• R.K. Li, H. Chen, Q. Du, T. Du, Y.-C. Du, Hua, J.F. Hua, W.-H. Huang, X. H. Lu, J. Shi, C.-X. Tang, H. S. Xu, L.X. Yan
  TUB, Beijing

Re­cent years have wit­nessed rapid ad­vances of MeV ul­tra­fast elec­tron diffrac­tion (UED), in which high qual­i­ty, ul­tra­short, MeV elec­tron puls­es from a pho­to­cath­ode RF gun are em­ployed as probes for ul­tra­fast struc­tural dy­nam­ics. We've built a pro­to­type MeV UED sys­tem at the Ac­cel­er­a­tor Lab­o­ra­to­ry of Ts­inghua Uni­ver­si­ty, op­ti­mized the the elec­tron pulse pa­ram­e­ters as well as hard­ware per­for­mances, and achieved high qual­i­ty sin­gle-shot diffrac­tion pat­terns. More­over, MeV UED can be op­er­at­ed in a so-called 'con­tin­u­ous­ly time-re­solved (CTR)' mode, in which an RF de­flect­ing cav­i­ty streaks the elec­tron pulse thus each diffrac­tion pat­tern con­sti­tutes an 'atom­ic movie'. We re­port our ex­per­i­men­tal progress on MeV UED in this paper.

• D. Qiang, Y.-C. Du, W.-H. Huang, C.-X. Tang, L.X. Yan
  TUB, Beijing

The Ts­inghua Thom­son scat­ter­ing X-ray source (TTX) has a strict laser-elec­tron syn­chro­niza­tion re­quire­ment and a com­pre­hen­sive sys­tem struc­ture in­clud­ing dual high-pow­er laser sys­tem, RF sys­tem and beam di­ag­nos­tic in­stru­ments, etc. Re­cent­ly, a syn­chro­niza­tion and con­trol sys­tem is de­vel­oped to meet these re­quire­ments, which in­cludes a laser-RF syn­chro­niz­er with 100fs time jit­ter, a FPGA based event gen­er­a­tor for laser and RF sys­tems with 250ps time res­o­lu­tion, and an EPICS based con­trol sys­tem for sys­tem in­te­gra­tion and re­mote mon­i­tor and con­trol. The elec­tron bunch ar­rival time jit­ter is care­ful­ly mea­sured and an­a­lyzed with the help of a RF de­flect­ing cav­i­ty. This paper re­ports the de­vel­op­ment sta­tus, tech­ni­cal im­ple­men­ta­tion, and mea­sure­ment re­sults of the syn­chro­niza­tion and con­trol sys­tem.

• W. Liu, W.-H. Huang, C.-X. Tang, D. Wu
  TUB, Beijing

Smith-Pur­cell ra­di­a­tion (SPR) is emit­ted when an elec­tron pass­es near the sur­face of a pe­ri­od­ic metal­lic grat­ing. The ra­di­a­tion wave­length λ ob­served at the angle θ mea­sured from a di­rec­tion of sur­face grat­ing is de­ter­mined by λ=D/|n|(1/β cosθ), Where D is the grat­ing pe­ri­od, βc is the elec­tron ve­loc­i­ty, c is the speed of light, and the in­te­ger n is the spec­tral order. This ra­di­a­tion mech­a­nism is wide­ly ap­pli­cat­ed to THz ra­di­a­tion source, for which can be de­vel­oped into tun­able and com­pact one. In this paper, the ra­di­a­tion char­ac­ter­is­tics of ter­a­hertz (THz) SPR gen­er­at­ed from the ul­tra­short elec­tron beam are an­a­lyzed with the three-di­men­sion­al par­ti­cle-in-cell sim­u­la­tion. For ob­tain­ing the in­tense THz ra­di­a­tion, the grat­ing pa­ram­e­ters and that of ul­tra­short elec­tron beam are op­ti­mum. The ra­di­a­tion power and en­er­gy are ob­tained by the PIC sim­u­la­tion. The band width of train bunch­es is com­pared with that of sin­gle bunch. The for­ma­tion fac­tors in­clud­ing the lon­gi­tu­di­nal and trans­verse are cal­cu­lat­ed. Through this study, we ob­serve that the ra­di­a­tion power is en­hanced and the band width can be ad­just­ed.

• L.X. Yan, Q. Du, Y.-C. Du, Hua, J.F. Hua, W.-H. Huang, C. Liao, C.-X. Tang
  TUB, Beijing

Ul­tra­short rel­a­tivis­tic elec­tron beam can be ap­plied to pro­duce high power co­her­ent THz ra­di­a­tion by mech­a­nisms such as FEL, CSR, CTR et al. The THz mod­u­lat­ed elec­tron beams, or THz-rep­e­ti­tion-rate ul­tra­short elec­tron bunch trains ex­hib­it fur­ther en­hance­ment of co­her­ent THz ra­di­a­tion. This ar­ti­cle will re­port the ex­per­i­men­tal re­sults on the ul­tra­short elec­tron bunch train pro­duc­tion by cop­per based pho­to­cath­ode RF gun via di­rect UV laser pulse stack­ing using bire­frin­gent α-BBO crys­tal se­ri­als at our lab­o­ra­to­ry. The tem­po­ral pro­file of the elec­tron beam was mea­sured by de­flect­ing cav­i­ty. Space charge ef­fect down­stream the pho­to­cath­ode is sim­u­lat­ed. This shap­ing method of laser pulse by α-BBO crys­tals can also be ap­plied to form quasi flat­top UV laser pulse for re­duc­ing the ini­tial emit­tance of the elec­tron beam from the pho­to­cath­ode RF gun.