Learning Objective: This module introduces frequency tuning of superconducting accelerating cavities.

Skills:

  • Understand why frequency tuning is necessary
  • Learn about the general procedure for frequency tuning 
  • Know the devices used for frequency tuning including their design and limitations
  • Understand the procedure for how to take a cavity off the beam

Introduction:

SRF accelerating cavities are able to store energy for accelerating beam when the resonant frequency of the cavity matches the drive frequency of the power source (dedicated SSA in the case of LCLS SC Linac). The resonant frequency of a cavity may be varied by changing the geometry of the cavity. As the SRF cavities change temperature, the geometry will also change due to thermal expansion and the cavity frequency will become detuned from the frequency of the RF source. Tuners are used to compress the cavity to bring the frequency back on resonance as well as to maintain the resonant frequency during operation. Standard operating frequency is 1.3 GHz (3.9 GHz for cavities in the harmonic linearizer cryomodules HL1 and HL2).

Automated cavity tuning is implemented via the LLRF system. 

Hardware

There are two types of tuners in each cavity, steppers and piezos. Both push on the tuner ring (pink in the figures above) which then pushes on the cavity to shorten the cavity length. Shortening the length changes the cavity geometry, which changes the resonant frequency. 

Stepper Motors

The stepper motors are the coarse tuners we use to get within 50 Hz of the desired frequency. They are manual operation only because they have the potential to permanently damage the cavity if they push too far. 

Piezo Tuners

The piezos are the fine grain tuners that function as a frequency feedback. They can engage when we're within 50 Hz of the desired frequency and are designed to keep us at the desired frequency (being off frequency means an inability to accelerate beam). 

Controls

There are quite a few places to find a given cavity's tuner controls, but the ones most likely to be useful are the summary in the Overview tab and the expert display in the Tuners tab.

Frequency Tuning

If the frequency is outside the capture range for the piezo, we need to tune it using the stepper motors. The typical procedure is to:

  1. Turn off the RF and set the piezo to manual
  2. Set the Drive Level to 15%
  3. Put the cavity in chirp mode
  4. Turn on the RF and verify stability
  5. Open a strip tool with the detune frequency
  6. Choose a number of steps to move the stepper (we've found that the calibration is roughly 1.4 Hz/256 steps)
  7. Check whether you're in a 1.3 GHz cryomodule or a 3.9 GHz one (H1 and H2 are 3.9, all the others are 1.3)
    1. 1.3 GHz: Click the right arrow to lower the frequency (or the left arrow to increase it)
    2. 3.9 GHz: Click the right arrow to increase the frequency (or the left arrow to lower it)
  8. Walk the detune as close to 0 as you can get it (within at least 50 Hz of 0)
  9. Set the piezo to feedback
  10. Reestablish previous RF settings

As an aside, the abort button works on this screen if you think that you'll overshoot before the motor stops moving. The motor will not accept a new move command until the motor has finished it's last one (as represented on the status bits)

Parking a Cavity

When a cavity is exhibiting serious performance issues, we will need to take it off the beam. In contrast to the normal conducting machine (where taking a klystron off the beam is accomplished by shifting the timing), we will take a cavity off the beam by shifting its frequency. Janice Nelson made a really great screen designed to walk you through the steps, but it's currently missing in the ether so I'm putting a screenshot here for now while we look for it.

Procedure

  1. Turn off the beam or stop it upstream.
  2. Note the change in frequency per step of stepper motor (dF/dStep)
  3. Calculate how many steps are needed to change the frequency by 1 kHz (called TotalSteps below)
  4. Turn on the cavity in chirp mode.
  5. Put the Piezo Tuner feedback to manual if in Feedback mode
  6. Disable the Piezo Tuner voltage if enabled.
  7. Find the detune frequency readout value.
  8. Reset the stepper motor step count (signed)
  9. Set the stepper motor step size to 10000 steps. Press the left-pointing arrow/triangle if dF/dStep is positive, right-pointing if dF/dStep is negative.
  10. Watch the frequency. Is it going negative? If so, send TotalSteps-10000 more steps in the same direction. If not, send TotalSteps+10000 steps in the other direction.
  11. When the stepper is done moving ("Motor Done" bit will light), is the frequency approximately -1 kHz?
    1. If it's less than -200 Hz (i.e. closer to -infinity), then go to the next step.
    2. If it's greater than -200 Hz, that's a problem. You could try sending more steps or call the area physicist.
  12. Check the box to denote that the cavity is now parked.
  13. Tell the abstraction lyaer that this cavity is now part of the None of the above group.
  14. Push the step count to the nsteps_park PV.
  15. Turn the cavity off
  16. Turn off the SSA
  17. Make an entry in the physics log that you parked this cavity, why, how many steps you sent, and the new frequency.
  18. Run HLO.
  19. LEM.
  20. Restore beam.
  21. Hope whatever problem you were having is now gone with the cavity parked.
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