f Sergei Nagaitsev (FNAL) Webex meeting Oct ICD-2 chopper requirements and proposal #1

f Agenda 1.Chopper requirements and idea #1 – S. Nagaitsev 2.RFQ and idea #2 -- John Staples 3.Discussion  At this pre-CD0 stage of the project we can still have many ideas and discussion…  but need a plausible concept for the ICD2 document and for the cost-range estimate.  Goals of this meeting:  Inform collaboration about (evolving) chopper requirements and proposals  Collect comments and ideas ICD2 chopper - Nagaitsev 2

f Known cw chopper conceptual ideas 1.Broad-band chopper, single ion source 2.Narrow-band chopper, single ion source 3.Two ion sources and RFQ’s 4.Laser stripping (H- only) ICD2 chopper - Nagaitsev 3

f ICD2 schematic  Two different time periods during operation:  Long cycle: 5-ms pulse to RCS every 100 ms  Short cycle (2 µs or less) for the 2-GeV program ICD2 chopper - Nagaitsev 4

f Known bunch structure requirements (short cycle)  Must provide these bunch structures concurrently  Muon conversion defines the short-time period time cycle. There could be multiple bunches within the 100- ns beam pulse width.  Kaon experiments require a single short bunch per pulse. ICD2 chopper - Nagaitsev 5

f Long cycle timeline  Pulsed dipole rise time (fall time) is about 0.5 ms  This pulse repeats every 100 ms ICD2 chopper - Nagaitsev 6 Pulsed dipole OFF ON Beam current, mA 0 1 Time, ms 4.3 ms flattop Chopping for injection Chopping for 2-GeV program

f Chopping for injection  RCS rf frequency at injection: MHz  Harmonic number:98  Revolution freq.: MHz  Chopper patterns: 1.Chop a 200 ns gap (10 buckets) at MHz for the RCS kicker 2.The chopper is synchronized with the RCS RF and chops off particles located outside of the ±7.3 ns window (relative to bucket center) corresponding to 73% of the bucket length. ICD2 chopper - Nagaitsev 7

f Example of chopping for injection  Bunches (red) are at MHz  40% of bunches are removed by chopper (buckets + kicker gap)  RCS RF at MHz ICD2 chopper - Nagaitsev 8 Chop RCS RF voltage

f Chopping for the 2-GeV program  We are proposing to run three experiments concurrently. Thus, two rf splitters are possible: 1.Option 1: (n+1/4)f_bunch 2.Option 2: (n+1/3)f_bunch  Option 1:  One experiment gets 2/4 bunches, the other two get 1/4 bunches each.  Option 2 (like at CEBAF):  Option A: 1/3 bunches to each experiment  Option B: 2/3 bunches to one, 1/3 to another, 0 to third ICD2 chopper - Nagaitsev 9

f Option 1  For bunches at MHz the RF splitter frequency is MHz ICD2 chopper - Nagaitsev 10 Transverse RF kick Bunches

f Option 2a ICD2 chopper - Nagaitsev 11 Transverse RF kick Bunches

f Option 2b ICD2 chopper - Nagaitsev 12 Transverse RF kick Bunches

f ICD2 choice  For ICD2 we selected Option 1 in order to balance beam power between experiments requiring two different bunch structures  Mu2e would get 2/4 bunches but NOT all bunches are present. ICD2 chopper - Nagaitsev 13

f Example of bunches at MHz Ion source runs at 3.24 mA Chopper removes bunches to make 1 mA average current ICD2 chopper - Nagaitsev 14 Time, µs Bunch intensity, e7 ppb Mu2e – 400 kW, each other 800 kW at 20 MHz

f Other possible bunch structures for 2 GeV  No chopping, bunches at MHz, 3.85e7 ppb  Ion source at 1 mA  After rf splitter: 50% of bunches at 81 MHz, and 2 x 25% at 40.5 Mz  90% chopping, bunches at MHz, 3.85e8 ppb  Ion source at 10 mA  Other bunch structures possible… As long as the average beam current remains at 1 mA.  Bottom line: chopper needs to be able to remove individual bunches at MHz ICD2 chopper - Nagaitsev 15

f Chopping for RCS injection  I propose to adjust the ion source current during the 500 µs rise (fall) time of the pulsed dipole.  During 500 µs chopper removes all bunches. This can be done by a separate “slow” chopper in LEBT or MEBT.  Then, set ion source to 1.7 mA and remove 40% (see Slide 6) of bunches for injection  1 mA average current after chopping  After injection, reset the ion source and chopper to 2-GeV program. ICD2 chopper - Nagaitsev 16

f Bunch parameters after RFQ  Simulations by J. Staples (LBNL) for a MHz RFQ at 6 mA, 2.5 MeV  Beam non-Gaussian (longitudinally)  rms bunch length: 6° or 0.1 ns  rms energy spread: 11.7 keV  Assume that 1 ns contains 100% of beam ICD2 chopper - Nagaitsev 17 time, ns MHz

f Chopper parameters  For MHz the bunch spacing is 6.15 ns  Kicker pulse flat-top ~1.5 ns  Deflected beam needs to be moved transversely by more than 4 sigmas.  Assume sigma=4mm and the distance of 50 cm, the total kick (with a single kicker) needs to be >30 mrad  For a kicker plate length of 30 cm, this corresponds to the electric field of 5 kV/cm (for a single kicker)  probably very difficult ICD2 chopper - Nagaitsev 18

f Chopper Proposal #1  Use an AC-coupled amplifier  Example: 1-kW, BW: 80 MHz – 1 GHz  Suppose 50 Ohm impedance, 1-kW results in 300 V per plate  Two plates, 3-cm apart, 30 cm long -> 1.2 mrad kick ICD2 chopper - Nagaitsev 19

f Simple model with 7 kickers  No longit. dynamics and no space charge (yet)  1-sigma beam sizes ICD2 chopper - Nagaitsev 20

f Simple model with 7 kickers  +/- 300 V per plate, kicker gap 3 cm  trajectory with +/- 1 sigma envelope ICD2 chopper - Nagaitsev 21

f Normalized displacement  in units of beam sigma  Final displacement >4.5 sigma ICD2 chopper - Nagaitsev 22

f Summary  Chopper requirements – work in progress. Need to talk more to users.  Will need one “slow” and one “fast” chopper  Potentially need to change ion source current for RCS injection in about 0.5 ms  At least 3 ideas for a fast chopper exist: 1.Multi-kicker broad-band 2.Periodic (narrow-band) 3.Multi- source (and RFQ) and an rf funnel 4.Laser stripping ???  For idea #1 work to do:  use realistic amplifier parameters (B. Webber)  longit modeling and space-charge (Eliana, SN)  Kicker plate design and max power (Robyn) ICD2 chopper - Nagaitsev 23