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COULOMB ’05 Experiments with Cooled Beams at COSY A.Lehrach, H. J.Stein, J. Dietrich, H.Stockhorst, R.Maier, D.Prasuhn

COULOMB ’05 Experiments with Cooled Beams at COSY A.Lehrach, H. J.Stein, J. Dietrich, H.Stockhorst, R.Maier, D.Prasuhn, V.Kamerdjiev , COSY, Juelich , I.Meshkov, Yu.Korotaev, A.Sidorin, A.Smirnov, JINR, Dubna. Contents 1. Introduction: Electron cooling at COSY

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COULOMB ’05 Experiments with Cooled Beams at COSY A.Lehrach, H. J.Stein, J. Dietrich, H.Stockhorst, R.Maier, D.Prasuhn

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  1. COULOMB ’05 Experiments with Cooled Beams at COSY A.Lehrach, H.J.Stein, J. Dietrich, H.Stockhorst, R.Maier, D.Prasuhn, V.Kamerdjiev,COSY,Juelich, I.Meshkov, Yu.Korotaev, A.Sidorin, A.Smirnov, JINR, Dubna

  2. Contents 1. Introduction: Electron cooling at COSY 2.“Electron heating” 3. Coherent instability 4. Ion cloud in an electron cooling system

  3. COSY Accelerator Facility • Ions: (pol. & unpol.) p and d • Momentum: 300/600 to 3700 MeV/c for p/d, respectively • Circumference of the ring: 184 m • Injection: • 45 MeV • H-, D- stripping injection • Intensity 8 mA: 1011 protons • coasting beam • Electron Cooling at injection • Stochastic Cooling above 1.5 GeV/c • 4 internal and 3 external experimental areas

  4. COSY Electron Cooling system Design values Cooling section length 2 m Electron current 4 A Beam diameter 2.54 cm Energy 100 keV Normal operation Energy 25 keV Current 100 – 250 mA Magnetic field 800 G • Applications • On-turn extraction using diagnostics kicker (JESSICA) • Increase of the beam quality for slow extraction (TOF) • Increase of polarized beam intensity (cooling-stacking)

  5. Typical graphs at injection in COSY Beam shrinks and decays Initial losses “Coherent”losses The dependence on time (a) neutrals generation rate and (b) proton beam intensity (1.275·1010 protons/div).

  6. 2. «Electron heating» «Measurements of electron cooling and «electron heating» at CELSIUS» D.Reistad et al. Workshop on Beam Cooling, Montreux, 1993 In presence of the electron beam the ion beam lifetime is much shorter: 50 - 100 sec without electron beam 0.5 - 1 sec at electron current of 100 mA

  7. COSY, detuned electron beam Ie = 0 Ie = 45 mA Ie = 98 mA Ie = 243 mA

  8. Equilibrium beam emittance At small intensity equilibrium between electron cooling and IBS leads to  N0.6 At large intensity Heating by high order resonances r  I-0.5 H0 profiles  •  N Q = const

  9. Nonlinear field of the electron beam CELSIUS: Ion beam cross-section 70 x 58 mm electron beam diameter 20 mm COSY: Ion beam cross-section 40 x 75 mm electron beam diameter 25.4 mm Two-beam instability V.Parkhomchuk, D.Pestrikov, Coherent instabilities at electron cooling, Workshop on Beam Cooling, Montreux, 1993

  10. 3.Coherent instability at COSY Single injection in COSY Initial losses Coherent oscillation start (no losses!) Oscillations “jump” (see next slide) H0(t) Ip(t)

  11. Coherent instability development 1 (t = 0) 2 (t=8 s) 3 (t = 16 s) Qx = 3.62 Qy =3.66 1 injection (t = 0), 2 horizontal betatron oscillations start (t=8 s), 3 “jump” to vertical oscillations (t = 16 s), tjump< 0.5 s

  12. 5.Coherent instability H. Stockhorst Optimised setting of sextupoles “Standard” setting of sextupoles COSY: Sextupole correction As result of correction accelerated beam increased in two times

  13. Schottky Spectrum Qx=3.609, Qy=3.694 x=−2.8,y=0.3 Qx=3.598,Qy=3.636 x=−2.4, y=−0.6

  14. Instability suppression Feedback system: LEAR: (CERN) bandwidth 500 MHz - 81010 protons COSY: bandwidth 70 MHz - 1011 stored protons Variation of electron beam energy, CELSIUS: Most effective square-wave modulation 50 V amplitude at 115 keV electron beam energy “Hollow beam”, Measuring a hollow electron beam profile,  A. V. Bubley, V. M. Panasyuk, V. V. Parkhomchuk and V. B. Reva, NIM A 532(October 2004)

  15. 4. Ion cloud in an electron cooling system P. Zenkevich, A. Dolinskii and I. Hofmann Dipole instability of a circulating beam due to the ion cloud in an electron cooling system,NIM A 532(October 2004) E.Syresin, K.Noda, T.Uesugi, I.Meshkov, S.Shibuya, Ion lifetime at cooling stacking injection in HIMAC, HIMAC-087, May 2004

  16. “Natural” neutralization Potential at the electron beam axis Neutralization measurements Potential depression by space charge 45V/100mA (theo.) 30V/100mA (meas.) Natural neutralization 34-37% Neutralization level due to variation of the vacuum chamber radius Vacuum chamber radius At gun and collector 3.25 cm At cooling section 7.5cm

  17. Control of the neutralization level Trapped residual gas ions oscillate in the solenoid magnetic field and electric field of the electron beam: “Shaker” – resonant excitation of the ion oscillations Ie = 250 mA 18 harmonics Change of neutralization leads to the shift in proton revolution frequency

  18. Transverse shaking A/Z of residual gas ions stored in electron beam 28 N2+ H+ 16 CO+ 40 Xe+ Constant beam revolution frequency Longitudinal shaking Ie = 170 mA Revolution frequency shift is compensated by change of cathode voltage Ions traveling along cooler

  19. Shaker is off Non resonance excitation Resonance 100-120 kHz Resonance 130-150 kHz

  20. Conclusion • Electron cooling permits to form ion beams at high phase space density, howeverthe problems of beam stability specific for electron cooler ringsappear. 2. First problem relates to interaction of an ion circulating in the ring with nonlinear field of cooling electron beam. 3. Second problem is connected with development of coherent instability in cooled ion beam. 4. The thresholdof this instability can be reduced when “secondary” ions of residual gas are being stored in the cooling electron beam. 5. The thresholdof this instability can beincreasedwhen feedback system and control of “the natural neutralization” (with a shaker, for instance) are applied.

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