)
  • select a bunch, and give an Exc Start = bunch number - 25 (bunch number = RF bucket number/10)
  • select Exc strength (normally ~0.2)
  • select Excitation mode = MULTI-PULSE
  • select Exec length = 11245
  • select Repetition= 10
  • Click on SEND APPLICATION VALUES TO LSA AND HW
  • Now to start the blowup just click on  START BLOW-UP. An RBAC role RF-ADT-BLOWUP is necessary. The role administrators are D. Valuch and S. Redaelli.

 

Notes on the excitation length:

The blowup sequence length is programmed in number of turns by parameter "Exc Length" (11245 turns equal to 1 second). Minimum sequence length is 1 turn, maximum 99 999 turns. 

The sequence can be executed once (SINGLE_PULSE), multiple times (MULTI_PULSE), or run continuously (CONTINUOUS). The repetition mode is selected by "Excitation mode" pull down menu. In case the multi_pulse is selected, the repetition number is controlled by "Repetition" parameter. Minimum repetition is 0, maximum 255.

Typical settings for collimator loss maps would be Mode: Multi_pulse, Exc Length: 11245, Repetition: 5..10. Start by pressing the button "Start blow-up", the excitation will stop automatically after the number of repetitions is reached or by "Stop blow-up" button.

Typical settings for a very slow, long spill would be Mode: Continuous, Exc Length: 11245, Repetition: not important. Start by pressing the button "Start blow-up", the excitation will be stopped by "Stop blow-up" button.

Loss Maps: Traditional Method

Injection loss maps:

Collimator loss map with 1-3 nominal bunches, one beam at the time. No need for relaxed SBFs (which would not change anything).

  • For Qv loss map, move Qv across 1/3.
  • For Qh loss map, first cross the tunes to move Qh > Qv, then move Qh across 1/3.
  • For dp/p loss map trim RF frequency by 500 to 1000 Hz.

Steps to take:

  • Mask : RF, Beam excursion, BLMs. In SIS there is nothing to mask: when SBF is true the orbit/TCDQ interlocks are now automatically deactivated.
  • Open BLM displays and be ready to capture a map, activate auto-save. The auto save directory is /user/slops/data/LHC_DATA/OP_DATA/LHCCollimators/BLMLHC/RawData/
  • Don't forget to move injection protection collimators OUT !
  • Don't forget to do the BPM calibration for single bunches.
  • For betatron loss maps: Only do loss maps in one plane per beam per fill, as there is some coupling between the planes
  • For the betatron loss maps: If doing the horizontal plane, swap horizontal and vertical tunes before moving the horizontal tune over the 3rd order ressonnace.
  • Once finished, re-do the BPM calibration.

Flat top loss maps:

The setup is done with 2 nominal bunches per beam in order to be able to find the collisions (for stable beams). The steps to be followed are:

  • Ramp 2x2 8-9E10, and follow the standard sequence until collisions.
  • Unforce the SBFs in the SMP application (LHC tab).
  • Relax the SBF threshold. Note for this you need  the RBAC role of SMP-THRESHOLD-EXPERT. Jorg, Markus Z and Rudiger are the only ones who can assign this role
  • Mask : RF, Beam excursion, BLMs. In SIS there is nothing to mask: when SBF is true the orbit/TCDQ interlocks are now automatically deactivated.
  • Open BLM displays and be ready to capture a map, activate auto-save. The auto save directory is /user/slops/data/LHC_DATA/OP_DATA/LHCCollimators/BLMLHC/RawData/

For betatron loss maps:

  • Move Qv across 1/3. Record losses.
  • Move Qh across 1/3. Record losses.

For off momentum loss maps:

  • Trim freq_RF by 500-1000 Hz. For dp/p < 0 trim frequency up, for dp/p > 0 trim frequency down.

You are here