• T. Higo, Y. Higashi, S. Matsumoto, K. Yokoyama
  KEK, Ibaraki
• C. Adolphsen, V.A. Dolgashev, A. Jensen, L. Laurent, S.G. Tantawi, F. Wang, J.W. Wang
  SLAC, Menlo Park, California
• S. Döbert, A. Grudiev, G. Riddone, W. Wuensch, R. Zennaro
  CERN, Geneva

A CERN-SLAC-KEK col­lab­o­ra­tion on high gra­di­ent X-band ac­cel­er­a­tor struc­ture de­vel­op­ment for CLIC has been on­go­ing for three years. The major out­come has been the demon­stra­tion of sta­ble 100 MV/m gra­di­ent op­er­a­tion of a num­ber of CLIC pro­to­type struc­tures. These struc­tures were fab­ri­cat­ed ba­si­cal­ly using the tech­nol­o­gy de­vel­oped from 1994 to 2004 for the GLC/NLC lin­ear col­lid­er ini­tia­tive. One goal has been to re­fine the es­sen­tial pa­ram­e­ters and fab­ri­ca­tion pro­ce­dures need­ed to re­al­ize such high gra­di­ent rou­tine­ly. An­oth­er goal has been to de­vel­op struc­tures with stronger dipole mode damp­ing than those for GLC/NLC. The lat­ter re­quires that sur­face tem­per­a­ture rise dur­ing the puls­es be high­er, which may in­crease the break­down rate. Struc­tures with heavy damp­ing will be test­ed in late 2009/early 2010, and this paper will pre­sent these re­sults to­geth­er with some of the ear­li­er re­sults from non-damped struc­tures and struc­tures built with a quad­rant ge­om­e­try.

• L. Faillace, R.B. Agustsson, P. Frigola, A.Y. Murokh
  RadiaBeam, Marina del Rey
• V.A. Dolgashev
  SLAC, Menlo Park, California
• J.B. Rosenzweig
  UCLA, Los Angeles, California

There is grow­ing de­mand from the in­dus­tri­al and re­search com­mu­ni­ties for high gra­di­ent, com­pact RF ac­cel­er­at­ing struc­tures. The com­mon­ly used S-band SLAC-type struc­ture has an op­er­at­ing gra­di­ent of only about 20 MV/m; while much high­er op­er­at­ing gra­di­ents (up to 70 MV/m) have been re­cent­ly achieved in X-band, as a con­se­quence of the sub­stan­tial ef­forts by the Next Lin­ear Col­lid­er (NLC) col­lab­o­ra­tion to push the per­for­mance en­ve­lope of RF struc­tures to­wards high­er ac­cel­er­at­ing gra­di­ents. Cur­rent­ly how­ev­er, high power X-band RF sources are not read­i­ly avail­able for in­dus­tri­al ap­pli­ca­tions. There­fore, Ra­di­a­Beam Tech­nolo­gies is de­vel­op­ing a short, stand­ing wave S-band struc­ture which uses fre­quen­cy scaled NLC de­sign con­cepts to achieve up to a 50 MV/m op­er­at­ing gra­di­ent at 2856 MHz. The de­sign and pro­to­type com­mis­sion­ing plans are pre­sent­ed.

• V.A. Dolgashev, S.G. Tantawi, A.D. Yeremian
  SLAC, Menlo Park, California
• Y. Higashi
  KEK, Ibaraki
• B. Spataro
  INFN/LNF, Frascati (Roma)

We re­port re­sults of on­go­ing high power tests of sin­gle cell stand­ing wave struc­tures. These tests are part of an ex­per­i­men­tal and the­o­ret­i­cal study of rf break­down in nor­mal con­duct­ing struc­tures at 11.4 GHz. The goal of this study is to de­ter­mine the ac­cel­er­at­ing gra­di­ent ca­pa­bil­i­ty of nor­mal-con­duct­ing rf pow­ered par­ti­cle ac­cel­er­a­tors. The test setup con­sists of reusable mode-launch­ers and short test struc­tures pow­ered by SLAC's XL-4 klystron. We have test­ed struc­tures of dif­fer­ent ge­ome­tries, cell join­ing tech­niques, and ma­te­ri­als, in­clud­ing hard cop­per al­loys and molyb­de­num. We found that the be­hav­ior of the break­down rate is re­pro­ducible for dif­fer­ent struc­tures of the same ge­om­e­try and ma­te­ri­al. The break­down rate de­pen­dence on peak mag­net­ic fields is stronger than on peak sur­face elec­tric fields for struc­tures of dif­fer­ent ge­ome­tries.

• A.E. Vlieks, V.A. Dolgashev, S.G. Tantawi
  SLAC, Menlo Park, California
• S.G. Anderson, F.V. Hartemann, R.A. Marsh
  LLNL, Livermore, California

In sup­port of the T-REX pro­gram at LLNL and the High Gra­di­ent re­search pro­gram at SLAC, a new X-band mul­ti-cell RF gun is being de­vel­oped. This gun, sim­i­lar to an ear­li­er gun de­vel­oped at SLAC for Comp­ton X-ray source pro­gram, will be a stand­ing wave struc­ture made of 5.5 cells op­er­at­ing in the pi mode with cop­per cath­ode. This gun was de­signed fol­low­ing cri­te­ria used to build SLAC X-band high gra­di­ent ac­cel­er­at­ing struc­tures. It is an­tic­i­pat­ed that this gun will op­er­ate with sur­face elec­tric fields on the cath­ode of 200MeV/m with low break­down rate. RF will be cou­pled into the struc­ture through a sym­met­ric final cell with a shape op­ti­mized to elim­inable both dipole and quadru­ple field com­po­nents. In ad­di­tion, ge­om­e­try changes to the orig­i­nal gun, op­er­at­ed with Comp­ton X-ray source, will in­clude a wider RF mode sep­a­ra­tion, re­duced sur­face elec­tric and mag­net­ic fields.

• A.D. Yeremian, V.A. Dolgashev, S.G. Tantawi
  SLAC, Menlo Park, California

SLAC par­tic­i­pates in the U.S. High Gra­di­ent col­lab­o­ra­tion whose char­ter in­cludes basic stud­ies of rf break­down prop­er­ties in ac­cel­er­at­ing struc­tures. These stud­ies in­clude ex­per­i­ments with dif­fer­ent ma­te­ri­als and con­struc­tion meth­ods for sin­gle cell stand­ing wave ac­cel­er­at­ing struc­tures. The most com­mon­ly used method of join­ing cells of such struc­tures is the high tem­per­a­ture bond­ing and/or braz­ing in hy­dro­gen and/or vac­u­um. These high tem­per­a­ture pro­cess­es may not be suit­able for some of the new ma­te­ri­als that are under con­sid­er­a­tion. We pro­pose to build struc­tures from cells with an rf choke, tak­ing the cell-to-cell junc­tion out of the elec­tro­mag­net­ic field re­gion. These cells will be clamped to­geth­er in a vac­u­um en­clo­sure, the choke joint en­sur­ing con­ti­nu­ity of rf cur­rents. Next, we pro­pose a struc­ture with a choke joint in a high gra­di­ent cell and a view port which may allow us mi­cro­scop­ic, in-situ ob­ser­va­tion of the metal sur­face dur­ing high power tests. And third, we de­scribe the de­sign of a TM01 choke flange for these struc­tures.