Report Setting of RATN in 1L26Tom Powers, Anna Solopova, and Clyde Mounts

Data taken on 16 Oct. 2018

On 16 Oct. 2018 a collaborative effort was made to take klystron saturation data on zone 1L26 with a goal of better understanding the process; increasing the available klystron power and well as the limitations that were encountered during the process. RATN is a step attenuator which was added within the C100 field control chassis (FCC) between the output of the Receiver/transmitter card and the front panel of the chassis. The purpose of the device is to limit the RF power to the klystron preamplifer as well as to insure maximum use of the dynamic range of the FCC digital/analog transmitter channel. Historically the approach was to ramp the output voltage (GASK), which is a 0 to 10 signal proportional to the RF voltage of the output RF chassis, between 0 and 3.5 while observing the output power (CRFP), klystron body current (KBCU) and cavity window temperature sensor (CWWT). Based on the simultaneous plots of CRFP and GASK value of RATN was applied such that the output power started to (LOOKING FOR WORD meaning saturate some but not fully) as RATN was increased in a linear fashion. Alternately CWWT and KBCU were used as limitations of the system and RATN was set such that the system would not fault on those interlocks. The process used during this test was to plot the forward power as indicated in EPICS as a function of GASK2. In an ideal situation this would be a linear function. In this case there are three sources of nonlinearity. The output of the FCC has a known compression of 0.5 to 1 dB when GASK is 10. The klystron preamplifier has about 0.5 dB of compression at about 100 mW output power and the klystron has a saturated RF output power of about 12 kW. This data was used to determine the GASK2 value where the indicated forward power was a maximum. The plots were made by downloading archived data and using Excel to plot the data. Once the corresponding GASK2 value was determined a value of RATN was calculated using the following formula:

RATN=FCCCoerection+10 log(GASK Sa turation2

100 )where FCCCorrection is compensation for the compression of the FCC added to the preamplifer output. A value of

(-1 dB was used for this variable) and GASK Saturation2 is the value of GASK2 where the CRFP no longer increased as

GASK was increased. Below is a table of the results and graphs of the before and after data plots are provided Following that.

Limitations in achieving 12 kW on all of the Systems

(A) Insufficient drive in order to determine the saturated power. The following figure shows what one would expect for a typical klystron saturation curve. It was from cavity 1L26-4. Once GASK2 reaches 5.5 the the forward power starts to decrease with increasing RF drive. This was the only channel that exhibited this behavior when RATN was set to 0 and the GASK was limited to 3.5 V.

Figure 1. Forward Power as a function of GASK2 for cavity 1L26-4.

(B) Cavitiy Waveguide Window Temperature fault (CWWT). This is the magnitude of a thermalpile detector (7 to 14 um “optical” sensor) after it has been amplified and digitized. For the C100 program it was, somewhat arbitrarily set to 4 based on past experience with C50 cavities which use a similar sensor. The C100 waveguide window assembly was designed to operate CW with 12 kW klystrons, full power in full reflection. Thus it seems that the setpoint that was chosen was overly conservative. A program has been initiated to determine the proper setpoint using the SRF window test stand which has thermal imaging cameras so that one can determine the relationship between actual window temperature and the thermalpile readback.

(C) Klystron Body Current Fault (KBCU). This is the leakage current from the klystron beam to the body of the klystron. The setpoint for this variable is based on manufactures recommendations and practical experience. There are klystron setup parameters, such as solenoid current, as well as other minor system modifictions, e.g. filtering or circulators between the preamplifier and the klystron input which have the potential to improve this. These will be investigated as time and resources permits.

Other Issues:

RATN was put into the system in order to better match the output of the FCC to the klystron drive signal without saturating the klystron drive. Adjustments to RATN directly impact the loop gain. In the past month values of RATN were adjusted about 10 times in the C100 systems. It was not clear why there someone adjusted them but, in the future, adjustment of RATN should only be done to (A) adjust the FCC maximum output in order to provide a maximum amount of RF power without having negative gain on the system (B) to limit the RF drive on systems that are limited by klystron body current until such time as the RF group can make adjustments to the klystron systems in order to reduce the body current. Further care must be taken to appropriately adjust the loop gains to compensate for changes in RATN. Also in the future it would be very useful to produce live plots of CRFP as a function of GASK2 when taking the data so that one can clearly see the saturation effects.

Cavity Previous Max CRFP (kW)

Max RF PowerAfter (kW)

RF past saturation

(Y/N)

RATN Before

RATNAfter

OverallLimitation

With RATN SetLimitation

1 9.5 11.9 CRFP12.4 CRRP N 9.0 7.5 CWWT CWWT

GASK=10

2 9.8 10.8 CRFP10.8 CRRP N 11.5 11 KBCU KBCU

GASK=9

3 10.8 10.5 CRFP10 CRRP N 13.5 8.5 CWWT CWWT

GASK=8.5

4 10.8 11.2 CRFP9.1 CRRP Y 10 12.5 CWWT CWWT

GASK=10

5 11.8 12.9 CRFP10.4 CRRP Y 10 10 CWWT CWWT

GASK=10

6 11.1 11.8 CRFP7.0 CRRP N 10 8.5 GASK=10 GASK=10

7 12.99 CRFP9.6 CRRP N 8 10.5 CWWT GASK = 8

8 10.1 CRFP8.4 CRRP N 8.5 12 KBCU GASK = 8

KBCUTable 1. Summary of results for setting up RATN for zone 1L26 including the limitations on a cavity by cavity basis.

Figure 2: Cavity 1, EPICS time domain plots (upper), CRFP as a function of GASK2 with RATN set to 0 (middle left), CRFP as a function of GASK2 with RATN at its final value (middle right) and CRFP as a function of GASK2 as extracted from waveform harvester data.

Figure 3: Cavity 2, EPICS time domain plots (upper), CRFP as a function of GASK2 with RATN set to 0 (middle left), CRFP as a function of GASK2 with RATN at its final value (middle right) and CRFP as a function of GASK2 as extracted from waveform harvester data.

Figure 3: Cavity 2, EPICS time domain plots (upper), CRFP as a function of GASK2 with RATN set to 0 (middle left), CRFP as a function of GASK2 with RATN at its final value (middle right) and CRFP as a function of GASK2 as extracted from waveform harvester data.

Figure 5: Cavity 4, EPICS time domain plots (upper), CRFP as a function of GASK2 with RATN set to 0 (lower left), and CRFP as a function of GASK2 with RATN at its final value (lower right)

Figure 6: Cavity 5, EPICS time domain plots (upper), CRFP as a function of GASK2 with RATN set to 0 (middle left), CRFP as a function of GASK2 with RATN at its final value (middle right) and CRFP as a function of GASK2 as extracted from waveform harvester data.

Figure 7: Cavity 6, EPICS time domain plots (upper), CRFP as a function of GASK2 with RATN set to 0 (middle left), CRFP as a function of GASK2 with RATN at its final value (middle right) and CRFP as a function of GASK2 as extracted from waveform harvester data.

Figure 8: Cavity 7, EPICS time domain plots (upper), CRFP as a function of GASK2 with RATN set to 0 (lower left), and CRFP as a function of GASK2 with RATN at its final value (lower right).

Figure 9: Cavity 8, EPICS time domain plots (upper), CRFP as a function of GASK2 with RATN set to 0 (middle left), CRFP as a function of GASK2 with RATN at its final value (middle right) and CRFP as a function of GASK2 as extracted from waveform harvester data.