Baričevič, B.

Cargnelutti, M.

Mostacci, Andrea

Pioli, Stefano

Serio, M.

Stella, A.

Variola, A.

English

Baričevič, B.

MOSTACCI, Andrea

PIOLI, STEFANO

Archivio della ricerca- Università di Roma La Sapienza

DESIGN AND SIMULATIONS OF THE CAVITY BPM READOUT ELECTRONICS FOR THE ELI-NP GAMMA BEAM SYSTEM M. Cargnelutti, B. Baričevič, Instrumentation Technologies, Solkan, SloveniaA. Mostacci, Sapienza Università di Roma, Rome, ItalyS. Pioli, A. Stella, A. Variola, M.Serio, INFN-LNF, Frascati, ItalyAbstractThe  Extreme  Light  Infrastructure  –  Nuclear  Physics (ELI-NP) facility will provide a high intensity laser and a very intense gamma beam which will be used in a broad range  of  experiments.  The  gamma  beam  is  obtained through  incoherent  Compton  back-scattering  of  a  laser light off a high brightness electron beam provided by a 720MeV warm LINAC. Electrons are accelerated in trains with up to 32 bunches, each one separated by 16ns. In the laser-electron interaction region, every bunch needs to be monitored with a resolution below 1μm RMS. To  achieve  this  performance,  a  low-Q  cavity  beam position  monitor  will  be  used  in  combination  with  a dedicated data acquisition system able to perform bunch-by-bunch  beam  position  measurements  with  sub-μm resolution. Using fast A/D converters and specific digital filtering,  the  readout  system  proposes  an  alternative measurement concept. The requirements of the system, its design  and  the  results  from  the  simulations  will  be presented.INTRODUCTIONThe relativistic electron beam required to generate the ELI-NP high-intensity gamma beam will be provided by a warm  LINAC.  The  accelerator  consists  of  an  S-band photo-injector followed by C-band accelerating structures, combining a  conservative  injector  design  with  compact accelerating  structures  [1].  At  every  injection  up to  32 electron  bunches  are  accelerated  and  delivered  to  two interaction  points  (IPs)  with  energies  of  280MeV  and 720MeV.  The  parameters  describing  the  electron  beam characteristics are listed in Table 1.Table 1: ELI-NP LINAC Beam ParametersParameter Value UnitBeam Energy 80-720 MeVBunch charge 25-400 pCBunch separation 16.1 nsBunches per pulse ≤ 32 unitsBunch energy spread 0.04-0.1 %Focal spot size > 10 μmPulse repetition rate 100 HzIn order to align the beam at the IP, the position of the electrons in the range of  ±1mm has to be measured for each bunch with sub-μm resolution. THE BPM16 CAVITY BPMTo  achieve  the  required  high-resolution  position measurement, four low-Q cavity beam position monitors (BPMs) are used. The cavity pick-up is the BPM16 model which is also used in the SwissFEL facility [2], consisting of one reference cavity and two position cavities. Its main parameters are presented in Table 2. Table 2: Parameters of the BPM16 CavityParameter ReferenceCavityPositionCavityMaterial Stainless Steel Stainless SteelResonant mode TM010 TM110 Resonant frequency 3.284GHz 3.284GHzLoaded Q 40 40Bandwidth 82.1MHz 82.1MHzSignal sensitivity 135V/nC 7.07V/mm/nCInner aperture 16mm 16mmThe  cavity  perfectly  fits  in  the  ELI-NP  accelerator layout since its aperture is the same and the low quality factor enables the independent measure of each bunch: the signal excited by one bunch will decay fast enough to not interfere with the signal coming from the next bunch (will reach 10% of its initial value in 8.92ns). Figure  1  shows  a  simulation  of  the  signal  shapes expected  from  the  pick-up  when  a  bunch  train  passes through it, where the third bunch is missing on purpose. The amplitude is equalized for both cavities using some attenuators before inputting the signal into the electronics.Figure 1: BPM16 expected output signal. MOPMR017 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-2264Copyright©2016CC-BY-3.0andbytherespectiveauthors06 Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationDESIGN OF THE ELECTRONICSDifferent processing techniques have been proposed to handle  the  cavity  BPM  signals  [2-6]:  the  approach depends on the cavity parameters and machine operation mode.  The  RF  front-end  typically  includes  filters  to remove  the  unwanted  frequency  components  from  the signal,  variable  attenuators  to  optimize  the  signal  level depending on the beam conditions (e.g. charge, position) and mixers to down-convert the cavity signal to a lower intermediate frequency (IF) which is easier to digitize.The  proposed  solution  uses  the  same  approach:  a simplified block diagram of the RF front-end for only one input channel is presented in Figure 2. A reference signal at the bunch repetition frequency (62.02MHz) is provided by the timing system to two Phase Locked Loops (PLLs) which  generate  the  ADC  sampling  frequency  and  the down-conversion  mixer  components.  The  cavity  input signal  is  adjusted  with  the  attenuators  and  down-converted  to  375MHz.  Later  on  a  filter  with  116MHz bandwidth  limits  the  signal  harmonic  content  to  the second Nyquist zone preparing it for the A/D conversion, done with 500MSps – 14bit ADC converters.Figure 2: Block diagram of the electronics RF front-end.In the case of bunches separated by only 16.1ns, this conversion  provides  8  ADC  samples  per  bunch  from which the position is later extracted.Once the signals are digitized, the samples belonging to different bunches have to be separated. This is done with a calibration procedure which associates 8 ADC samples to each bunch and assures that the center of mass of the bunches is centred in the vector of samples – see Figure 3.A where the red circles are the limits of each bunch.Figure 3: Digitized input signal after calibration (A) and deconvolution filter (B).Later  on  a  special  deconvolution  filter  extracts  from each bunch the expected signal response and compresses the signal towards the center in order not to have coupling between them. This is implemented with a FIR filter with 100 taps – see its output in Figure 3.B. Position CalculationOnce the data of each bunch is separated, it is possible to calculate the amplitude (Vx, Vy, Vr) associated to each cavity  signal  through  a  sum-of-squares  formula  and  to calculate the absolute position value with the equations:where  Kx,  Ky are calibration constants which depend on the sensitivity of the cavities and on specific value of the variable attenuators. To resolve the sign of the position it is  necessary  to  study  the  phase  relation  between  the position  and  the  reference  cavity,  and  this  is  done  in parallel through an I/Q demodulation.  PERFORMANCE SIMULATIONSAccording with the expected input signals, the structure of  the  RF  front-end  and  the  discussed  processing algorithms it is possible to simulate the performance of the system in different conditions like single- bunch and bunch-train operating modes. As  a  first  measurement,  a  single-bunch  with  900μm offset  from the beam pipe center  is considered.  Several measurements are simulated and the position resolution is estimated  as  their  standard  deviation.  Figure  4 presents how the position resolution depends on the bunch charge and the variable attenuator setting: sub-μm resolution can be achieved for every charge level. Figure 4: Position resolution in single-bunch mode.Depending  on the beam charge  and position also the position measurement range changes. This dependence is shown in Figure 5, again parametrized depending on the variable attenuator setting. Finally if the beam is moved in a certain position range, the  position  measurement  resolution  remains  relatively stable with the exception of the cavity center where the X=K xV xV rY=K yV yV rSimulations in Single-bunch Mode RF Front-end and A/D Conversion Bunch Decoupling and Deconvolution Filter Proceedings of IPAC2016, Busan, Korea MOPMR01706 Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationISBN 978-3-95450-147-2265 Copyright©2016CC-BY-3.0andbytherespectiveauthorsdipole signal produced by the position cavity is weaker and dominated by the noise.  Figure  5:  Position  measurement  range  depending  on bunch charge and attenuators setting.The simulations in  bunch train  mode are  carried  out with the purpose to evaluate the capability of the system to  measure  each  bunch  position  in  the  train,  without coupling effects. For this reason a train of 32 bunches is simulated  with 400pC bunch charge  and an attenuation setting of  15dB. The position of  each  bunch along the train  is  modulated  as  a  cosine  signal  in  the  range  of ±100μm, see the blue circles in Figure 6.A. The result of the  position  calculation  is  represented  with  the  green crosses in the same picture.Figure 6: Position measurements in bunch train mode.The red crosses in Figure 6.B represent the resolution of each bunch measurement, in line with what presented in Figure 4 (~300μm). In the same figure the blue circles show  the  measurement  accuracy,  which  proves  to  be lower only when the bunches cross the pipe center.FIRST PROTOTYPE RESULTSThe  development  of  the  electronics  for  the  ELI-NP project is carried on by Instrumentation Technologies and is organized in two phases. Purpose of the first phase is to prove the design and performance of the RF front-end the fast A/D conversion, while the second phase proves the digital signal processing and the final performance. The first  development  phase  resulted  in  a  first  prototype consisting of  evaluation boards interconnected  together: FPGA and ADC boards, two PLL boards and a prototype of the RF front-end – see Figure 7.Figure 7: First development phase prototype.Part of the tests carried out with the first prototype were functional,  with the purpose to demonstrate  that  all  the blocks  can  work  together.  Performance  measurements were done on the acquired ADC signals to evaluate the noise contribution of the RF front-end, and proved to be in line with the expected results.CONCLUSIONSThe electronics for the ELI-NP electron LINAC cavity BPM system are currently being developed. The system is required to measure the position of each electron bunch independently and with sub-μm resolution in the range of ±1mm from the pipe center. The proposed design consists of an RF front-end which down-converts  the  cavity  input  signal  preserving  each individual  bunch  contribution,  fast  A/D  converters  to extract  enough information from the signal  and custom FPGA algorithms to decouple the acquired bunch samples and compute the beam position.  The measurements done with the prototype developed during the first project phase confirmed the feasibility of the development and the performance of the RF front-end. REFERENCES[1] L.  Serafini  et  al.,  “Technical  Design  Report  - EuroGammaS proposal for the ELI-NP Gamma beam System”, July 14th, 2014.[2]  B.  Keil  et  al.,  “Design  of  the  SwissFEL  BPM System”, IBIC2013, Oxford, UK.[3] D. Lipka, “Cavity BPM Designs, Related Electronics and Measured Performances”, DIPAC09, Basel, CH[4] A. Young et al., “Performance Measurements of the New  X-Band  Cavity  BPM  Receiver”,  IBIC2013, Oxford, UK.[5] H.  Maesaka et  al.,  “Performance of  the  RF Cavity BPM  at  XFEL/SPRING-8  'SACLA'”,  FEL2011, Shanghai, CN.[6] Y.  I.  Kim  et  al.,  “Cavity  beam  position  monitor system for  the  Accelerator  Test  Facility  2  ”,  Phys. Rev. ST Accel. Beams 15, 042801 (2012).Simulations in Bunch Train Mode MOPMR017 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-2266Copyright©2016CC-BY-3.0andbytherespectiveauthors06 Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and Instrumentation

Design and simulations of the cavity BPM readout electronics for the ELI-NP gamma beam system

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Design and simulations of the cavity BPM readout electronics for the ELI-NP gamma beam system

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