Report of workshop of Operating SRF Systems reliably in a “Dirty” Machine @ HZB (Berlin) Hiroshi Sakai (KEK) (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. 23 pages LCWS2017 in Strasbourg, 2017.Oct.24 1
Transcript
Report of workshop ofOperating SRF Systems reliably in a “Dirty” Machine @ HZB (Berlin)
Hiroshi Sakai (KEK)
(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.
23 pages
LCWS2017 in Strasbourg, 2017.Oct.241
Program of this workshop
Details about clean works
Introduction of E-XFEL clean work for assembly and vacuum .
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.
What kinds of problem about SRF operation were faced in the existing acceleratorsand how do we recover the problems.
CEBAF
CEBAF/KEK
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(Lesson 1) Learn from long-term beam operation and how to overcome ?
• SRF cavity performances for a long-beam operation are show as follows • CEBAF @JLAB
• KEKB @KEK
• How to overcome the performance degradation of SRF cavity in cryomodules.• Pulse processing
• He processing
• Plasma processing
• Horizontal HPR
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CEBAF introduction
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
CH
L-2
Upgrade
magnets
and
power
supplies
Upgraded cavityCavity: 0.7 m, “Low loss”
cell shape, 1.497 GHz8 cavities/cryomodule
CEBAF
CEBAF field emission onset(C100 cryomodule)
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C.Reece
Example of dirty cavity inside during long operation and Degradation made radiation damage
Cleaning essential not only cavity but also beam line.
Damaged by radiation produced by F.E
Particle sampling inside the cavity and vacuum chamberIn one cryomodule in CEBAF (FRANKLIN)
Many particles come from assembly work and ceramic ?
5
C.Reece
He processing
Increase by He processing
Procedure : Add He to 2KProcess by RF under He pressure around 0.05-0.1 mTorrFor about 1 hour.Warm up again to 40 K.
See the radiation onset before and after He processing.
total
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 .
One example
6
C.Reece
Reference of CEBAF (see ref. in detail)
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509 MHz 8 Nb Single-cell Cavities, 4.4 K Operation
RF-Voltage : 1.5MV/cav, unloaded-Q > 1e9@2MV
Re-use of existing SRF system in SuperKEKB
Issues in SuperKEKB for SCCLarge HOM power is expected due to twice highbeam 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.
Experience of KEKB and recovery to Super KEKB Michiru Nishiwaki (KEK)
SC-Cavity(HER)
HER(e-): 7 GeV, 2.6ALER(e+) : 4 GeV, 1.6A
NC-cavity(LER)
Super KEKB Highest luminosity machineUnder commissioning from 2016
Q0 at 1.5 MV > 1e+09 => Acceptable for SupeKEKBFurther degradation make the operation difficult. Need to recover the cavity performance8
Both cavity performances were successfully recovered in Super-KEKBIt 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 wayto recovery performance in principle. But is it applicable to ILC cavity (9-cell, long string assembly) ?
Horizontal HPR applied to Degraded Cavity
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Rotating Water Jets
• 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 andGV, 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.
108
109
1010
0 0.5 1 1.5 2 2.5Vc [MV]
108
109
1010
0 0.5 1 1.5 2 2.5Vc [MV]
CA-B03 CA-B04
BeforeLeak
Degradedat Repairing work
after HHPR
Degradedby Vacuum Trouble
after HHPRafter Aging in Tunnel Results of HHPR
(Lesson 2) Controlling Particulates and Dust in Vacuum Systems
Lutz LiljeDESY, 14.9.2017
(Lesson 2) Learn from the experience of EURO-XFEL construction (Lutz@DESY)
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Before we start: Introduction and definition of the Problem
• 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
Lutz
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Mass of Particulates
• 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
By Original uploader was Sullivan.t.j at English Wikipedia. - The description as originally from Wikipedia., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2249027
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Lutz
Avoiding Particulate Transport during Pumpdown and Vent
• 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
K. Zapfe, J. Wojtkiewicz
SRF2007, WEP74
Slow vent
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Lutz
Venting and Pump Down of “Particle Free” Sections
• 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 works14
Lutz
Example XFEL: Overview of “Particle-Free” sections
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 *
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 ?
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Lutz
Particle Cleanliness: Segmentation• 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.16
Lutz
Particle Cleanliness: Segmentation of Accelerator Modules
Dust stick to the surface: (by Van der Vaals) Cleaned by Ionized nitrogen blowing.
Make local clean booseWith laminer flow
Before opening the flanges, particulates must be blowedto keep 0 particle by particle counters.
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Lutz
Mobile Clean Rooms: Example XFEL HOM-Absorber
• 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 fortransports
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Lutz
Mobile Clean Rooms: Injector Version
• 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 boose19
Lutz
XFEL „Particle-free“ Highlights in Numbers (include)
• 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)
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Lutz
XFEL Module Gradient Performance
Module performance well
above XFEL specs.
and visible
improvement with time
Tunnel installation used
sorting of modules
based on AMTF
performance
vertical test (clipped at 31 MV/m)module performance
Remarks:Clipping at 31 MV/m is done due to max. available RF power; limit given by waveguide distribution.XM98 is a scavenger module.
Ncavs Average RMS
VT 815 28.3 MV/m 3.5
CM 815 27.5 MV/m 4.8
XFEL Spec23.6 MV/m
see talk by D. Kostin “European-XFEL summary: Cavities/Modules performance” in SRF session, Tue 24th AM
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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.
By Nick Walker(Presented by Denis Kostin)on this workshop.
Now . First Lasing at European XFEL – 2nd of May 2017
6.3 GeV electron beam energy500 pC buch charge, 1 Hz, single bunch→ 0.8 nm SASE wave length
SASE intensity vs. compression(lower Pyro signal = higher compression)
SASE image on OTR station
No beam
Compression OFF
Compression ON
Now:
14 GeV, 30 Bunches,
1.3 Angström, 9.3 keV, upto 1mJ
22We did not see the significant degradation in E-XFEL. need to continue measuring SRF performance
Lutz
Summary from E-XFEL experience about clean work
• 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.
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For ILC up to now, keep clean clean clean clean as much as possible.
Lutz
Thank you
Participants of the workshop 24
backup
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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 7800 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
TRANSIENT BEAM LOSSES IN THE LHC INJECTION KICKERS FROM MICRON
