040220LCPAC 2004 Structure (T. Higo)1 X-band accelerator Structure LCPAC Feb. 20, 2004 T. Higo KEK

040220LCPAC 2004 Structure (T. Higo)2 The most essential problem to be solved in accelerator structure Stable operation at a high field is described in ICFA International Linear Collider Technical Review Committee Second Report 2003 Ranking 1 (R1) : R&D items needed for feasibility demonstration of the machine For JLC-X/NLC, the validation of the presently achieved performance (gradient and trip rate) of low group velocity structures—but with an acceptable average iris radius, dipole detuning and manifolds for damping— constitutes the most critical Ranking 1 R&D issue. Tests of structures with these features are forseen in 2003.

040220LCPAC 2004 Structure (T. Higo)3 Practical R&D target Gradient much more than 65MV/m was already obtained in a detuned structure but with very small a/ and without slots and manifolds. Stable operation (namely 0.1BD / 60Hz X 1hr) at 65MV/m should be proved in a structure –with a/ as large as about 0.18 –with all slotted cells

040220LCPAC 2004 Structure (T. Higo)4 FY2003 Target items In collaboration with SLAC Prove the high field performance to meet ITRC R1 requirement Prove wake field control in 60cm high-power structures By KEK Invoke high field test of structure at KEK Fabricate actual-size structure in Japan

040220LCPAC 2004 Structure (T. Higo)5 Evolution from RDDS1 to H60VG4S17 Until m  0.9m 2  /3  5  /6 In m  0.6m a/ 0.18  0.17

040220LCPAC 2004 Structure (T. Higo)6 Structure parameter evolution in 2003 L s a/  v g /c P in E nl Structure m % MW MV/m Reduce structure length(1.8  0.9) for low vg (12%  5~3%) 2003 Feb (H90VG5N etc.) Reduce structure length, reduce power & pulse heating 2003 ~Spring: H60VG3S18 Reduce a/ for R sh up to reduce power & reduce upstream field 2003 ~Fall: H60VG4S17

040220LCPAC 2004 Structure (T. Higo)7 Parameter evolution in 60cm structures for further improving high-field performance VG3A18 ILT Round iris VG3S18 Elliptical iris VG3S17 Elliptical iris VG4S17 Elliptical iris Naming 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/ = H60VG3S18 2.Iris round  elliptical 3.a/ 0.18  V g /c 3%  start at 4% 4  5

040220LCPAC 2004 Structure (T. Higo)8 FY2003 Actual activities Refinement and production of HDDS cells for structures tested at NLCTA –H60VG3S18 and H75VG4S18 –H60VG4S17-I, II and H60VG4S17-III –Prepare for H60VG4SL17-A,B Fabrication of 60cm structure in Japan Setup of GLCTA high power test area by ATF Establishment of calculation of cell frequencies Initialization of HDDS cell production studies by parties other than KEK

040220LCPAC 2004 Structure (T. Higo)9 High field test of 60cm structures For HDDS evaluation 4 -- H60VG3N-6C 4 -- H60VG3S H60VG4S17-I 5 -- H60VG4S17-III For fabrication by FNAL 4-- H60VG3R18(FXB) 5-- H60VG3S17(FXC) High power test all at NLCTA Structures are SLAC/KEK SLAC made FNAL made For early tasting of a/ =0.17 H60VG3R17 H60VG4R17 Keep close collaboration with FNAL structure group in addition to SLAC

040220LCPAC 2004 Structure (T. Higo)10 High field studies of 60cm 5  /6-mode structures in which KEK cells are/were used Structure code In Coupler Results or status 4 H60VG3N-6C ILT High power test finished 4 H60VG3S18 MC High power test finished H75VG4S18 MC Completed 5 H60VG4S17-I MC High power test finished 5 H60VG4S17-II WG Cells were produced 5 H60VG4S17-III(1) WG Under assembly 5 H60VG4S17-III(2) WG Cell under production H60VG4SL17 A,B WG For wake field control suspended Input coupler type: ILT=in line taper, MC=mode converter, WG=waveguide

040220LCPAC 2004 Structure (T. Higo)11 HDDS cell production

040220LCPAC 2004 Structure (T. Higo)12 Basic specification for HDDS cell We 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  ~5MHz Concentricity better than 5 microns  Alignment tolerance of several microns becomes feasible.

040220LCPAC 2004 Structure (T. Higo)13 Electric field / Magnetic field We 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 E surface H surface   T pulse

040220LCPAC 2004 Structure (T. Higo)14 Ensuring junction between milling and turning connecting at a small angle Use of tapered milling tool Precise vertical positioning Checking profile by stylus Established cell fabrication technique based on precision milling + diamond turning Profile measurement Junction

040220LCPAC 2004 Structure (T. Higo)15 Burrs are safely removed SEM 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.

040220LCPAC 2004 Structure (T. Higo)16 Scratches are smoothed by chemical etching process SEM 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

040220LCPAC 2004 Structure (T. Higo)17 Summary of breakdown rate of recent structures Following 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.

040220LCPAC 2004 Structure (T. Higo)18 C. Adolphsen (SLAC) Jan Structure High Gradient Performance Summary (Breakdown Rate -vs- Unloaded Gradient with 400 ns Square Pulses)

040220LCPAC 2004 Structure (T. Higo)19 H60VG3S18 Processing history First full-HDDS cell structure

040220LCPAC 2004 Structure (T. Higo)20 H60VG3S18 BD position 65MV/m 70MV/m Breakdowns located mainly at upstream cells Data from C. Adolphsen SLAC

040220LCPAC 2004 Structure (T. Higo)21 Reduce surface field at upstream end a/ 0.18  0.17 and v g /c 3%  4% H60VG3S18 H60VG4S17 Increase Rs 59  61M  /m Reduce power 69  59MW Reduce magnetic field Reduce pulse temperature rise

040220LCPAC 2004 Structure (T. Higo)22 C. Adolphsen SLAC H60VG4S17 Process history Data from C. Adolphsen SLAC Eacc [MV/m]

040220LCPAC 2004 Structure (T. Higo)23 Pulse shapes for high power test at NLCTA Input power with ramp for Beam loading compensation Time (ns) Input power with flat pulse Time (ns) from C. Adolphsen SLAC Data from C. Adolphsen SLAC

040220LCPAC 2004 Structure (T. Higo)24 Dependence on input pulse shape Various 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 structure Data from C. Adolphsen SLAC

040220LCPAC 2004 Structure (T. Higo)25 C. Adolphsen (SLAC) Jan Structure High Gradient Performance Summary (Breakdown Rate -vs- Unloaded Gradient with 400 ns Square Pulses)

040220LCPAC 2004 Structure (T. Higo)26 High field test result summary Structure 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.

040220LCPAC 2004 Structure (T. Higo)27 Proof of wake field suppression Need to prove Wake field suppression by damped-detuned scheme  Frequency control Structure alignment based on HOM monitoring  Structure 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 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.

040220LCPAC 2004 Structure (T. Higo)28 Frequency smoothness in H60VG4S17 production and feasibility to control F1 Std dev = ~0.5MHzStd dev = ~1MHz It seems that we have a feasibility to control within 1MHz sigma. To be confirmed in wake-field test structure.

040220LCPAC 2004 Structure (T. Higo)29 Fabrication of 60cm structure KX01 (H60VG3N) Diamond turning by an industry Chemical etching at KEK Diffusion bonding and brazing in hydrogen furnace at a company Vacuum baking in a klystron factory Now tuning and high-power test at KEK

040220LCPAC 2004 Structure (T. Higo)30 Actual-size structure fabrication Gain experience of fabrication of actual-size structure Study high-field performance at GLCTA Try 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

040220LCPAC 2004 Structure (T. Higo)31 GLCTA structure point of view Activities of 2003 –Moved high-field setup from AR-south to ATF area –Refurbished the system in control and data taking –Restarted high power study Goal of 2004 –Actually process structures to high field during long- term (>>100hours) –Breakdown related data are recorded and analyzed through operation –Various components will be developed and used, such as waveguide flanges, low-loss waveguides, RF load, etc.

040220LCPAC 2004 Structure (T. Higo)32 GLCTA at present Two 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.

040220LCPAC 2004 Structure (T. Higo)33 Conclusion of 2003 structure development Established 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.

040220LCPAC 2004 Structure (T. Higo)34 Developments in early 2004 Acquire 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.

040220LCPAC 2004 Structure (T. Higo)35 Developments in second half of 2004 Once 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.