X-band accelerator Structure

X-band accelerator StructureLCPAC Feb. 20, 2004

T. HigoKEK

LCPAC 2004 Structure (T. Higo)

The most essential problem to be solved in accelerator structureStable operation at a high fieldis described inICFA International Linear Collider Technical Review Committee Second Report 2003Ranking 1 (R1) : R&D items needed for feasibility demonstration of the machineFor JLC-X/NLC, the validation of the presently achieved performance (gradient and trip rate) of low group velocity structuresbut with an acceptable average iris radius, dipole detuning and manifolds for dampingconstitutes the most critical Ranking 1 R&D issue. Tests of structures with these features are forseen in 2003.


Practical R&D targetGradient much more than 65MV/m was already obtained in a detuned structure but with very small a/l and without slots and manifolds.

Stable operation (namely 0.1BD / 60Hz X 1hr) at 65MV/m should be proved in a structure with a/l as large as about 0.18with all slotted cells


FY2003 Target itemsIn collaboration with SLACProve the high field performance to meet ITRC R1 requirementProve wake field control in 60cm high-power structures

By KEKInvoke high field test of structure at KEKFabricate actual-size structure in Japan


Evolution from RDDS1 to H60VG4S17Until 20031.8m 0.9m2p/3 5p/6In 20030.9m 0.6ma/l 0.18 0.17


Structure parameter evolution in 2003 Ls a/l vg/c Pin Enl Structure m % MW MV/m

Reduce structure length(1.8 0.9) for low vg (12% 5~3%)2003 Feb. 3 0.9 0.18 5-3 75 65 (H90VG5N etc.)

Reduce structure length, reduce power & pulse heating2003 ~Spring: 4 0.6 0.18 3-1 69 65 H60VG3S18

Reduce a/l for Rsh up to reduce power & reduce upstream field2003 ~Fall: 50.6 0.17 4-1 59 65 H60VG4S17


Parameter evolution in 60cm structures for further improving high-field performanceVG3A18 ILT Round irisVG3S18 Elliptical irisVG3S17 Elliptical irisVG4S17 Elliptical irisNaming for example on H60VG4S17 H High phase advance structure (150deg/cell) 60 60cm long VG Group velocity at upstream side 4 Percent of light speed S With slots and manifolds 17 a/l=0.17H60VG3S18Iris roundellipticala/l 0.180.17Vg/c 3%start at 4%4 5


FY2003 Actual activitiesRefinement and production of HDDS cells for structures tested at NLCTAH60VG3S18 and H75VG4S18H60VG4S17-I, II and H60VG4S17-IIIPrepare for H60VG4SL17-A,BFabrication of 60cm structure in Japan Setup of GLCTA high power test area by ATFEstablishment of calculation of cell frequenciesInitialization of HDDS cell production studies by parties other than KEK


High field test of 60cm structures For HDDS evaluation4 -- H60VG3N-6C4 -- H60VG3S185 -- H60VG4S17-I5 -- H60VG4S17-III For fabrication by FNAL-- H60VG3R18(FXB)-- H60VG3S17(FXC)High power test all at NLCTAStructures are SLAC/KEK SLAC made FNAL made For early tasting of a/l=0.17H60VG3R17H60VG4R17Keep close collaboration with FNAL structure group in addition to SLAC


High field studies of 60cm 5p/6-mode structuresin which KEK cells are/were used Structure code In Coupler Results or status

4 H60VG3N-6C ILT High power test finished4 H60VG3S18 MC High power test finishedH75VG4S18 MC Completed5 H60VG4S17-I MC High power test finished5 H60VG4S17-II WG Cells were produced5 H60VG4S17-III(1) WG Under assembly 5 H60VG4S17-III(2) WG Cell under productionH60VG4SL17 A,B WG For wake field control suspended

Input coupler type: ILT=in line taper, MC=mode converter, WG=waveguide


HDDS cell production


Basic specification for HDDS cellWe design the fabrication based on precision milling + diamond turning.Turning precision is specified as 2 microns in diameter. Dimensions among milled 3D geometries are within 10 microns.Frequencies can be controlled within a few MHz, meeting dipole frequency tolerance of s~5MHzConcentricity better than 5 microns Alignment tolerance of several microns becomes feasible.


Electric field / Magnetic fieldWe need to avoid additional local field enhancement due to non-smoothness especially at red areas.Special care is taken at points (2,3,4) where smooth junction is difficult due to the junction between milling and turning 1234EsurfaceHsurfaceDTpulse


Ensuring junction between milling and turning connecting at a small angleUse of tapered milling toolPrecise vertical positioningChecking profile by stylus Established cell fabrication technique based on precision milling + diamond turningProfile measurementJunction


Burrs are safely removedSEM from C. Pearson (SLAC)Not easy for the machining to be completely free from burrs but they are in low field area so that they can be hand deburred.


Scratches are smoothed by chemical etching processSEM from C. Pearson (SLAC)Present quality on surface scratches:

Scratches are smoothed by etching process as shown left. This example is much larger than our acceptance level inspected with a low magnification optical microscope.After chemical etching As of machined


Summary of breakdown rate of recent structuresFollowing page shows plots of the breakdown rates of structures tested in 2003 as function of accelerator field.The performance near the nominal field of 65MV/m without beam loading was studied.The pulse length is 400ns flat pulse, except for the point indicated as design pulse shape where the ramping pulse shape needed for the beam loading compensation.Requirement of 0.1 breakdown per hour is equivalent to 1 breakdown in 2 million pulses at the present repetition rate of 60Hz.


C. Adolphsen (SLAC) Jan. 2004Structure High Gradient Performance Summary(Breakdown Rate -vs- Unloaded Gradient with 400 ns Square Pulses)


H60VG3S18 Processing historyFirst full-HDDS cell structure


H60VG3S18 BD position65MV/m70MV/mBreakdowns located mainly at upstream cellsData from C. Adolphsen SLAC


a/l 0.18 0.17 and vg/c 3% 4%Reduce surface field at upstream endH60VG3S18

H60VG4S17Increase Rs 59 61MW/mReduce power 69 59MWReduce magnetic fieldReduce pulse temperature rise


H60VG4S17 Process historyC. Adolphsen SLAC 040126Data from C. Adolphsen SLACEacc [MV/m]


Pulse shapes for high power test at NLCTAInput power with ramp for Beam loading compensationTime (ns)Input power with flat pulseTime (ns)from C. Adolphsen SLACData from C. Adolphsen SLAC


Dependence on input pulse shapeVarious pulse shapeRamped pulse shapeBack to flat 400ns BD rate significantly decreased with ramped pulse shape BD localized, BD position switched to another cell Many soft events, later than 100ns similar to those due to pulse heating

Need to study with a normal structureData from C. Adolphsen SLAC


C. Adolphsen (SLAC) Jan. 2004Structure High Gradient Performance Summary(Breakdown Rate -vs- Unloaded Gradient with 400 ns Square Pulses)


High field test result summaryStructure with present HDDS cells show similar performance to those of non-slotted structures.Low surface electric field at upstream cells seems effective to reduce BD rate.BD rate scales exponentially as accelerator field, roughly 5MV/m per decade.Pulse shaping for beam loading compensation is effective to reduce BD rate.Rates scatteres from structure-to-structure probably related to fabrication/installation processes. Ii is important to reproduce good ones by improving practical processes with more structures under the present structure design.


Proof of wake field suppressionNeed to proveWake field suppression by damped-detuned schemeFrequency controlStructure alignment based on HOM monitoringStructure straightness and 3D geometry concentricity w.r.t. beam hole etc.

The feasibility of wake field control was already proved in 1.8m structure.This was not in the high priority now and we suspended the wake-field program in 2003.After establishing high field performance, we should come back to this issue and actually prove in our latest structure design.But there are some related experiences acquired in HDDS cell production as shown in the following two pages.


Frequency smoothness in H60VG4S17 production and feasibility to control F1Std dev = ~0.5MHzStd dev = ~1MHzIt seems that we have a feasibility to control within 1MHz sigma. To be confirmed in wake-field test structure.


Fabrication of 60cm structure KX01 (H60VG3N)Diamond turning by an industryChemical etching at KEKDiffusion bonding and brazing in hydrogen furnace at a companyVacuum baking in a klystron factoryNow tuning and high-power test at KEK


Actual-size structure fabricationGain experience of fabrication of actual-size structureStudy high-field performance at GLCTATry to find a way to improve high-field performance through fabrication technology

Made first 60cm HDS structure (KX01) with parameters H60VG3N and now under tuning


GLCTA structure point of viewActivities of 2003Moved high-field setup from AR-south to ATF areaRefurbished the system in control and data takingRestarted high power studyGoal of 2004Actually process structures to high field during long-term (>>100hours) Breakdown related data are recorded and analyzed through operationVarious components will be developed and used, such as waveguide flanges, low-loss waveguides, RF load, etc.


GLCTA at presentTwo klystrons are delivering ~60MW, 400ns to structure T53VG3F is sitting now Preparing control and measurement system Install KX01 soon SLAC helps such as acoustic meas.(now) low loss waveguides (in future) etc.


Conclusion of 2003 structure developmentEstablished HDDS cell fabrication technique.Typically 60cm structures show BD rate at nominal field of 65MV/m within a factor of 5 or so w.r.t. the requirement.On the other hand, almost all structures met BD rate requirement at 60MV/m. We understand it is worthwhile to study the feasibility to start with a moderately low field operation.BD rate nearly met the requirement when operated with a pulse shape for beam-loading compensation.Prepared a high power test facility GLCTA.


Developments in early 2004Acquire more statistics with the present design (H60VG4S17) structures with SLAC to have a clear view of the present design. This process is pursued by applying any improvement of fabrication, installation and processing procedures.Develop and really make GLCTA serve as high field study facility.


Developments in second half of 2004Once foresee the establishment of high field performance, we discuss the next study items such as Proving the wake field performance with SLAC in the present design structure.Or going into studies on mass production of cells and structures.Otherwise, we focus on establishing good high field performance.


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X-band accelerator Structure