• A. Brachmann, D. Dowell
  SLAC, Menlo Park, California

Sim­u­la­tions of ion mi­gra­tion in the LCLS RF gun and in­jec­tor A. Brach­mann On be­half of the LCLS com­mis­sion­ing team The mo­ti­va­tion for this work was the ob­served sur­face con­tam­i­na­tion of the first LCLS RF gun cop­per cath­ode. We will pre­sent the re­sults of sim­u­la­tions in re­gards to ion mi­gra­tion in the LCLS gun. Ions of resid­u­al gases will be cre­at­ed by in­ter­ac­tion of molec­u­lar gas species with the UV drive laser beam and by the elec­tron beam it­self. The larg­er part of those ion­ized molecules re­main in the vicin­i­ty of cre­ation, are trans­port­ed to­wards beam line walls or away from the cath­ode. How­ev­er a sig­nif­i­cant frac­tion gains enough ki­net­ic en­er­gy to be fo­cused by RF and mag­net­ic fields, reach­ing the cath­ode and pro­duc­ing an un­de­sir­able in­crease of the cath­ode's sur­face work func­tion. Al­though this frac­tion is small, dur­ing long term op­er­a­tion, this ef­fect be­comes a sig­nif­i­cant fac­tor lim­it­ing the source per­for­mance.

• A. Brachmann, F.-J. Decker, Y.T. Ding, D. Dowell, P. Emma, J.C. Frisch, A. Gilevich, G.R. Hays, P. Hering, Z. Huang, R.H. Iverson, H. Loos, A. Miahnahri, D. Nordlund, H.-D. Nuhn, P.A. Pianetta, J.L. Turner, J.J. Welch, W.E. White, J. Wu, D. Xiang
  SLAC, Menlo Park, California

Sur­face char­ac­ter­i­za­tion of the LCLS RF gun cath­ode A. Brach­mann On be­half of the LCLS com­mis­sion­ing team The first cop­per cath­ode in­stalled in the LCLS RF gun was used dur­ing LCLS com­mis­sion­ing for more than a year. How­ev­er, after high charge op­er­a­tion (~ 500 pC), the cath­ode showed a de­cline of quan­tum ef­fi­cien­cy due to sur­face con­tam­i­na­tion caused by resid­u­al ion­ized gas species pre­sent in the vac­u­um sys­tem. We re­port re­sults of SEM, XPS and XAS stud­ies that were car­ried out on this cath­ode after it was re­moved from the gun. X-ray ab­sorp­tion and X-ray pho­to­elec­tron spec­troscopy re­veal sur­face con­tam­i­na­tion by var­i­ous hy­dro­car­bon com­pounds. In ad­di­tion we re­port on the per­for­mance of the sec­ond in­stalled cath­ode with em­pha­sis on the spa­tial dis­tri­bu­tion of elec­tron emis­sion.

• J.C. Frisch, C. Bostedt, J.D. Bozek, A. Brachmann, R.N. Coffee, F.-J. Decker, Y.T. Ding, D. Dowell, P. Emma, A. Gilevich, G. Haller, G.R. Hays, P. Hering, B.L. Hill, Z. Huang, R.H. Iverson, E.P. Kanter, B. Kraessig, H. Loos, A. Miahnahri, H.-D. Nuhn, A. Perazzo, M. Petree, D.F. Ratner, T.J. Smith, S.H. Southworth, J.L. Turner, J.J. Welch, W.E. White, J. Wu, L. Young
  SLAC, Menlo Park, California
• R.B. Wilcox
  LBNL, Berkeley, California

In ad­di­tion to its nor­mal op­er­a­tion at 250pC, the LCLS has op­er­at­ed with 20pC bunch­es de­liv­er­ing X-ray beams to users with en­er­gies be­tween 800eV and 2 keV and with bunch lengths below 10 fs FWHM. A bunch ar­rival time mon­i­tor and tim­ing trans­mis­sion sys­tem pro­vide users with sub 100 fs syn­chro­niza­tion be­tween a laser and the X-rays for pump / probe ex­per­i­ments. We de­scribe the per­for­mance and op­er­a­tional ex­pe­ri­ence of the LCLS for short bunch ex­per­i­ments.

• F.-J. Decker, R. Akre, A. Brachmann, J. Craft, Y.T. Ding, D. Dowell, P. Emma, J.C. Frisch, Z. Huang, R.H. Iverson, A. Krasnykh, H. Loos, H.-D. Nuhn, D.F. Ratner, T.J. Smith, J.L. Turner, J.J. Welch, W.E. White, J. Wu
  SLAC, Menlo Park, California

The Linac Co­her­ent Light Source (LCLS) at SLAC is an x-ray Free Elec­tron Laser with wave­lengths of 0.15 nm to 1.5 nm. The elec­tron beam sta­bil­i­ty is im­por­tant for good las­ing. While the trans­verse jit­ter of the beam is about 10-20% of the rms beam sizes, the jit­ter in the lon­gi­tu­di­nal phase space is a mul­ti­ple of the en­er­gy spread and bunch length. At the lower en­er­gy of 4.3 GeV (cor­re­spond­ing to the longest wave­length of 1.5 nm) the rel­a­tive en­er­gy jit­ter can be 0.125%, while the rms en­er­gy spread is with 0.025% five times small­er. An even big­ger ratio ex­ists for the ar­rival time jit­ter of 50 fs and the bunch du­ra­tion of about 5 fs (rms) in the low charge (20 pC) op­er­at­ing mode. Al­though the im­pact to the ex­per­i­ments is re­duced by pro­vid­ing pulse-by-pulse data of the mea­sured en­er­gy and ar­rival time, it would be nice to un­der­stand and mit­i­gate the root caus­es of this jit­ter. The thyra­tron of the high power sup­ply of the RF klystrons is one of the main con­trib­u­tors. An­oth­er sus­pect is the mul­ti-pact­ing in the RF loads. Phase mea­sure­ments down to 0.01 de­gree (equals 10 fs) along the RF pulse were achieved, giv­ing hints to the im­pact of the dif­fer­ent sources.