1. C100 Operational Performance
Curt Hovater
Discussing input from many

2. Outline C100 Cryomodule Performance Operational Experience Summary
11/9/2018

3. C100 Performance Limitations
Faults
Definition
Outcome/Issues/Comments
Weekly time lost
Cryogenic Cavity Detuning
He pressure and or cavity boiling causes cavity to detune and fault
Trip and recovery time, availability
TBD
Microphonic Detuning
A sudden increase in microphonics (thunder, Cat D7) causes cavity to detune and fault
Anomalous Quench
Real quench that causes the cavity to trip.
Not well known. Is there a cavity damage issue
 TBD
Unknown
Unknown trip that doesn’t show up on the once and future fault logger
Trip and recovery time, availability. Needs fault logger!!
Vacuum Faults
Vacuum faults caused by field emission
Cryomodule recovery
Gradient Performance
 Outcome/Issues/Comments
Energy Loss
Field Emission/Radiation
Gradient reduced to decrease the radiation loading
Reduced gradient, vacuum faults, radiation damage to cables and nearby components,
23.1 MeV
RF Bath Heating/cryo issues
Gradient reduced because of boiling and or cryo issues
Reduced gradient, He pressure and level not stable
TBD MV
Gradient reduced to keep cavity with in the klystron power when detuned.
Most noticed on pre-tuner plate modification and specifically on cavities 4 and 5.
Quench/Field Emitter/Boiling/Arc/Window
Original SRF commissioning limitation.
Inflated gradient performance. Cavities commissioned at max gradient in SEL have to be recalibrated in GDR. Most cavities were beyond 19 MV/m so small effect.

4. C100 Performance Limitations Cont.
Recovery
Definition
Outcome/Issues/Comments
Recovery Time*
Cavity Turn On
Speed to recover a single cavity
Availability
27.6 seconds
Cryomodule Turn On
Speed to recover a cryomodule
55 sec
(Both gradient dependent due to ramping)
*Best time …actual recovery times maybe slower …..

5. C100 Operational Performance with Beam
98 MeV/C100 NL SL
this plot that shows the C100 performance since 2012, The purple line represents 98 MeV per C100.   The small dots are the energy gain for an 8h period with CW beam delivery.   The large points are the operational point for the three periods chosen.   The plot shows several things: We tried to push the C100s hard between 2014 and 2016, especially Fall2015 and Spring2016.
The push to high gradient is followed by a retreat, unfortunately.
The South linac shows a modest improvement, probably due to all the C100s being Helium processed.  North Linac only NL22 was HeProc.

6. C100 Cryomodule Microphonic Issues
Tuner mechanism (*stepper and piezo)
acts as an acoustic antenna for microphonic ground motion
Waveguides and other mechanical couplings channel
acoustic noise (thunderstorms, cooling tower fans etc.)
onto the cavities
Cavities detune beyond a controllable point (RF power)
and the RF system faults

7. C100 Susceptibility to Construction, Truck Traffic, LCW Cooling Fans, Lawnmowers, Thunder ...
C100 Cavity gradients
The drops show the cavity faulting during the day due to construction and truck traffic really any low frequency acoustic noise.
ESH&Q Building construction halted
Reason: RF Power could not compensate for the rapid detuning
C Cavity Gradients

8. Coupled Cavities
C100-4 Cavities 4, 6, 7, 8 responding to an applied PZT step voltage change from 4 to 3 volts in cavity 5
Adjacent Cavity
coupling is ~ 10%
between 1-4 and
5-8 cavities
Cavities 4 and 5
have a “quasi”
mechanical
support between
them.
Ringing is the
21 Hz mechanical
Mode
Cavity 5 PZT
moved 460 Hz
Cavity Gradients
10 MV/m
Locked in GDR Mode
Klystron had the
overhead to keep
cavities locked
Stepper Motor
kicks in to tune
the cavities

9. Cryomodule Heaters
Increased microphonics were observed when heaters were on. The cryostat riser became a choke point as additional heat was applied.
Out of the eight installed, only the even heaters (4) were initially used. When an odd cavity would turn off, additional heat would be supplied to the even cavities to compensate ..then
Presently we are testing an eight heater system on one of the C100s.
He Liquid Level in a cryomodule as
heat was applied.

10. Operational Experience - Field Emission
H&V nested Air Core correctors over BPM
Field emission heats beamline
Vacuum pump faults
Vacuum interlock drops cryomodule out of RF
Quad
Viewer & pump drop cross
End of Cryomodule
Cavity Gradients impacting Beamline Vacuum activity
BEAMLINE VACUUM
C Cav 6
C Cav 7
GMES
MV/m
Radiation Damage

11. NL C100 FE Vacuum Trips Feb/Mar 2016

12. C100 & He Processing
It was kind of a mixed bag ….

13. C100 & He Processing Next Steps
Proceed with caution in 1L23
Added thermal cycle upfront
Review procedures

14. RF Firmware/Software Focus
Firmware focus has been, since the first commissioned C100s, on mitigating C100 performance issues
Faults/Detuning
Identifying troubled cavities quickly and softening the blow to the rest of the cryomodule
Microphonic Detuning
Improved algorithm for absorbing periodic microphonic excursions
Cavities switch to SEL when detuning angle is > 60o
Recovery
Single Cavity
Whole Cryomodule

15. C100 Cavity Fault & Recovery Solution
C100s will never be fully immune from microphonic detunings
Algorithm
Identify cavity and switch to Self Excited Loop (SEL) at gradient
Pull FSD
Once microphonic has passed (driven by, stopped digging what ever…), automatically switch back to locked condition
Alert operator and resume beam operations.
Benefits
C100s automatically recover, operations does not have to assist the C100 RF system.
Potential for automated beam recovery too…. .

16. C100 Recovery Time vs Time
Courtesy Bob Legg

17. Piezo Tuner
JLAB cavity operating at ~ 15 MV/m with the PZT off and then on.
Substantial improvement for slow detuning ( helium pressure drift or slow microphonics)
Control bandwidths above 2 Hz cause oscillations
Operated 2L25 for two months with Piezos.
Piezo study has been a slow process
Engineer left who was leading the investigation
Competing projects have slowed this down
PZT Off PZT On
Slow tuner turns
On to re-center
The cavity
PZT bandwidth was limited to 2 Hz
Suggest to implement piezo tuners on “non stiffened” tuner C100s ….with possibility of full implementation

18. Summary C100 average energy at the end of the Winter/Spring Run
NL ~ 89 MV
SL ~ 87 MV
Above 90 MV a combination of issues make operating problematic
He bath and pressure issues contribute to microphonic detunings
Cryomodules susceptible to ground motion and acoustic pickup
FE vacuum trips bring down whole cryomodule
There is Hope
LLRF and more recently the Gradient Team have focused on many C100 Issues.
Recovery times have steadily improved from 2012 to now
RF Issues (HPA, Algorithmic etc.) have been fixed and are improving
SRF is taking a closer look at C100 microphonic Issues
New Heater implementation
See Bob Legg’s Gradient Team talk for more hope….
Questions
Can we get back to the 108 MV, 465 mA “high water” mark of 2012?
Do we need to put more resources onto the C100s to get to 108 MV?
The recent influx of resources (Gradient team) has improved C100 operations.

19. Clyde Mounts, Rick Nelson who contributed to this presentation
Special thanks to
Trent Allison, Rama Bachimanchi, Ed Daly, Mike Drury, George Lahti, Bob Legg,
Clyde Mounts, Rick Nelson
and Tomasz Plawski
who contributed to this presentation

20. Mechanical Tuner Modification
Design allows for 25 Hz Peak Detuning
Microphonic
Detuning*
C100-1
C100-4
RMS (Hz)
2.985
1.524
6s(Hz)
17.91
9.14
Actual peak detuning (21 Hz) was higher than expected in first cryomodules
A detailed vibration study was initiating which led to the following design change.
Cavity C
Cavity C
A minor change to the tuner pivot plate substantially improved the microphonics for the CEBAF C100 Cryomodules.
While both designs meet the overall system requirements the improved design results in a larger RF power margin

21. Cavity Fratricide
Cavity Fratricide occurs when one cavity faults (typically microphonic detuning from He instability or ground motion) and the Lorentz force detuning of the faulted cavity detunes the adjacent cavities resulting in the cavity faulting too.
Adjacent cavity has a 10% coupling to its brother/sister!
Adjacent cavity was operating at 5 MV/m so the klystron had the overhead to absorb the detuning

22. Cavity/Cryomodule Turn On
A digital self-excited loop (SEL) tracks the cavity frequency and quickly restores the cavity to its operational gradient.
A firmware application then tunes the cavity and switches to Generator Driven Resonator (GDR) mode, locking the cavity to the reference.
The procedure has evolved to the point that it is a “single button” automated turn on for the high gradient cryomodules. Each cavity is turned on in the SEL mode at some nominal gradient (18 MV/m for example) and then tuned to the reference frequency. Once on resonance the automated algorithm locks the cavity to the reference and then adjusts the gradient for the accelerator needs.
Transition from SEL to GDR