Back J

Faircloth D

Gabor C

Jolly S

Letchford A

Plostinar C

Pozimski J

English

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PARTICLE DYNAMICS CALCULATIONS AND EMITTANCEMEASUREMENTS AT THE FETS *J. Pozimski1+2 #,A. Letchford2 , J. Back3,  Dan Faircloth2, S. Jolly1 C. Gabor4, C. Plostinar41Imperial College London, United Kingdom,  2Rutherford Appleton Laboratory, Oxford, UnitedKingdom, 3Warwick University, Coventry, 4ASTeC United KingdomAbstractIn order to contribute to the development of high powerproton accelerators in the MW range, to prepare the wayfor an ISIS upgrade and to contribute to the UK designeffort  on neutrino factories [1,2],  a  front  end test  stand(FETS) is being constructed at the Rutherford AppletonLaboratory (RAL) in the UK [3]. The aim of the FETS isto demonstrate the production of a 60 mA, 2 ms, 50 ppschopped  beam at  3  MeV with  sufficient  beam quality.The  results  of  numerical  simulations  of  the  particledynamics  from the  charge  separation dipole  behind  theion source  to  the  end  of  the  MEBT will  be  presented.Previous measurements showed that the emittance of thebeam  delivered  by  the  ion  source  exceeded  ourexpectations by more than a factor of three [4]. Since thenvarious  changes  in  the  beam extraction/post  acceleratorregion reduced the beam emittance by nearly a factor oftwo.  Simulations  of  the  particle  dynamics  in  the  FETSbased on distributions gained from recent measurementsof the transversal beam emittance behind the ion sourcewill  be  presented  and  the  results  for  different  inputdistributions discussed.Figure 1: Schematic layout  of the FETS set up. For thepositions  indicated  (P1  behind  post  acceleration  at  theentrance of the LEBT [z=0mm], P2 at the entrance of theRFQ [z=1770mm], P3 at the RFQ exit [z=5770mm] andP4  at  the  end  of  the  MEBT [z=10580mm])  the  phasespace distribution of the beam has been determined withGPT and TRACEWIN.INITIAL PARTICLE DISTRIBUTIONThe  ion  source  development  program,  based  on  thehighly successful ISIS H− ion source at RAL, has alreadyshown encouraging results. The aim is to increase the ioncurrent from 35mA to 70mA, to increase the pulse lengthfrom 250μs to 2ms and to improve the beam quality [5].In order to compare the recent status of the improvementof the ion source emittance and the consequences for thebeam transport through the FETS end-to-end simulationshave been performed for an idealized (shown in figure 2)and a real  measured space phase distribution (shown infigure 3) based on pepper-pot emittance data.Figure 2: Input  particle distribution into the  LEBT  (P1)for  an  ideal  waterbag  beam  (input  1)  (εx,rms=0.25πmmmrad; εy,rms=0.25 πmmmrad).Figure 3: Input  particle distribution into the  LEBT  (P1)for  a  measured  beam  distribution  (input  2)  (εx,rms=0.58πmmmrad; εy,rms=0.52 πmmmrad).LEBT SIMULATION AND RESULTSA 3 solenoid LEBT system similar to the one used atthe ISIS injector is under construction and the calculated3D field distribution of the solenoids (figure 4) is used forparticle transport simulation of the LEBT using the GPTcode [6, 7].Figure 4: Left: one of three LEBT solenoids for FETS.Right: Magnetic  field distribution in  the z direction fordifferent radii.Proceedings of LINAC08, Victoria, BC, Canada MOP088Extreme Beams and Other Technologies 4D - Beam Dynamics, Computer Simulation, Beam Transport281The field strengths of the three solenoids are chosen tooptimise beam injection into the RFQ. The result of theseoptimisations for input distribution 1 is shown in figure 5.The  estimated  acceptance  of  the  RFQ  is  drawn  as  anellipse in the transversal phase space plots.  There is  notransversal  beam  loss  in  the  LEBT  and  the  totaltransmission  (including  stripping)  into  the  RFQacceptance is predicted to be above 90%.Figure  5:  Phase  space  distributions  in  the  transversalplane at the exit of the LEBT (P2) for input 1 (εx,rms=0.33πmmmrad; εy,rms=0.33 πmmmrad).The  beam  envelope  in  both  transversal  planes  in  theLEBT using distribution 2 is shown in figure 6 and thephase space distribution is shown in figure 7. Again theacceptance of the RFQ is drawn. Figure 6:Beam envelope along the length z of the LEBTusing  the  input  distribution  from  the  measured  beam.Dotted vertical lines show the drift and solenoid sections.Due  to  the  relatively  large  size  of  the  beam  in  thesolenoids  compared  to  their  aperture  (80%)  non  linearfields cause aberrations (already indicated by the slightlyS-shaped  phase  space  distribution  in  figure  5)  andemittance  growth.  While  the  absolute  increase  of  theemittance  for  the  waterbag  input  distribution  withΔεx,y,rms=0.08 πmmmrad is smaller than for the real beam (Δεx,rms=0.11  and  Δεy,rms=0.12  πmmmrad),  the  penaltyseems to be rather moderate.  Figure  7:  Phase  space  distributions  in  the  transversalplane at the exit of the LEBT (P2) for input 2 (εx,rms=0.69πmmmrad; εy,rms=0.64 πmmmrad).Even  for  the  presented  very  preliminary  first  data  theresult is surprisingly good and a total transmission intothe  RFQ  acceptance  is  predicted  to  be  50%.   Thesimulations performed also show that, independent of thedegree  of  space  charge  compensation  and  using  theavailable measured beam emittances, it is always possibleto inject the core of the beam into the RFQ by just tuningthe solenoid strengths.PARTICLE TRANSPORT CALCULATIONIN THE RFQA preliminary design of the 324 MHz RFQ has beenused for particle dynamics simulations. The variation ofthe RFQ parameters along z is  shown in Figure 8. Thedesign was made for an optimized transmission of morethan 90% under the estimate of an input beam emittanceof 0.25  πmmmrad. In a first run the particle distributiongained  from  the  GPT  simulations  of  the  LEBT  withgenerated data was used for input into the RFQ.  Figure 8: Development of the main RFQ parameters forthe FETS along z.The output distributions of this simulation are shown infigure 9. The transmission through the RFQ was ~96% forall  energies  and  ~91%  for  an  energy  of  3MeV.  Thereduction  of  the  emittance  by  15%  is  caused  by  theparticle  losses.  Considering  this  input  distribution  thedesign is likely to fulfil the requirements for the FETS. Figure  9:  Particle  distribution  behind  the RFQ (P3)  forinput 1 (εx,rms=0.28 πmmmrad; εy,rms=0.27 πmmmrad). Using the second data set  and taking into  account  that,while  improved by nearly  a  factor  of two  over  the  lastyear, the input emittance is still a factor of two larger thanthe  one  used  for  the  design,  the  achievable  RFQtransmission of ~52% for all energies (50.5% for 3 MeV)seems  to  be  reasonable  but  fails  to  meet  the  FETSspecifications.  The  transversal  output  distributions  areshown in figure 10. The emittance is reduced by up to 44% due to the large particle losses  but still  30 % largercompared to the results for the waterbag inputMOP088 Proceedings of LINAC08, Victoria, BC, CanadaExtreme Beams and Other Technologies2824D - Beam Dynamics, Computer Simulation, Beam TransportFigure 10: Particle distribution behind the RFQ (P3) forinput 2 (εx,rms=0.46 πmmmrad; εy,rms=0.47 πmmmrad).PARTICLE TRANSPORT CALCULATIONIN THE MEBTA preliminary design  of  the  MEBT consisting  of  11quadrupoles,  4  bunching  cavities  and  a  slow and  fastchopper  with  accompanying  beam  dumps  is  shown  inFigure 11 [8].Figure 11: Design of the MEBT. The output distributions of the simulation are shown infigure 12 and 13. For the ideal case the emittance growthis  ~7% in  x,x'  and  ~20%  in  y,y'.  The  transmission  is~98%.Figure 12: Particle distribution at the end of the MEBT(P4)  for  input  distribution  1  (εx,rms=0.30  πmmmrad;εy,rms=0.34 πmmmrad)For the real beam the emittance growth is -13% in thex,x' space (due to losses) and 4 % in the y,y' space. Thetransmission is reduced to ~89%.Figure 13: Particle distribution at the end of the MEBT(P4)  for  input  distribution  1  (εx,rms=0.40  πmmmrad;εy,rms=0.49 πmmmrad).Figure 14: Development of Transmission (solid, circles),and  transversal  emittances  (x  is  dashed,  triangles,  ydotted,  squares),  along  beam  propagation  for  ideal(green),  initial  (red)  and  new  (blue)  real  inputdistributions.DISCUSSIONThe first  simulations  from the  ion source  exit  to  theoutput of the MEBT show a mixed picture. The presentedresults based on an artificial input distribution are quiteencouraging and with a total transmission from the sourceto the exit of the MEBT of >90% (100% in the LEBT,95% in the RFQ and 98% in the MEBT, see figure 14) thegoals are nearly reached. This result further proves thatthe design of the individual sections is sound and the jointperformance is satisfactory.  On the other hand, the latestresults  using  a  measured  initial  distribution  show  adramatic  increase  of  the  beam  emittance  with  fatalconsequences  on  beam  transmission.  For  the  currentstatus the transmission is expected to be ~46% (100% inthe LEBT, 52% in the RFQ and 89% in the MEBT). Dueto the large beam losses in the RFQ the total emittancegrowth is moderate (-10% in x and 23% in y).  Seriousefforts to reduce the ion source emittance and improve theRFQ  acceptance  are  under  way  and  recent  work  hasalready increased the transmission through the RFQ by10%, but further progress is requiredREFERENCES[1] T R Edgecock, 6th Int. Workshop Neutrino Factories& Superbeams (NuFact04), July/August 2004, Osaka[2] Neutron News, vol. 15 (2004), ISSN 1044-8632. Seealso http://www.isis.rl.ac.uk/.[3] Alan Letchford et al., EPAC06, June 2006, Edinburgh[4]  J.  Pozimski  et  al,  Proceedings  of  LINAC  2006,Knoxville, Tennessee USA, p 403 ff[5]  S  Lawrie  et  al,  Proceedings  of  NIBS2008,  Aix  enProvence, France, to be publ. (AIP)[6] John Back et al, EPAC2006, June 2006, Edinburgh [7]GPT  User  Manual,  Pulsar  Physics,http://www.pulsar.nl/gpt/[8]  C.  Plostinar  et  al.,  Proc.  of  PAC'07,  Albuquerque,NM, USA, June 2007, p 1646 ff..Proceedings of LINAC08, Victoria, BC, Canada MOP088Extreme Beams and Other Technologies 4D - Beam Dynamics, Computer Simulation, Beam Transport283

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Particle dynamics calculations and emittance measurements at the FETS

Particle dynamics calculations and emittance measurements at the FETS

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