• T. Mastorides, J.D. Fox, C.H. Rivetta, D. Van Winkle
  SLAC, Menlo Park, California
• P. Baudrenghien, A.C. Butterworth, J.C. Molendijk
  CERN, Geneva

Radio Fre­quen­cy (RF) ac­cel­er­at­ing sys­tem noise and non-ide­al­i­ties can have detri­men­tal im­pact on the LHC per­for­mance through lon­gi­tu­di­nal mo­tion and lon­gi­tu­di­nal emit­tance growth. A the­o­ret­i­cal for­mal­ism has been de­vel­oped to re­late the beam and RF loop dy­nam­ics with the bunch length growth [1]. Mea­sure­ments were con­duct­ed at LHC to val­i­date the for­mal­ism, de­ter­mine the per­for­mance lim­it­ing RF com­po­nents, and pro­vide the foun­da­tion for beam dif­fu­sion es­ti­mates for high­er en­er­gies and in­ten­si­ties. A brief sum­ma­ry of these re­sults is pre­sent­ed in this work.

[1] T. Mastorides et. al., "RF system models for the LHC with Application to
Longitudinal Dynamics", prepared for submission to Physical Review ST-AB.

• D. Van Winkle, J.D. Fox, T. Mastorides, C.H. Rivetta
  SLAC, Menlo Park, California
• P. Baudrenghien, A.C. Butterworth, J.C. Molendijk
  CERN, Geneva

The LHC Low Level RF sys­tem (LLRF) is a com­plex mul­ti-loop sys­tem used to reg­u­late the su­per­con­duc­tive cav­i­ty gap volt­age as well as to re­duce the impedance pre­sent­ed by RF sta­tions to the beam. The RF sys­tem can have a pro­found im­pact on the sta­bil­i­ty of the beam; a mis-con­fig­ured RF sys­tem has the po­ten­tial of caus­ing lon­gi­tu­di­nal in­sta­bil­i­ties, beam dif­fu­sion and beam loss. To con­fig­ure the RF sta­tion for op­er­a­tion, a set of pa­ram­e­ters in the LLRF mul­ti-loop sys­tem have to be de­fined. Ini­tial sys­tem com­mis­sion­ing as well as on­go­ing op­er­a­tion re­quires a con­sis­tent method of com­put­er based re­mote mea­sure­ment and mod­el-based de­sign of each RF sta­tion feed­back sys­tem. This paper de­scribes the suite of Mat­lab tools used for con­fig­ur­ing the LHC RF sys­tem dur­ing the start up in Nov2009-Feb2010. We pre­sent a brief overview of the tool, ex­am­ples of com­mis­sion­ing re­sults, and ba­sics of the mod­el-based de­sign al­go­rithms. This work com­ple­ments our pre­vi­ous pre­sen­ta­tion [1], where the al­go­rithms and method­ol­o­gy fol­lowed in the tools were de­scribed.

[1] D. Van Winkle et. al. 'Feedback Configuration Tools for LHC Low Level RF System,' PAC'09, Vancouver, Canada, May 2009, THZCH03, p. 249 (2009); http://www. JACoW.org.

• J.-L. Vay, J.M. Byrd, M.A. Furman, G. Penn, R. Secondo, M. Venturini
  LBNL, Berkeley, California
• J.D. Fox, C.H. Rivetta
  SLAC, Menlo Park, California

Elec­tron clouds im­pose lim­i­ta­tions on cur­rent ac­cel­er­a­tors that may be more se­vere for fu­ture ma­chines, un­less ad­e­quate mea­sures of mit­i­ga­tion are taken. Re­cent­ly, it has been pro­posed to use feed­back sys­tems op­er­at­ing at high fre­quen­cy (in the GHz range) to damp sin­gle-bunch trans­verse co­her­ent os­cil­la­tions that may oth­er­wise be am­pli­fied dur­ing the in­ter­ac­tion of the beam with am­bi­ent elec­tron clouds. We have used the sim­u­la­tion pack­age WARP-POSINST to study the growth rate and fre­quen­cy pat­terns in space-time of the elec­tron cloud driv­en trans­verse in­sta­bil­i­ty in the CERN SPS ac­cel­er­a­tor with, or with­out, an ide­al­ized feed­back model for damp­ing the in­sta­bil­i­ty. We will pre­sent our lat­est sim­u­la­tion re­sults, con­trast them with ac­tu­al mea­sure­ments and dis­cuss the im­pli­ca­tions for the de­sign of the ac­tu­al feed­back sys­tem.

Slides

• J.D. Fox, A. Bullitt, T. Mastorides, G. Ndabashimiye, C.H. Rivetta, O. Turgut, D. Van Winkle
  SLAC, Menlo Park, California
• J.M. Byrd, M.A. Furman, J.-L. Vay
  LBNL, Berkeley, California
• R. De Maria
  BNL, Upton, Long Island, New York
• W. Höfle, G. Rumolo
  CERN, Geneva

The SPS at high in­ten­si­ties ex­hibits trans­verse sin­gle-bunch in­sta­bil­i­ties with sig­na­tures con­sis­tent with an Ecloud driv­en in­sta­bil­i­ty. We pre­sent re­cent MD data from the SPS, de­tails of the in­stru­ment tech­nique and spec­tral anal­y­sis meth­ods which help re­veal com­plex ver­ti­cal mo­tion that de­vel­ops with­in a sub­set of the in­ject­ed bunch trains. The beam mo­tion is de­tect­ed via wide-band ex­po­nen­tial taper striplines and delta-σ hy­brids. The raw sum and dif­fer­ence data is sam­pled at 50 GHz with 1.8 GHz band­width. Slid­ing win­dow FFT tech­niques and RMS mo­tion tech­niques show the de­vel­op­ment of large ver­ti­cal tune shifts on por­tions of the bunch of near­ly 0.025 from the base tune of 0.185. Re­sults are pre­sent­ed via spec­tro­grams and rms bunch slice tra­jec­to­ries to il­lus­trate de­vel­op­ment of the un­sta­ble beam and time scale of de­vel­op­ment along the in­ject­ed bunch train. The study shows that the grow­ing un­sta­ble mo­tion oc­cu­pies a very broad fre­quen­cy band of 1.2 GHz. These mea­sure­ments are com­pared to nu­mer­i­cal sim­u­la­tion re­sults, and the sys­tem pa­ram­e­ter im­pli­ca­tions for an Ecloud feed­back sys­tem are out­lined.