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Chopper for the MEBT

Chopper for the MEBT. Chopper for the MEBT. Point of the chopper: Linac Bunches are spaced by 325 MHz ( 3.1ns ) In MI, bunch spacing is ~ 53 MHz (~19ns ) Don’t want bunches in the 53 MHz seperatrix =>Chop out ~1 of every 6 bunches Additional complication: 325 ≠n∙53

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Chopper for the MEBT

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  1. Chopper for the MEBT

  2. Chopper for the MEBT • Point of the chopper: • Linac Bunches are spaced by 325 MHz (3.1ns) • In MI, bunch spacing is ~53 MHz(~19ns) • Don’t want bunches in the 53 MHz seperatrix =>Chop out ~1 of every 6 bunches Additional complication: 325≠n∙53 =>Sometimes chop 1, sometimes 2

  3. Chopper for the MEBT • Only 3ns between bunches => use a travelling wave deflector which follows the beam • Since b=0.073 @2.5 MeV, need to slow down pulse => use meander wave structure: • Georgio obtained a prototype at CERN (from Fritz Caspers)

  4. Trace3d output from Gennady Chopper goes here; Allowed length TBD 28 cm long 70 cm

  5. A few numbers • Electrostatic deflection:d=(V/a)L2/2bp a=plate spacing, V=voltage, L=length • ~1kV,~2ns rise @53 MHz is very challenging • From CD0 param list: a=1.8cm, L=45cm, V=±1.8kV (assuming 3MeV beam) Chopperplates Begin solenoid

  6. A few more numbers • Allow beam to hit chopper plates • Spacing can be smaller, V requirement lower • For instance: with V=±885V, d=1.2cm, L=45cm: Chopperplates Begin solenoid

  7. CERN prototype • Ag /Moly-Mn on alumina • Layout for 3 Mev (b=0.08) • Only (10-15um of Ag (prototype) • Measured S21, consistent with CERN measurements

  8. Measurements CERN D. Wildman/R. Madrak 19 cm prototype

  9. More measurements S21 beyond 1 GHz: First resonance @1.5GHz Complex impedance: @53 MHz, 48.996Ω, 600pH

  10. Reminder: what it has to do • Bunch spacing = 3ns, bunch width=0.5ns • width <~5.5ns, including rise and fall • At full voltage for >~ 0.5 ns • 53 MHz rep rate, DF of 1ms*10Hz = 1% • Minimal(?) ringing after pulse • Measurements show the structure has no problem with this. • The most challenging part is the pulser

  11. Response to ‘the perfect’ low V pulse First try: fast scope calibration pulse, thru heliax->meander third try: fancy 2ns pulser (not available for use) Fourth try: borrowed HP8130A pulse gen 2nd try: same as above, Thru RG58

  12. Response to imperfect higher V pulse Input waveform: 100V, 400ps rise 1.3 GHz ringing After meander: 1 ns rise, 890 MHz ringing

  13. High Voltage Amplifier: what’s been done • SNS fast chopper: array of ~1ns risetime DEI FET’s to achieve ±2350V, 10ns risetime • SPL: two tubes providing high/low freq. components • Were not able to achieve 8ns (low) risetime with 45 MHz rep rate • RAL: seem to have a pulse generator (made by Kentech, U.K.), (but rep rate of only 2.4MHz):

  14. Forseeable options for pulser • FET’s (DEI/IXYS) • Risetime almost sufficient, falltime not (can solve with additional FET?) • Rep rate/duty factor worrisome for burnout • A module designed by DEI? • Kentech? • High power amplifier • E.g. IFI 1kHz-1GHz solid state and tetrode tube combo • D. Wildman got a quote of ~$60k • Probably need more than 1

  15. Measurements with DEI FET • Pulse width limited to 8ns by driver • Got up to 200k pulses @20MHz (10ms total); repeated at 3Hz • Need to heat sink next time

  16. To consider • How much space is available? • Chopped beam • Dump into chopper? • And/or target? • What about solenoids? • Tolerable residual V on meander (do not want to disturb chopped beam) • Pulser issues

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