1. Report of workshop of Operating SRF Systems reliably in a “Dirty” Machine @ HZB (Berlin)
Hiroshi Sakai (KEK)
23 pages
(Purpose of this workshop ）
A workshop to explore the challenges of running SRF systems in an accelerator environment that may not meet the stringent requirements for high voltage SRF operation.
The aim of the workshop is to gather together the expert community to compile experience with these operating conditions and develop recipes for the reliable operation of high-voltage SRF.
(For ILC)
As quick results of this workshop, Cavity performance during beam operation was mainly degradated by field emission which was produced by the small particle contamination.
For ILC, How do we keep the high gradient in beam operation ?
That is, what clean condition and care are needed ?
We learned from this workshop.
For example, it is crucially important for ILC to understand what clean condition is needed for not only assembly work of cavity and cryomodule but also the other vacuum components around cryomodule especially during beam operation.
LCWS2017 in Strasbourg, 2017.Oct.24

2. Program of this workshop
Introduction of E-XFEL clean work for assembly and vacuum .
Details about clean works
DESY
What kinds of problem about SRF operation were faced in the existing accelerators and how do we recover the problems.
Workshop was held on HZB on 14-15th September organized by J.Knobloch. Participants are 62 peoples, who interested in operation SRF cavities at the existing accelerator (HZB, DESY, HZDR, GSI, ESS, SOLEIL, JLAB, SLAC, CLS, DIAMOND, PLS, IHEP, TPS, IFMIF, KEK, RIKEN) all over the world.
CEBAF/KEK
CEBAF

3. (Lesson 1) Learn from long-term beam operation and how to overcome ?
SRF cavity performances for a long-beam operation are show as follows
How to overcome the performance degradation of SRF cavity in cryomodules.
Pulse processing
He processing
Plasma processing
Horizontal HPR

4. Upgrade magnets and power supplies
CEBAF introduction
CHL-2
Upgrade magnets and power supplies
CEBAF
C.Reece
Upgraded cavity
Cavity: 0.7 m, “Low loss” cell shape, GHz
8 cavities/cryomodule
Field emission is one of the main issue of gradient limiting and this was come from not only string assembly but also during beam operation
CEBAF field emission onset
(C100 cryomodule)

5. Particle sampling inside the cavity and vacuum chamber
In one cryomodule in CEBAF (FRANKLIN)
Example of dirty cavity inside during long operation and Degradation made radiation damage
Many particles come from assembly work and ceramic ?
Damaged by radiation produced by F.E
C.Reece
Cleaning essential not only cavity but also beam line.

6. He processing C.Reece One example total Procedure : Add He to 2K
Process by RF under He pressure around mTorr
For about 1 hour.
Warm up again to 40 K.
See the radiation onset before and after He processing.
Increase by He processing
1MV gain per cavity with 100 cavities by He processing. One solution is to recover the cavity performance !
But this mainly effectively work for lower cavity and sometimes degrade the cavity during processing .

7. Reference of CEBAF (see ref. in detail)

8. Experience of KEKB and recovery to Super KEKB Michiru Nishiwaki (KEK)
SC-Cavity
(HER)
509 MHz 8 Nb Single-cell Cavities, 4.4 K Operation
RF-Voltage : 1.5MV/cav, unloaded-Q >
Re-use of existing SRF system in SuperKEKB
Issues in SuperKEKB for SCC
Large HOM power is expected due to twice high beam current and shorter bunch length.
Degradation of RF performance of Qo for many years.
Degradation might be due to particle contamination during
repair of vacuum leak.
replacement of coupler gaskets to change Qext.
NC-cavity
(LER)
HER(e-): 7 GeV, 2.6A
LER(e+) : 4 GeV, 1.6A
Super KEKB
Highest luminosity machine
Under commissioning from 2016
Thank you for inviting me to talk about Horizontal HPR in SRF module of SuperKEKB.
Q0 at 1.5 MV > 1e+09 => Acceptable for SupeKEKB
Further degradation make the operation difficult.
Need to recover the cavity performance

9. Horizontal HPR applied to Degraded Cavity
New horizontal HPR with ultrapure water system was developed.
Input coupler and both end groups, including ferrite HOM damper, taper chamber, bellows chamber, ion pump, vacuum gauges and GV, are removed before HHPR in a clean booth.
Water in the cell is pumped up by aspiration system during rinsing.
Only cell and iris area are rinsed.
Horizontal HPR applied to Degraded Cavity
Rotating Water Jets
CA-B03
CA-B04
after Aging in Tunnel
Before
Leak
after HHPR
Results of HHPR
after HHPR
Degraded
by Vacuum Trouble
Degraded
at Repairing work
Before HHPR, input coupler and HOM dampers were dismount in a clean booth.
After that, HHPR system is set in SBP side.
Both cavity performances were successfully recovered in Super-KEKB
It suggests, the causes of FE are not only gas absorption but also dust particles on the surface.
This HHPR worked well because we could remove all particles inside the cavity. It is also effective way to recovery performance in principle. But is it applicable to ILC cavity (9-cell, long string assembly) ?

10. (Lesson 2) Learn from the experience of EURO-XFEL construction (Lutz@DESY)
(Lesson 2) Controlling Particulates and Dust in Vacuum Systems Lutz Lilje DESY,

11. Before we start: Introduction and definition of the Problem
Lutz
Dust is composed of particulates.
A particle accelerator accelerates particles: electrons, protons, ions
Particulates make field emission even though this was very small (~ 0.1 um).
When people aim for „particle-free“ vacuum systems, they mean a vacuum system with the lowest possible count of particulates
A truly particulate-free accelerator is difficult – if not impossible – to achieve.
Need to much reduce introducing the particulates to the cavity during assembly work, especially vacuum work.
From previous slides of long-term operation, dusts might come from outside of the dirty components during beam operation.
Keep clean not only inside the cavity but also other components near the cavity.
It is desirable to clean all vacuum components, especially for ILC accelerators.
Some information is available in the recent 2017 CERN Accelerator School on vacuum

12. Mass of Particulates They have mass In vacuum they fall down
By Original uploader was Sullivan.t.j at English Wikipedia. - The description as originally from Wikipedia., CC BY-SA 3.0,
Lutz
They have mass
6,2 mg per m2 per day …
In vacuum they fall down
A particle needs a few ten milliseconds to traverse a typical beam pipe (if only accelerated by gravity)
BUT: They have only a small mass
They will be transported in gaseous media at pressures above 1 mbar.
Opening an angle valve at 10 mbar while pumping a system

13. Avoiding Particulate Transport during Pumpdown and Vent
Lutz
Particle transport will be avoided by careful operation of the vacuum system
Avoiding vibrations is mandatory in general
When there is no vacuum, this is even more important
Use laminar gas flow while venting and pumping vacuum systems
Avoid turbulences
Even with laminar gas flow mechanical vibrations can lead to long distance particle transport
Slow vent
K. Zapfe, J. Wojtkiewicz
SRF2007, WEP74

14. Venting and Pump Down of “Particle Free” Sections
Lutz
There are and always will be particles in the vacuum system!
Developments of slow pumping / venting procedures by means on in-vacuum particle counter. No particles are transported if either:
Flow ≤ 3 ln/min, or
Pressure < 1 mbar
Automatic pumping / venting units developed
Constant flow of 3 ln/min of nitrogen or argon, by means of mass flow controllers.
Units have been widely used for XFEL.
This is important not to move the particle under pumping and venting during all cavity & vacuum works

15. Example XFEL: Overview of “Particle-Free” sections
Lutz
I1, I1D
Injector, Injector dump
BC0, BC1, BC2
Bunch compressor 0, 1, and 2
BC1D, BC2D
Bunch compressor 1 and 2 dumps
L1, L2, L3
Superconducting linacs (4, 12, and 80 accelerator modules) CL
Collimation
T1, T3, U1, T5, U2, T5D
Electron transfer lines of the southern branch up the XSDU1 dump
TL, TLD
Switch-Yard, electron transfer line to XS1 dump S2
SASE section of the southern branch
T2,T4, T4D
Electron transfer lines of the northern branch up the XSDU2 dump
T6, T7, T8
Photon transfer lines of the southern branch *
S1, S3
SASE sections of the northern branch
T9, T10
Photon transfer lines of the northern branch *
And the X-ray optics areas, too.
(not covered here)
Around 1.5 km need to be set “particle-free” section from injector to main-linac (cryomodule section)
At least ISO 5 level cleaness are needed on these area.  How to make in acclelerator tunnel ?

16. Particle Cleanliness: Segmentation
Lutz
Segmentation is important
Pumpdown time needs to be acceptable
Efficient leak searches
Cleaning of subsections in an optimised environment i.e. normal clean room
Steel flanges , plastic caps
Reduction of types and number of flange connections to be made in the tunnel
Better reproducibility
Time saving
Reduce open parts and time in accelerator tunnel.

17. Particle Cleanliness: Segmentation of Accelerator Modules
Lutz
Before opening the flanges, particulates must be blowed
to keep 0 particle by particle counters.
Make local clean boose
With laminer flow
Dust stick to the surface: (by Van der Vaals) Cleaned by Ionized nitrogen blowing.

18. Mobile Clean Rooms: Example XFEL HOM-Absorber
Lutz
Air flow from below
Access from both sides
Handling system for the component (No need access by human, who produce particulates.)
Stairs can be removed for transports
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19. Mobile Clean Rooms: Injector Version
Lutz
Filter system attached to blower unit and can be tilted
Access with two people possible
Injector is a complex area. It is difficult to smart clean boose

20. XFEL „Particle-free“ Highlights in Numbers (include)
Lutz
1 RF Gun including cathode handling system
Very delicate components, very little space, very demanding requirements
38 girders with 5 m each
More than 500 different pieces
Tubes, pumps, compensators, diagnostics incl. transverse deflecting structure,
Laser heater chicane
3 Bunch Compressor Chicanes
Long chambers
Large flanges
6 pressure stages at warm cold transition
3 dump lines
60 m warm beam line at the end of the cold linac as a buffer zone
For the 100 accelerator modules
200 all metal date valves
800 cavity and 800 coupler bellows
200 coupler pump lines with TSP, sputter ion pump
100 HOM Absorbers
Plus testing, installation etc.
More than 1,5 km of vacuum chambers have been cleaned (without the cavities)

21. XFEL Module Gradient Performance
By Nick Walker
(Presented by Denis Kostin)
on this workshop.
XFEL Module Gradient Performance 15
Module performance well above XFEL specs. and visible improvement with time
Tunnel installation used sorting of modules based on AMTF performance
XFEL Spec
23.6 MV/m
see talk by D. Kostin “European-XFEL summary: Cavities/Modules performance” in SRF session, Tue 24th AM
vertical test (clipped at 31 MV/m)
module performance
Ncavs
Average
RMS VT
815
28.3 MV/m
3.5 CM
27.5 MV/m
4.8
Slide 15
Remarks:
Clipping at 31 MV/m is done due to max. available RF power; limit given by waveguide distribution.
XM98 is a scavenger module.
We have ~10 % degradation from VT to CM. But we could keep the cavity requirements of E-XFEL of 23.6 MV/m.
 Elaborate clean assembly works for E-XFEL at string-assembly kept the cavity performance.

22. Now . First Lasing at European XFEL – 2nd of May 2017
Lutz
Compression ON
Compression OFF
No beam
SASE intensity vs. compression
(lower Pyro signal = higher compression)
SASE image on OTR station
6.3 GeV electron beam energy
500 pC buch charge, 1 Hz, single bunch
→ 0.8 nm SASE wave length
Now:
14 GeV, 30 Bunches,
1.3 Angström, 9.3 keV, upto 1mJ
We did not see the significant degradation in E-XFEL.  need to continue measuring SRF performance

23. Summary from E-XFEL experience about clean work
Lutz
Remember the basic rules as follows
Avoid particulates at every stage and include it in the mechanical design
Remove particulates at every stage possible
Do not produce particulates especially during installation and operation
Never transport particulates
Layout and mechanical design need to take into account cleaning and installation
Transport of particulates must be avoided
Methods to avoid turbulent flows during pump-down and venting are available
Large-scale infrastructures can be built “particle-free”
A large part of the European XFEL accelerator vacuum system is „particle-free“
… and it works!
ILC needs higher gradient of more than 10 times larger SRF scale than E- XFEL. These clean work of E-XFEL must be learned more and brushed up.
For ILC up to now, keep clean clean clean clean as much as possible.

24. Thank you
Participants of the workshop

25. backup

26. Particulate-related Degradations: LHC Beam Losses
TRANSIENT BEAM LOSSES IN THE LHC INJECTION KICKERS FROM MICRON SCALE DUST PARTICLES
B. Goddard et al., IPAC2012, TUPPR092
Particulate-related Degradations: LHC Beam Losses
2010 and 2011 beam losses leading to 35 protection beam dumps were observed
With improved diagnostics information of about suspicious events detected
UFOs !
Unidentified Falling Objects
6% of these events could be attributed to the injection kickers which are only 0,06% of the LHC length
Other kickers were not showing is anomaly
Clear correlation to kickers being pulsed

27. Particulate-related Degradations: LHC Injection Kicker
Added beam loss monitors confirm location
Kicker vibrates when pulsed
Aluminum oxide particles were found in abundance after one item removed from the accelerator
Other kickers have metallic coating

28. Vacuum systems of the XFEL accelerator Warm beam line vacuum
Requirements from beam dynamics
Except μr and RF-shielding absolutely new requirements on an accelerator vacuum system
Material properties
alignment
RF shielding
Vacuum (**)
conductivity
rel. magnetic permeability μr
R + 50*O
max. step at flanges
max. longitudinal gap
Flanges, bellows, pumps, valves
Particle free
Average pressure
mWcm nm mm
mbar
RF-gun (up to L0) 75
<1.01
---
ISO 5
< 10-10
I1, BC0, BC1-, BC2- chicane 3
1.01
1250
0.2
0.5
YES
< *
I1, BC1, BC2 dump
1.05
2000 NO
<
BC1,2
CL, TL, T1, T2, T3. T4, T4D, T5, T5D, U1, U2
S1, S2, S3
550
0.1
<
Right in front of main dumps
(*) close to SRF modules mbar
(**) In addition all components have to be in accordance to the DESY vacuum specification

29. E-XFEL Cavities : from VT to (current) operation
By Nick Walker
(Presented by Denis Kostin)
on this workshop.
E-XFEL Cavities : from VT to (current) operation 24
ILC 35 MV/m
ILC 31.5 MV/m
ILC 31.5 MV/m
Single Cavity Vertical Test (VT)
Single cavity CW
<E> ~ 30 MV/m
<E> ~ 28 MV/m clipped at 31 MV/m
Cryomodule Test (CM) Single Cavity (pulsed)
<E> ~ 27 MV/m
Current XFEL operation (approximate)
<E> ~ 22 MV/m
Work in progress – expect to do better
slide 24
goal
31.5MV/m is limited by RF power source in CM test