1. 12 GeV CEBAF Performance summary and plan to achieve robust reliable operations at the design energy
Outline:
Recent CEBAF performance
JLAB-TN-022
Energy Reach Plan
Near Term
Long Term
Update on gradient degradation
Arne Freyberger
Monday, April 15, 2019

2. Recent CEBAF Performance: Summer SAD and 2018-Aug/Sep troubles
During Summer2018 shutdown two modules, P1 & F100 moved from LERF to CEBAF
Estimated gain to CEBAF: +40 MeV
Desired to increase gradient headroom and lower RF trips at 1050 MeV/linac
Result:
Frozen N2 gas in neighboring module, had to terminate restoration efforts and thermal cycle affected cryomodules.
P1 and F100 have not performed as expected in CEBAF
Net loss of ~10-30 of MeV
<RF trip> barely below 10 trips/h
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3. Recent CBAF performance: Q1FY19
Post Sept. troubles:
Oct/Nov operations at 1050 MeV/linac trippy but productive
Decemeber operations at 930 and 805 MeV/linac demonstrate the wisdom of operating with 100 MeV/linac (or more) of gradient headroom.
Trip rate dropped as expected
Ability to by-pass cavities and even complete cryomodules reduced (eliminated) the need to interrupt the program to fix RF systems.
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4. Q1FY19 DOE Performance Metric: Accelerator Reliability
First quarter in 12 GeV era with reliability > 80 %
930 MeV/linac (first half of December) reliability impacted (in a negative way) by the low current operations to B & D which led to many MPS trips.
805 MeV/linac, last eight days of the run, had reliability > 98%
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5. CEBAF Energy Reach Plan
Goal: Operate CEBAF at design energy, 1090 MeV/linac with 100 MeV of margin in each linac.
100 MeV of margin allows for an entire C100 to be by-passed
Emergent problematic cavities and modules can be by-passed without impact to the program (except higher trip rate).
In order to achieve this goal, there are two issues:
CEBAF energy reach is presently below the 1090 MeV/linac design value (1050 MeV/linac).
CEBAF energy reach degrades at a rate of 34 MeV/pass/year (JLAB-TN-022) or 48 MeV/pass/year (preliminary analysis of operations).
We have to catch up, we have to continue to add gradient, and we have to arrest the degradation.
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6. CEBAF Performance Plan (CPP)
(JLAB-TN )
Long term strategy to improve and maintain CEBAF performance through:
Identify and purchase Critical Spares to mitigate the impact of single point failures.
Replenish consumed hardware spares (I.e. Klystrons)
Increase Energy Reach to support the design energy with robust energy margin:
C75 Refurbish 8 original C20 modules, includes new cavities and digital controls.
C100 Develop and execute C100 refurbishment plan.
Particulate Control Clean warm girder regions and upgrade vacuum systems.
Upgrade original CEBAF systems to mitigate obsolescence issues in a timely manner (I.e. before reliability degrades)
Procure equipment to minimize future maintenance duration (to support up to 34 weeks-per-year of CEBAF operation). 34

7. CPP: Energy Reach
Saw tooth lines represent the energy setting that results in ~ 10 RF trips/h
Over 20 years of operational experience with CW SRF operations
Analysis of SRF performance over this period has found:
~17 MeV/linac/year of gradient degradation
Degradation is not correlated with operating weeks
Thermal cycles tend to increase the amount of degradation
Recent SRF R&D has found a large amount of particulate on cavities surfaces that have been removed from CEBAF/LERF.
Present energy is 4% below design
CPP includes improved in-tunnel clean rooms, SRF processing of warm girder sections and NEG pumps
In-tunnel vacuum procedure of SRF system has been improved with new mobile clean rooms
When degradation is arrested, the Energy Reach plan will be adjusted.

8. Cryomodule C75-01 Cavity Delays from Vendor
Contract awarded in Feb 2018 to vendor based on capability to meet technical requirements at best value
After award, original delivery date was agreed to be Aug. 08, 2018 for all (8) cavities
In the monthly status meetings, the vendor continually reported delays in delivering
In June, delivery delayed to Oct 2018, then delayed to Nov 12, 2018 in August meeting, then to Nov 20, 2018 in Sept meeting
Letter of Concern issued in early Sept 2018
Delays were due to fabrication issues encountered during deep drawing and welding of stiffening rings
Team lost confidence in vendor’s delivery dates, ability to deliver C75-01 in Summer of 2019 was at risk
Decision made in late Sept 2018 to refurbish C for installation in Summer 2019
Work began in October 2018 to move C to the Test Lab for disassembly 
Latest delivery date for C75 cavities is March 08, 2019

9. Status and Plans for C100-06R Cryomodule Refurbishment
C100-06R Status
The cryomodule was removed from the linac as part of the summer down and set aside to “cool down”
Disassembly of the cryomodule was completed in November
Several cavities have been reprocessed and tested
Some cavities have required more than one round of cleaning and testing to eliminate field emission
4 our of 8 cavities meet the requirements and are ready for cavity string assembly
C100-06R Plans
Complete remainder of cavity qualification by Feb 1, 2019
Build cavity string in February 2019
Complete cryomodule assembly by end of July 2019
Complete cryomodule acceptance test by Labor Day
Install and commission in September 2019
The design gradient (108 MeV/module) is 19.3 MV/m.
Cavity performance post high pressure rinse is well above the operational limit.
All this good work must be retained in the assembly and installation of the module

10. Update on Cavity Operational Gradient as a function of time
Cavity operational gradients change with time
Field emitters come (mostly) and go (sometimes)
Frozen gases come (unfortunately) and go with thermal cycles
Analysis of the CEBAF cavity performance during is on-going
Results presented here are preliminary
CEBAF energy was constant during this period
Dataset corresponds to 315 days of CEBAF MeV/linac
No Helium Processing occurred during this period
Sundry unplanned warm-ups did occur (typical)
Overall loss of gradient margin has been observed (higher trip rate) while the delivered beam energy has remained constant
C20 gradients increased (trip rate increases) to accommodate lower gradients in C50 and C100 cavities.
Changes due to issues external to the cavity are not included in this analysis
Control system, klystron issues, frozen tuners…

11. Nov-2015 -> Nov-2018 Gradient Slope (Preliminary)
C20 cavity analysis uses the C20 cavity fault models (trip/rate as a function of gradient) to determine the cavity gradients for a ensemble C20 trip rate of 8 trips/h.
Five model updates in this period
Not all C20 have good models, those cavities slopes are set to zero.
Similar to the JLAB-TN analysis
C50/C100 cavity analysis uses operational gradients during this two-year period
CW beam must be present in the machine
2019 (preliminary)
(MV/m)/year
C20
-0.25
C50
-0.13
C100
-0.24

12. Update on Cavity Operational Gradient as a function of time
CEBAF Performance-Energy Reach Plan uses the 2012 gradient slopes found in JLAB-TN
These values result in ~-34 MeV/pass per year of lost energy
No loss in the C100 is presently accounted for in the Energy Reach Plan
2019 Analysis is a significant increase in data on the C50s and C100s cavities.
Stable CEBAF conditions in terms of operating energy allow for a more systematic analysis of C50 performance.
2012 analysis (JLAB-TN )
2019 (preliminary)
(MV/m)/year
C20
-0.14 ± 0.05
-0.25 ± 0.04
C50
-0.70 ± 0.08
-0.13 ± 0.10
C100 NA
-0.24 ± 0.18
2019 preliminary analysis results in ~-47±12 MeV/pass per year
Consistent with the observation of the loss of gradient margin (~120 MeV margin in Fall2016) to near zero margin at present (at linac energy =1050 MeV/linac).
C100 loss rate is consistent with observations, although there is large difference in the loss between the North (bad) and South (not so bad) C100s.
Gradient Slope
Number of Cavities/pass
Cavity Length
<DE/pass>
((MV/m)/year)
(m)
(MeV/year)
C20
-0.25 ± 0.04
224
0.5
-28 ± 4.5
C50
-0.13 ±0.10 96
-6.2 ± 4.6
C100
-0.24 ± 0.18 80
0.7
-13.3 ± 10.0
Preliminary
Energy reach plan will use 34 MeV/pass (2012 value) until 2019 analysis is complete

13. Energy Reach Chart
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14. Summary
Recent low energy operation (Dec. 2018) has validated the CPP goal for 100 MeV of margin in each linac.
C100 refurbishment consisting of a ultra pure high pressure rinse has yielded field emission free cavities beyond the operational limit.
First C100 refurbished module, C100-6-R is scheduled for installation in 2019-Sept, will participate in the 2019-Fall run.
Analysis of cavity performance during operations confirms cavity degradation
Preliminary value of energy loss of 47 ± 12 MeV/pass/year.
The energy reach plan (JLAB-TN ) used the JLAB-TN value of 34 MeV/pass/year
C75 cavities are delayed and should arrive and be qualified by the end of the year.
We continue to adjust the CPP program to optimize against NP schedule, funding, and lab resources.
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15. Thank You
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16. Gradient Slope as a function of CEBAF Zone
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17. Gradient slope by cavity type
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