1. Overview of cavity reliability at Diamond Light Source
3 GeV 300 mA third generation light source
Operating for users since 2007
NC linac and full energy booster, SC storage ring RF

2. Storage Ring RF
Space for up to three CESR-B cavities in the RF straight
Usually operating with two cavities at any on time
Voltages 1.1 MV and 1.4 MV, powers 180 kW and 240 kW
Cavity pressure ~5x10-10 mbar with beam

3. Storage ring RF history
Diamond has four CESR-B cavities
Two in operation at any one time
Cavity failure is a major disruption
Mean Time Between Failures for RF system has improved
20 hours in 2007
200 hours in 2017
Cavity
Failure date
Detail A
none B
2009, 2014
UHV leak C
2006
Insulation vacuum leak D
2015
Window failure

4. Cavity “fast vacuum trips”
Sequence of events
Discharge in high field near coupling tongue
Discharge is fed by desorbed gas
Electrons are detected
Arc crowbars cavity
Reflected power trips amplifier
Pressure spike follows

5. Reliability and cavity voltage
Trips initially dominated by fast vacuum trips
MTBF is strongly dependent on cavity voltage
Each cavity has a “safe” operating voltage below which it is unconditionally stable
Trip rate is independent of power
Reliable operating voltages
Cavity A
1.1 MV
Cavity B
1.2 MV
Cavity C
1.4 MV
Cavity D
0.8 MV
Trips eliminated by reducing voltage
Effective
Why can’t we operate at higher voltage?
Not always free to drop voltage
Impact on lifetime
Special modes of operation, e.g.. Low α
SRLEm3ps: 1.7MV, 3.5ps, stable
SRLETHz: 1.7MV, 3.5ps, bursting
SR21: 1.7 MV, 16ps, stable

6. Cavity warm-ups
Cavities have been subjected to warm-ups to room temperature in the past
Occasional full warm-ups tried in early days and with problematic cavities
Gas is cleared from surfaces and identified by RGA
More frequent partial warm-ups have been used with all cavities
Every warm-up carries the risk of disaster!
Very little benefit at Diamond
Cavities are no better in run following warm-up
Cavities are no worse in run following no warm-up
No degradation of reliability if partial warm-ups are stopped for several runs
Short term improvement apparent following partial warm-up
No trips in first few days
Trips resume when vacuum returns to normal

7. Cavity conditioning Cavity “probe blips”
Pulse conditioning every week in run-time
2.3 MV peak voltage,10% duty cycle (10 ms/100 ms)
Detune angle scanned to sweep standing wave
Carried out when work is going on elsewhere in SR
X-ray emission reduced after conditioning
Two cavities simplify “with-beam” conditioning
Sweep cavity phases to move power between cavities
Beam is restored after violent conditioning events
Never tried an extended period without conditioning
Cavity “probe blips”
Probe blips
Appear as fall in cavity voltage
LLRF increases power and power level trip follows
Add filter to reduce bandwidth
220 kW
270 kW
Before
After
1µs
Pfwd
Cavity Probe
Prev
Blip recognition and handling to be addressed in new digital LLRF
Development of ALBA µTCA-based system
First DLLRF has been tested with NC cavity in booster and RF test bunker
Blip recognition not yet implemented

8. Recent cavity failures
2014: Leak from helium can into cavity UHV
Failed during cool-down from room temperature
No more warm-ups unless absolutely necessary
Indium seal at waveguide flange
Returned to RI in December 2014
Cavity returned to DLS in 2016
Failed acceptance test with leak at indium seal on FBT
Repair is to resume shortly
2015: Failure of ceramic-metal braze at window
During normal operation after standard conditioning
Repaired on-site at RAL in February 2016
Installed spare window assembly
Used RAL Space satellite assembly cleanroom
ISO class 5 cleanroom with 5 tonne crane
Tested to 2.1 MV operation in RF test facility

9. Normal conducting cavities for resilience
Cannot risk interruption to operations arising from a lengthy repair of a superconducting cavity
The first of two normal-conducting cavities has been installed in the storage ring
Installed outside the RF straight
Will allow operation with only one superconducting cavity in ring
Will support SC cavities, not replace them
Keeping the cavities permanently cold
Cavities are now only warmed up when absolutely necessary
Cryogenic plant is regularly serviced
We must warm up the cold box
Can we keep the cavities cold?
One cryostat of helium lasts longer than one day
Pass cold helium gas through cryostat until Dewar is depleted
How long will this give us?
How long does a service require?

10. Summary and conclusions
Cavity reliability has improved enormously since operation started in 2007
Cavity fast vacuum trips have been eliminated
Largely down to a reduction in operating voltage
Different cavities have different safe operational voltages
Fundamental studies are ongoing
Cavity warm-ups have been discontinued
A risky operation with no obvious benefit
Can we keep the cavities cold permanently?
Regular pulsed conditioning continues
Scheduled around weekly machine interventions
Cavity failure has long-term impact
Normal conducting cavities are being introduced to ensure machine continuity
The Diamond RF Group
Chris Christou
Shivaji Pande
Pengda Gu
Adam Rankin
Matt Maddock
David Spink
Peter Marten
Anton Tropp