• A. Kanareykin, C.-J. Jing, A.L. Kustov, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio
• A. Altmark
  LETI, Saint-Petersburg
• W. Gai, J.G. Power
  ANL, Argonne

Beam breakup (BBU) ef­fects re­sult­ing from par­a­sitic wake­fields are a se­ri­ous lim­i­ta­tion to the per­for­mance of di­elec­tric struc­ture based ac­cel­er­a­tors. We re­port here on nu­mer­i­cal stud­ies and ex­per­i­men­tal in­ves­ti­ga­tions of BBU and its mit­i­ga­tion. An ex­per­i­men­tal pro­gram is un­der­way at the Ar­gonne Wake­field Ac­cel­er­a­tor fa­cil­i­ty that will focus on BBU mea­sure­ments in di­elec­tric wake­field de­vices. We ex­am­ine the use of ex­ter­nal FODO chan­nels for con­trol of the beam in the pres­ence of strong trans­verse wake­fields. We pre­sent cal­cu­la­tions based on a par­ti­cle-Green's func­tion beam dy­nam­ics code (BBU-3000) that we are de­vel­op­ing. We will re­port on new fea­tures of the code in­clud­ing the abil­i­ty to treat space charge. The BBU code is being in­cor­po­rat­ed into a soft­ware  frame­work that will sig­nif­i­cant­ly in­crease its util­i­ty (Beam Dy­nam­ics Sim­u­la­tion Plat­form). The plat­form is based on the very flex­i­ble Boinc soft­ware en­vi­ron­ment de­vel­oped orig­i­nal­ly at Berke­ley for the SETI@​home pro­ject. The pack­age can han­dle both task farm­ing on a het­ero­ge­neous clus­ter of net­worked com­put­ers and com­put­ing on a local grid. User ac­cess to the plat­form is through a web brows­er.

• W. Gai, M.E. Conde, J.G. Power
  ANL, Argonne
• C.-J. Jing
  Euclid TechLabs, LLC, Solon, Ohio

In this paper, we pre­sent a lin­ear col­lid­er con­cept based on drive beam gen­er­a­tion from an RF pho­toin­jec­tor, and em­ploy­ing di­elec­tric struc­tures for power ex­trac­tion and ac­cel­er­a­tion. The col­lid­er is based on a mod­u­lar de­sign with each mod­ule pro­vid­ing 100 GeV net ac­cel­er­a­tion. A high cur­rent drive beam is pro­duced using a low fre­quen­cy RF gun (~ 1GHz), and sub­se­quent­ly ac­cel­er­at­ed to ~1 GeV using con­ven­tion­al stand­ing wave cav­i­ties. High fre­quen­cy (20 GHz) RF power, ex­tract­ed from the drive beam using a low impedance di­elec­tric struc­ture, is used to power the main linacs, which are based on high impedance high gra­di­ent di­elec­tric load­ed ac­cel­er­at­ing struc­tures. We en­vi­sion this scheme will pro­duce high gra­di­ents (300 MeV/m), lead­ing to a very com­pact de­sign. The mod­u­lar­i­ty of the de­sign will allow a staged con­struc­tion that will en­able ex­ten­sion to mul­ti-TeV en­er­gies.

• S.P. Antipov, W. Gai, O. Poluektov
  ANL, Argonne
• C.-J. Jing, A. Kanareykin, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio

As the di­men­sions of ac­cel­er­at­ing struc­tures be­come small­er and beam in­ten­si­ties high­er, the trans­verse wake­fields driv­en by the beam be­come quite large with even a slight mis­align­ment of the beam. These de­flec­tion modes can cause in­ter-bunch beam breakup and in­tra-bunch head-tail in­sta­bil­i­ties along the beam path. We pro­pose a built-in tun­able ab­sorp­tion mech­a­nism for damp­ing the par­a­sitic trans­verse modes with­out af­fect­ing the op­er­a­tional modes in di­elec­tric load­ed ac­cel­er­at­ing (DLA) struc­tures and wake­field power ex­trac­tors. The new prin­ci­ple for HOM ab­sorp­tion is based on elec­tron para­m­ag­net­ic res­o­nance. The di­elec­tric tube of the DLA has to be doped with a ma­te­ri­al ex­hibit­ing high EPR, for ex­am­ple ruby, Al2O3 over­doped ~1% with Cr³⁺. The ab­sorp­tion fre­quen­cy can be tuned by an ex­ter­nal DC mag­net­ic field to match the fre­quen­cy of the trans­verse mode. At the res­o­nance imag­i­nary part of per­me­abil­i­ty be­comes sig­nif­i­cant and the di­elec­tric tube acts as an ab­sorber for the trans­verse modes. The ex­ter­nal DC mag­net­ic field is solenoidal and has to have a mag­ni­tude of about 3 kG. This con­fig­u­ra­tion in fact is de­sir­able to focus the beam and pro­vide ad­di­tion­al con­trol of beam break up.

• W. Liu, W. Gai
  ANL, Argonne
• L. Rinolfi
  CERN, Geneva
• J. Sheppard
  SLAC, Menlo Park, California

We pro­pose a vi­able positron source scheme that uses cir­cu­lar­ly po­lar­ized gamma rays gen­er­at­ed from the main 250 GeV elec­tron beam. The beam pass­es through a he­li­cal su­per­con­duct­ing un­du­la­tor with a mag­net­ic field of ~ 1 Tesla and a pe­ri­od of a few cen­time­ters. The gam­ma-rays pro­duced in the un­du­la­tor in the en­er­gy range be­tween ~ 3 MeV ~ 100 MeV will be di­rect­ed to a ti­ta­ni­um tar­get and pro­duces po­lar­ized positrons. The positrons are then cap­tured, ac­cel­er­at­ed and trans­port­ed to a damp­ing ring. De­tailed pa­ram­e­ter stud­ies of this scheme in­clud­ing positron yield, un­du­la­tor pa­ram­e­ter de­pen­dence and tar­get com­po­si­tion and ge­om­e­try will be pre­sent­ed. Ef­fects on the 250 GeV drive beam, in­clud­ing emit­tance, en­er­gy spread and en­er­gy loss from the beam pass­ing through the un­du­la­tor will also be dis­cussed.

• W. Liu, W. Gai
  ANL, Argonne

We pre­sent an up­date on the ANL ILC positron source study. We ex­am­ined the im­pact of dif­fer­ent drive beam en­er­gies on the positron yield and po­lar­iza­tion for the ILC RDR base­line un­du­la­tor. The e⁺ yield is found to drop rapid­ly as the drive beam en­er­gy is re­duced. We stud­ied dif­fer­ent un­du­la­tor pa­ram­e­ters for their ef­fect on the positron yield and po­lar­iza­tion when work­ing at lower drive beam en­er­gies. Using a lower K (B field level) can in­crease the pho­ton en­er­gy, but it is still very dif­fi­cult to bring the yield up for low drive beam en­er­gies. For 250 GeV drive beam op­tions, we stud­ied the RDR un­du­la­tor per­for­mance as a func­tion of K. In­stead of pow­er­ing off some sec­tions of the un­du­la­tor, one can also con­sid­er low­er­ing the B field to bring the positron yield back to the de­sired 1.5 e+/e-. We also stud­ied the liq­uid lead tar­get op­tion for ILC positron source and the en­er­gy de­po­si­tion in the ref­er­ence de­sign Ti tar­get wheel.

• J.G. Power, M.E. Conde, W. Gai
  ANL, Argonne
• Z. Li
  SLAC, Menlo Park, California
• D. Mihalcea
  Northern Illinois University, DeKalb, Illinois

We re­port on the de­sign of a 7 cell, stand­ing wave, 1.3 GHz LINAC cav­i­ty and the as­so­ci­at­ed beam dy­nam­ics stud­ies for the up­grade of the drive beam­line for the Ar­gonne Wake­field Ac­cel­er­a­tor (AWA) fa­cil­i­ty. The LINAC de­sign is a com­pro­mise be­tween sin­gle bunch op­er­a­tion (100 nC @ 75 MeV) and min­i­miz­ing the en­er­gy droop due to beam load­ning along the bunch train dur­ing bunch train op­er­a­tion. The 1.3 GHz drive bunch train tar­get pa­ram­e­ters are: 75 MeV, 10-20 ns macropulse du­ra­tion, 16x60nC mi­crobunch­es; this is equiv­a­lent to a macropulse cur­rent and beam power of 80 Amps and 6 GW, re­spec­tive­ly. Each LINAC struc­ture ac­cel­er­ates ap­prox­i­mate­ly 1000 nC in 10 ns by a volt­age of 11 MV at an RF power of 10 MW. Due to the short bunch train du­ra­tion de­sired (~10 ns) and the ex­ist­ing fre­quen­cy (1.3 GHz), com­pen­sa­tion of the en­er­gy droop along the bunch train is dif­fi­cult to ac­com­plish with the two stan­dard tech­niques: time-do­main or fre­quen­cy-do­main beam load­ing com­pen­sa­tion. There­fore, to min­i­mize the en­er­gy droop, our de­sign is based on a large stored en­er­gy LINACs. In this paper, we pre­sent our LINAC op­ti­miza­tion method, de­tailed LINAC de­sign, and beam dy­nam­ics stud­ies of the drive beam­line.

• M.E. Conde, S.P. Antipov, W. Gai, R. Konecny, W. Liu, J.G. Power, Z.M. Yusof
  ANL, Argonne
• C.-J. Jing
  Euclid TechLabs, LLC, Solon, Ohio

The AWA Fa­cil­i­ty is ded­i­cat­ed to the study of ad­vanced ac­cel­er­a­tor con­cepts based on elec­tron beam driv­en wake­fields. The fa­cil­i­ty em­ploys an L-band pho­to­cath­ode RF gun to gen­er­ate high charge short elec­tron bunch­es, which are used to drive wake­fields in di­elec­tric load­ed struc­tures, as well as in metal­lic struc­tures. Ac­cel­er­at­ing gra­di­ents as high as 100 MV/m have been reached in di­elec­tric struc­tures, and RF puls­es of up to 44 MW have been gen­er­at­ed at 7.8 GHz. In order to reach high­er ac­cel­er­at­ing gra­di­ents and high­er RF power lev­els, sev­er­al up­grades are un­der­way: (a) a new RF gun with high­er QE pho­to­cath­ode will re­place the pre­sent drive gun; (b) the ex­ist­ing RF gun will gen­er­ate a wit­ness beam to probe the wake­fields; (c) three new 25 MW L-band RF power sta­tions will be added to the fa­cil­i­ty; (d) five ad­di­tion­al linac struc­tures will bring the beam en­er­gy up from 15 MeV to 75 MeV. The drive beam will con­sist of bunch trains of up to 32 bunch­es, with up to 60 nC per bunch. The goal of fu­ture ex­per­i­ments is to reach ac­cel­er­at­ing gra­di­ents of sev­er­al hun­dred MV/m and to ex­tract RF puls­es with GW power level.

• C.-J. Jing
  Euclid TechLabs, LLC, Solon, Ohio
• S.P. Antipov, M.E. Conde, W. Gai, F. Gao, J.G. Power, Z.M. Yusof
  ANL, Argonne
• H. Chen, C.-X. Tang, S.X. Zheng
  TUB, Beijing
• P. Xu
  Tsinghua University, Beijing

Wake­field ex­ci­ta­tion has been ex­per­i­men­tal­ly stud­ied in a 3-cell X-band stand­ing wave Pho­ton­ic Band Gap (PBG) ac­cel­er­at­ing struc­ture. Major monopole (TM01- and TM02-like) and dipole (TM11- and TM12-like) modes were in­den­ti­fied and char­ac­ter­ized by pre­cise­ly con­trol­ling the po­si­tion of beam in­jec­tion. The qual­i­ty fac­tor Q of the dipole modes was mea­sured to be ~10 times small­er than that of the ac­cel­er­at­ing mode. A charge sweep, up to 80 nC, has been per­formed, equiv­a­lent to ~30 MV/m ac­cel­er­at­ing field on axis. A vari­able delay low charge wit­ness bunch fol­low­ing a high charge drive bunch was used to cal­i­brate the gra­di­ent in the PBG struc­ture by mea­sur­ing its max­i­mum en­er­gy gain and loss. Ex­per­i­men­tal re­sults agree well with nu­mer­i­cal sim­u­la­tions.

• C.-J. Jing, F. Gao, A. Kanareykin, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio
• M.E. Conde, W. Gai, R. Konecny, J.G. Power
  ANL, Argonne

High fre­quen­cy, high power rf sources are need­ed for many ap­pli­ca­tions in par­ti­cle ac­cel­er­a­tors, com­mu­ni­ca­tions, radar, etc. We have de­vel­oped a 26GHz high power rf source based on the ex­trac­tion of wake­fields from a rel­a­tivis­tic elec­tron beam. The ex­trac­tor is de­signed to cou­ple out rf power gen­er­at­ed from a high charge elec­tron bunch train travers­ing a di­elec­tric load­ed waveg­uide. The first high beam ex­per­i­ment has been per­formed at Ar­gonne Wake­field Ac­cel­er­a­tor fa­cil­i­ty. The ex­per­i­men­tal re­sults suc­cess­ful­ly demon­strate the 15ns 26GHz rf pulse gen­er­at­ed from the wake­field ex­trac­tor with a bunch train of 16 bunch­es. Mean­while, ~ 30MW short rf pulse has been achieved with a bunch train of 4 bunch­es. Beam Breakup has pre­vent­ed charge trans­port through the power ex­trac­tor be­yond 10nC. We are doing sim­u­la­tions and de­vel­op­ing meth­ods to al­le­vi­ate the BBU ef­fect.

• C.-J. Jing, A. Kanareykin, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio
• M.E. Conde, W. Gai, J.G. Power
  ANL, Argonne
• E. Nenasheva
  Ceramics Ltd., St. Petersburg

Di­elec­tric-Load­ed Ac­cel­er­at­ing (DLA) struc­tures are gen­er­al­ly lack of ap­proach­es to tune fre­quen­cy after the fab­ri­ca­tion. A tun­able DLA struc­ture has been de­vel­oped by using an extra non­lin­ear fer­ro­elec­tric layer. Di­elec­tric con­stant of the ap­plied fer­ro­elec­tric ma­te­ri­al is sen­si­tive to tem­per­a­ture and DC volt­age. Bench test shows the +14MHz/°C, and 6MHz fre­quen­cy tun­ing range for a 25kV/cm of DC bias field. A beam test is planned at Ar­gonne Wake­field Ac­cel­er­a­tor fa­cil­i­ty be­fore the IPAC con­fer­ence. De­tailed re­sults will be re­port­ed.