• K. Nanbu, H. Hama, F. Hinode, M. Kawai, F. Miyahara, T. Muto, Y. Tanaka
  Tohoku University, School of Scinece, Sendai
• S. Kashiwagi
  Tohoku University, Research Center for Electron Photon Science, Sendai

A thermion­ic RF gun has been de­vel­oped to gen­er­ate very short elec­tron bunch for a THz light source at To­hoku Uni­ver­si­ty. Bunch com­pres­sion scheme re­quires, in gen­er­al, lin­ear mo­men­tum dis­tri­bu­tion of the par­ti­cles with re­spect to the lon­gi­tu­di­nal po­si­tion, so that mea­sure­ment of lon­gi­tu­di­nal phase space is sig­nif­i­cant for bet­ter bunch com­pres­sion. How­ev­er, such mea­sure­ment for the low en­er­gy elec­trons is dif­fi­cult be­cause space charge ef­fect is so strong that longer drift space should not be in­clud­ed. Con­se­quent­ly, we have per­formed de­lib­er­a­tion for em­ploy­ing en­er­gy de­pen­dent an­gu­lar dis­tri­bu­tion of Cherenkov ra­di­a­tion. Though the en­er­gy de­pen­dence of emis­sion angle of Cherenkov ra­di­a­tion is rapid­ly get­ting small as in­creas­ing the beam en­er­gy, it is still 25 deg/MeV at an en­er­gy around 2.0 MeV when we use ra­di­a­tor hav­ing re­frac­tive index of 1.035. Thus the beam en­er­gy dis­tri­bu­tion can be mea­sured if we ob­serve Cherenkov ring with suf­fi­cient an­gu­lar res­o­lu­tion. Since this method needs only thin ra­di­a­tor, the drift space length will be min­i­mized. We will dis­cuss lim­i­ta­tion for res­o­lu­tions of both the time and the en­er­gy as well.

• N.Y. Huang, S.S. Yang
  NTHU, Hsinchu
• H. Hama
  Tohoku University, School of Scinece, Sendai
• W.K. Lau
  NSRRC, Hsinchu

De­sign of an in­tense but tight­ly fo­cused ul­tra­short elec­tron beam for pro­duc­tion of sub-hun­dred fem­tosec­ond x-ray puls­es that based on head-on in­verse Comp­ton scat­ter­ing (ICS) has been stud­ied. The three di­men­sion­al (3D) space charge dy­nam­ics has been tracked and op­ti­mized through­out the whole beam­line. It is found that the fo­cus­ing ul­tra­short elec­tron puls­es as short as 67 fs can be pro­duced by com­press­ing the en­er­gy-chirped beam from a thermion­ic cath­ode rf gun with an alpha mag­net and linac op­er­at­ing at in­jec­tion phase near zero cross­ing. This mul­ti-bunch elec­tron beam has an in­ten­si­ty of 30 pC per bunch and is ac­cel­er­at­ed to 27 MeV with an S-band linac struc­ture. The com­pressed elec­tron beam is fo­cused to 64 μm for scat­ter­ing with an 800 nm, 3.75 mJ laser in the laser-beam in­ter­ac­tion cham­ber. With this method, total peak flux of back-scat­tered x-ray pho­tons ex­ceeds 10¹⁸ pho­tons/sec is achiev­able with the short­est wave­length of 0.7 Å.

• F. Hinode, H. Hama, M. Kawai, F. Miyahara, T. Muto, K. Nanbu, H. Oohara, Y. Tanaka
  Tohoku University, School of Scinece, Sendai
• S. Kashiwagi
  Tohoku University, Research Center for Electron Photon Science, Sendai

A test ac­cel­er­a­tor for the co­her­ent ter­a­hertz source (t-ACTS) has been under de­vel­op­ment at Lab­o­ra­to­ry of Nu­cle­ar Sci­ence, To­hoku Uni­ver­si­ty*. In­tense co­her­ent ter­a­hertz ra­di­a­tion will be gen­er­at­ed by the very short elec­tron bunch less than 100 fs using a thermion­ic RF gun (ITC RF-gun). ITC RF-gun is de­signed to have two cells un­cou­pled with each other, so that it can be op­er­at­ed at var­i­ous com­bi­na­tions of dif­fer­ent rf-pow­er level and phase dif­fer­ence so as to op­ti­mize the lon­gi­tu­di­nal phase space dis­tri­bu­tion of the elec­tron beam for bunch com­pres­sion**. The gun em­ploys sin­gle-crys­tal LaB6 cath­ode with small di­am­e­ter of 1.8 mm to ob­tain a very small ini­tial emit­tance with suf­fi­cient­ly high cur­rent den­si­ty. The RF gun has been al­ready man­u­fac­tured and the mea­sure­ment of RF char­ac­ter­is­tics is now in progress. We will pre­sent the re­sults of low-pow­er mea­sure­ment and also dis­cuss the ef­fect of the cath­ode mis­align­ment on the beam pa­ram­e­ters such as trans­verse emit­tance and lon­gi­tu­di­nal phase space dis­tri­bu­tion.

* H. Hama et al., New J. Phys. 8 (2006) 292.
** H. Hama et al., Nucl. Instr. and Meth., A 528, (2004) 371.

• F. Miyahara, H. Hama, F. Hinode, M. Kawai, T. Muto, K. Nanbu, H. Oohara, Y. Tanaka
  Tohoku University, School of Scinece, Sendai

We have stud­ied pro­duc­tion of the very short-bunch elec­tron beam to gen­er­ate in­tense co­her­ent THz ra­di­a­tion*. The bunch length of 100 fs is re­quired to pro­duce CSR around 1 THz. The beam from the thermion­ic RF-gun is in­tro­duced into the bunch com­pres­sion sys­tem con­sist of an alpha mag­net and a linac. The alpha mag­net is often used as a bunch com­pres­sor for elec­tron en­er­gy of sev­er­al MeV. How­ev­er, for our sys­tem, the alpha mag­net plays a role of the lon­gi­tu­di­nal phase space ro­ta­tor and en­er­gy fil­ter. The bunch com­pres­sion is done in the linac em­ploy­ing ve­loc­i­ty bunch­ing. The beam is in­ject­ed on near the ze­ro-cross phase of the RF field in the linac, and then the beam phase slip to­ward the crest. The lon­gi­tu­di­nal phase space and beam phase with re­spect to RF field at the en­trance of the linac are op­ti­mized so that the bunch length would be min­i­mum. In cur­rent anal­y­sis using nu­mer­i­cal sim­u­la­tion based on the GPT code**, an rms bunch length of 30 fs has been ob­tained for a bunch charge of 20 pC. We will dis­cuss the bunch com­pres­sion scheme and the beam dy­nam­ics in the sys­tem. Prospect of the co­her­ent ra­di­a­tion from the beam will be also re­port­ed.

* H. Hama et al, Proc. Ultrashort Electron & Photon Beam Techniques and Applications, Xian, China (2009)
** General Particle Tracer (GPT), URL: http://www.pulsar.nl/gpt

• H. Hama
  Tohoku University, School of Scinece, Sendai
• N.Y. Huang
  NTHU, Hsinchu

In com­pact linac sys­tem, alpha mag­net seems to be a use­ful de­vice to ma­nip­u­late the lon­gi­tu­di­nal phase space. Par­tic­u­lar­ly com­bined use with thermion­ic RF gun has been re­gard­ed as a con­ve­nient sys­tem for bunch com­pres­sion. The alpha mag­net sim­ply acts to ro­tate the lon­gi­tu­di­nal phase space of the beam, be­sides en­er­gy se­lec­tion by an aper­ture in it. How­ev­er, by using the alpha mag­net, if we like to pro­duce high bril­liant elec­tron beam with con­sid­er­able charge, space charge force has to be care­ful­ly taken into ac­count to eval­u­ate the beam prop­er­ty for not only the lon­gi­tu­di­nal but also the trans­verse. Since the both trans­verse mo­tions and the lon­gi­tu­di­nal one are cou­pled with each other in the alpha mag­net, it is most­ly im­pos­si­ble to eval­u­ate the space charge ef­fect an­a­lyt­i­cal­ly. Mean­while, be­cause en­er­gies of the elec­trons from the thermion­ic RF gun are rang­ing from zero to the max­i­mum, a con­ven­tion­al way to count Coulomb force in the rest frame may be not sat­is­fac­to­ri­ly valid in nu­mer­i­cal sim­u­la­tions. We will dis­cuss space charge dom­i­nat­ed phase spaces de­rived from 3-D track­ing sim­u­la­tions* for the alpha mag­net. *GPT (Gen­er­al Par­ti­cle Trac­er) and an FDTD code de­vel­oped our­selves.