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Lecture 4

Stochastic cooling Of Bunched Beams (with examples of most recent experience in RHIC heavy ion collider at Brookhaven National Laboratory). Lecture 4. Physics of Bunched Beam Stochastic Cooling. outline History Basic ideas: ‘bunched beams’ are different from ‘coasting beams’

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Lecture 4

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  1. Stochastic cooling Of Bunched Beams (with examples of most recent experience in RHIC heavy ion collider at Brookhaven National Laboratory) Lecture 4

  2. Physics of Bunched Beam Stochastic Cooling outline History Basic ideas: ‘bunched beams’ are different from ‘coasting beams’ RHIC system and results for longitudinal cooling Signal processing, filtering and transmission Transverse Cooling Future plans at RHIC

  3. History Herr and Mohl reported cooling bunched beams in ICE (1978) Chattopadhyay develops bunched beam cooling theory and practice (1983) Stochastic cooling considered for SPS, RHIC and Tevatron (80s). Unexpected RF activity pollutes Schottky signal (85s). Transverse signal suppression seen in Tevatron (1995). Cooling of long bunches in FNAL recycler (2005). Proton cooling experiment in RHIC (2006). Operational cooling of gold in RHIC (2007). Pasquinelli, PAC95 (3 more?)

  4. Basic idea CI lecture series, April 2009 relative arrival time

  5. RHIC RF Rebucketing can lead to sharp edges in phase space density Lots of high frequency signal CI lecture series, April 2009

  6. Proton coherence is different due to micro-bunching. PRSTAB 7, 044402 (2004) CI lecture series, April 2009

  7. *=1m CI lecture series, April 2009 beam in a drift space Smaller  allows for smaller * without increasing beam size in triplets

  8. Solution to the Coherent Line Problem After gain it is repeated 4 more times, to 80 ns. The time between bunches. The 5 ns bunch signal is split and repeated 4 times, reducing the peak by 1/4 RHIC

  9. A hybrid fiber optic/electrical IIR filter is flat across the frequency band Two optical wavelengths avoid coherent interference Transverse Cooling needs a new type of filter (for the common mode) BNL - RHIC

  10. The double ridge waveguide-to-coax adapter is mounted outside a ceramic beam pipe The bandwidth is 5- 11 GHz (WRD- 475) They will be arranged counter phased so that they can be combined in zero degree hybrids Because Ions have strong Schottky signals the pickup does not need to move (3 inch beam pipe) New Pickups are needed to go beyond where we are now • 4 pickups: 6 dB • Ions/protons: 19 dB • 100/250 GeV: 4 dB • S/N, cable loss: 4 dB • Total = +33 dB Sum signal Difference signal BNL - RHIC Time domain signal

  11. Transverse Cooling system Similar cavities. Low level requires a notch filter (R&D) 40 Watt amplifiers are sufficient. 5-8 GHz keeps aperture reasonable.

  12. Longitudinal kicker needs to open during the ramp .. Tolerance for closing is 0.002” Individual cavities driven by 40 Watt amplifiers (250 W each for 6, 1k-Ohm kickers with 1 GHz bandwidth) Amplitude and delay are corrected every 5 minutes.

  13. We wanted to make a design that could be made commercially The cavity structure was essentially the same but the implementation was a new concept Mechanical tolerances at 8 GHz are challenging. 0.001” = 1 bandwidth We will try again to make a simpler design Building the First Transverse Kicker BNL - RHIC

  14. Yellow Vertical Kicker in the Tunnel BNL - RHIC

  15. We hope this design can be built commercially The key component (the cavity structure) is now being machined by an outside shop The same concept will work for horizontal kickers New Design of vertical transverse kickers for the Blue ring BNL - RHIC

  16. The actual details of signal processing is complicated.

  17. Principle of Pilot Tone RHIC

  18. Evolution of a 5 hour RHIC store with 1.E9 Au/bunch. Top, wall current monitor profiles taken one hour apart without cooling. IBS causes significant loss from the RF bucket. Bottom, cooling on. Beam loss is consistent with burn off in the interaction regions.

  19. In run FY07 the stochastic cooling system in the gold ring was commissioned The beam loss rate was reduced to the “burn off” level (beam is lost to collisions only) Longitudinal cooling stopped the debunching caused by IBS Stores with/without cooling show marked increase in store time A store without cooling A store with cooling Results RHIC

  20. Schottky spectra at 2.7 GHz, before and after cooling Synchrotron sidebands after cooling Schottky Spectra Show Momentum Cooling RHIC

  21. Cutting the chord is the key to longitudinal cooling “bad mixing” with 2/3 turn delay limits cooling to only the halo of the beam Using the microwave link saves big$ Beam Transfer Functions and Signal Suppression were measured with deuterons in the Blue ring Testing the Microwave Link with Protons Beam Transfer Function with Protons BNL - RHIC

  22. Now: test Yellow Vertical Summer 2009: Install Blue Vertical Upgrade Yellow longitudinal to 6-9 GHz, microwave link 2010, Build Blue and Yellow horizontal 2011/2012 run, Operate with all six planes Concurrent R & D on 12 GHz longitudinal Schedule BNL - RHIC

  23. Transverse and longitudinal interact via IBS Shortening the bunch increases IBS growth in transverse planes Cooling rates need to be balanced 40 x 1026 (cm-2s-1) luminosity is foreseen FY09 test with one transverse and longitudinal plane Projected Luminosity Improvement with Transverse and Longitudinal Cooling Simulations of luminosity for a 5 hour store Magenta=no cooling Red=with cooling Blue= with 56 MHz cavity black=1.5 x 109,12 GHz RHIC

  24. Our goal is to counteract IBS and prevent debunching Cool only particles near the separatrix Halo cooling > more mixing > faster cooling Greater reach in momentum Two-turn Filter and Halo Cooling RHIC

  25. Conclusions For ion beams in RHIC Longitudinal stochastic cooling worked. Lifetime was improved. Simulations show reasonable agreement with data. Transverse cooling looks straightforward. Expect a big payoff from transverse cooling.

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