Title

ID

Setup longitudinal intra bunch-train feedback (L-IBFB) at LLRF station A5

CP1-DP-022-OP

Author

Responsible

Release

date

Version

Date: 

Version: 3

Status

DRAFT


Scope and goals

This procedure should be used to setup the longitudinal intra bunch-train feedback (L-IBFB) for different beam regions. The L-IBFB is used to stabilize the arrival time along the bunch train.

Application

  1. If an experiment needs a stabilized arrival time, the feedback should be enabled as an add on at the end of the machine/SASE tuning process.

Roles and responsibilities 

RoleResponsibility
OperatorEntire operation of the accelerator from the BKR




Date of next review
Status should be

DRAFT  or  RELEASED  or  WITHDRAWN

Only 'Released' documents can be published on the XFEL Operations page. 

Tags

After finalizing the procedure please use the to add some keywords. It will help others to find the procedure more easily.


Procedure to setup longitudinal intra bunch-train feedback (L-IBFB) at LLRF station A5.L2

1 Preparation: Switch off the L-IBFB whenever

  1. The feedback is not needed annymore.

  2. Changing parameters upstream of the LLRF station where the L-IBFB is activated.

  3. An issue appears on the LLRF station where the L-IBFB is activated.

  4. Unexplained instabilities appear in SASE intensity/pointing. One can switch the L-IBFB on again if this is not the source of instabilities.

  5. Severe beam cuts appear! (This exception is not yet caught by the controller.)

(warning) Note: whenever the feedback has to be switched off or a set point adjustment is needed document this in the logbook and inform the experiment(s)!



  1. Check if you are at the right LLRF station A5.
  2. Choose the right beam region
  3. Switch off the L-IBFB




(warning) Note: check the arrival time setpoint before switching the feedback on again! (For details go to step 7.)

2 Start: Open the L-IBFB panel


Open the L-IBFB panel: Feedbacks → L-IBFB Panel


  1. Open the L-IBFB panel

3 Check the beam region settings


  1. Check SASE bunches and the corresponding beam region
    1. beam region 1: Usually SASE2 bunches
    2. beam region 2: Usually SASE1/3 bunches

  1. Check the beam regions settings via the bunch pattern builder (which SASE beamline belongs to which beam region)

(warning) Note: the beam region of interest is the beam region where the L-IBFB will be switched on.

4 Check that the beam region of interest is not linked

  1. Open the sum voltage control panel and select the L2 section
  2. Check that the beam region of interest is not linked otherwise uncheck the link.



  1. Check on status display or the sum voltage control panel for the L2 section that the beam region of interest is not linked to another beam region.

5 Switch off the BC2 energy and BAM feedback for the beam region of interest


  1. Use the status displays for the BC2 energy and BAM feedack.
  2. If necessary switch them off e.g. via the history plot panels or the SASE tuning panel



  1. Check if the BC2 energy and BAM feedback is switched off for the beam regions, where the L-IBFB will be switched on.

  2. Switch the BCM feedback on for the beam region of interest. 

6 Check the bunch arrival time monitor (BAM) data and performe an operation point reset


  1. Check that all arrival time feedbacks are off.
  2. Press the "Operation point reset" button.
  3. Check if Data valid flags are green and that no error info is shown. Otherwise make a logbook entry and send this to the SDiag mailing list.


  1. Use the BAM for operators panel. (Diagnostics → BAM for operators)

  2. If the tuning process is finnished and stable SASE operation is established press the "Operation point reset" button.
    (warning) NOTE: All the BAM feedbacks has to be switched off for that optimization (slow longitudinal FB and the L-IBFB)!

  3. After the operation point reset check if the BAMs are working properly. Especially the one of iterest. For the feedback loop at A5 the BAM 414.B2 is used.

(warning) Note: In case of failures do not switch on the L-IBFB and inform expert: SDiag mailing list (sdiag@desy.de).

7 Setup and switch the L-IBFB on for the LLRF station A5


  1. The colors of the plot mark the different beam regions. Blue = beam region 1, Red = beam region 2 and Green = beam region 3.
  2. The L-IBFB modulates the gradient of the corresponding LLRF station A5. The modulation of the amplitude should be near the field set point shown in green. If there is a huge difference between the set point and the read back value switch off the L-IBFB and check the arrival time set point again.


  1. Check if you are still at the right LLRF location A5.

  2. Choose the right beam region.

  3. Enter the arrival time set point for the chosen beam region. (a) uses the measured arrival time of the corresponding beam region or (b) use the MATLAB tool "Set all SP".
    (warning) Note: whenever the L-IBFB arrival time set point needs an adjustment switch off the slow longitudinal BAM feedback. After the adjustment set a new target for the slow longitudinal BAM feedback as well.

  4. Switch on the L-IBFB

8 Set a new target and switch the BC2 BAM feedback on for the beam region of interest


  1. E.g. open the BAM history of the corresponding flat top.
  2. Set a new target and switch on the BAM feedback of the BC2 section.

As an example the history plot of the first beam region.


  1. Set a new target and switch on the longitudinal BAM feedback for the beam region where the L-IBFB is activated. Make sure that the beam region is not linked.

  2. If the beam region link was unchecked for the first time do not forgett to switch on the BC2 compression feedback as well.  

Note: the longitudinal energy feedback has to be switched off for the corresponding section and beam region where the L-IBFB is enabled! It is not possible to run the energy feedback togehter with the BAM feedback.

9 Documentation

  1. Please, print the L-IBFB panel into the logbook whenever the feedback is switched on or an adjusment was necessary.

  2. In case of problems switch off the L-IBFB, add a description and send this to the Feedbacks mailinglist (l-ibfb@desy.de).

  3. Do not forgett to inform the experiment(s) if an adjustment of the arrival time set point was necessary or if L-IBFB is switched off due to problems.

Exception handling

  1. LLRF station A5 shows a huge difference between the readback value and the setpoint and maybe the limiter is active. There are two posibilities to solve the problem. Please inform the experiments before you start with the adjustments.
    1. First option:
      Switch off the L-IBFB, wait until everything is settled again (mainly the slow longitudinal feedbacks as well as the leraning feedforward of the LLRF station.),
      Proceed with step 5.
      (warning) Note: this will change the arrival time of the electron bunches.

    2. Second option:
      Switch off the slow longitudinal BC2 BAM feedback,
      Let the L-IBFB running and adjust the arrival time setpoint of the L-IBFB carefully until you reach the current value (see picture on the right hand side), 
      Change the sum voltage of the L2 section until the amplitude setpoint (green line) is in good agreement with The readback value (black line)
      Take a new setpoint for the slow BC2 BAM feedback and activate the feedback again.
      (warning) Note: this will not change the arrival time of the electron bunches.


Description of the MATLAB tools



Explanation of the MATLAB tools - comming soon 


Flow chart L-IBFB setup





























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