The 1.3 GHz CW RF is produced by the low level RF (LLRF) system then amplified by a solid state amplifier (SSA). Each cavity has its own SSA. The RF generated by the SSA is directed into the tunnel through rectangular waveguides and fed into the downstream end of the cavity through a fundamental power coupler (FPC) .
Mode of Operation
The SRF cavities in use at the LCLS SC-Linac have several different modes of operation. These modes are briefly described here to provide context for their mention in the rest of the article.
- Chirp - Quickly sweeps through a range of frequencies to find the resonant frequency of the cavity (primarily used for cavity turn-on.)
- Pulsed SEL - RF tracking cavity frequency with constant (2kW) SSA output and short duty cycle.
- SEL - RF tracking cavity frequency with constant (2kW) SSA output power.
- SELA - RF tracking cavity frequency; SSA output power adjusted to maintain constant cavity field.
- SELAP - Beam acceleration mode; RF drive is exactly at 1.3 GHz; Phase feedback loop keeps cavities in tandem; SSA output power fluctuates to keep cavity gradient constant.
SSA Turn-on and Calibration
You can access SSA controls for each cavity from the main cavity panel: LCLSHome → RF/LxB → Cavity Control → More...
The PV naming convention for SRF cavities starts with ACCL and includes the region, cryomodule and cavity number. For instance, the PV for the second cavity in the first L0B cryomodule is ACCL:L0B:0120 where the last four digits indicate the cryomodule (01) and the cavity (20).
Everytime an SSA is turned on, it will need to be calibrated in order to map the LLRF input to the output. You can run the calibration using the Chracterization → SSA tab of the panel above.
Follow the commissioning procedure or instructions from system experts to run the calibration and update the ratio of output to input values. If the calibration crashes, the "Plot.." button can help identify if there is a problem such as non-linear drive.
Frequency Tuning
After the SSA is calibrated, the frequency must be tuned to 1.3 GHz using the stepper motors with RF in Chirp mode. After the cavity is tuned, the piezo feedback deals with longterm drift to keep oscillations around 1.3 GHz.
Cavity Diagnostic Calibration
Once the cavity has been tuned to its nominal frequency, it is ready to be commissioned. A pulsed SEL calibration will give the cavity scale factor, which is the relationship between the signal on the RF pickup to the actual field in the cavity. The ratio between forward and reflected RF signals allows us to calibrate the RF pickup signal for when we are running in CW.
Loaded Q described cavity performance as part of the RF system, as opposed to the quality factor of an isolated cavity. In order to speed up beam loading, we slightly spoil cavity Q with FPC. The design loaded Q for LCLS = 4.1E07.