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Cooling of Hadrons at Relativistic Energies: Performance of FNAL’s Recycler Electron Cooler

Cooling of Hadrons at Relativistic Energies: Performance of FNAL’s Recycler Electron Cooler. BNL – Collider-Accelerator Department Accelerator Physics Seminar November 13 th , 2008 L. Prost , Recycler Dpt. personnel. Fermi National Accelerator Laboratory. Outline. Fermilab brief overview

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Cooling of Hadrons at Relativistic Energies: Performance of FNAL’s Recycler Electron Cooler

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  1. Cooling of Hadrons at Relativistic Energies:Performance of FNAL’s Recycler Electron Cooler BNL – Collider-Accelerator Department Accelerator Physics Seminar November 13th, 2008 L. Prost, Recycler Dpt. personnel Fermi National Accelerator Laboratory

  2. Outline • Fermilab brief overview • Luminosity history • Antiproton production and storage at FNAL • Role & Description of the Recycler ring • Current mode of operation • Electron cooling in operation • Electron cooling performance characterization • Performance • Conclusion BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  3. Proton source CDF Tevatron Main Injector\ Recycler Antiproton source D0 Fermilab complex • The Fermilab Collider is an Antiproton-Proton Collider operating at 980 GeV BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  4. Luminosity performance • Luminosity increase due to: • Antiproton Production • Injector Chain: more beam on target • Pbar source improvements • Integration of Recycler into operations • Electron Cooling • Tevatron improvements for higher beam intensity Ecool installation Ecool installation BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  5. Antiprotons and Luminosity • Integration of the Recycler into Collider operations • Final storage ring for antiprotons • Improve average accumulation rate • Implementation of electron cooling • Remove 1/N cooling rate limitation of stochastic cooling • Double Antiprotons available to the collider Recycler Only Accumulator + Recycler Pbars are also used more efficiently in the Tevatron Accumulator Only BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  6. Fermilab’s antiproton production chain Nickel target 1108 = 1 1108 = 300 61011 = 2108 41012 = 4109 Proton beam 71012 protons every 2 sec 120 GeV 1 TeV 150 GeV Tevatron Main injector 8 GeV 8 GeV Accumulator (stochastic cooling) Debuncher (stochastic cooling) until 2005 8 GeV Recycler (stochastic and electroncooling) Electron cooling in the Recycler Ring eliminates one of the bottlenecks in the long chain of the antiproton production 6 BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  7. Antiprotons production and storage • Every 2.2 seconds: • 1-2 x 108 pbars are transferred from the Debuncher to the Accumulator • 5-8 x 1012 120 GeV protons strike an Inconel target • 8 GeV pbars are focused with a lithium lens • 1-2 x 108 pbars are collected in the Debuncher • Every N hours: • transfer pbars from Accumulator to Recycler • N to maximize operational performance • Every M hours: • Transfer pbars from Recycler to Tevatron • M to maximize operational performance BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  8. Antiprotons flow (Recycler only shot) - Illustration Tevatron Transfers from Accumulator to Recycler Shot to TeV Recycler Accumulator 35 mA 3000 e9 400 e10 100 mA 17 hours BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  9. Recycler – Main Parameters • Recycler: • Fixed energy storage ring (uses strontium ferrite permanent magnet) • Goal of cooling in the Recycler • Increase longitudinal (and transverse) phase space density of the antiproton beam in preparation for • Additional transfers from the Accumulator • Extraction to the Tevatron Recycler Main Injector BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  10. Recycler stochastic cooling system – Main features • Longitudinal: • 0.5 - 1 GHz and 1 - 2 GHz • 1-2 GHz now a horizontal system • Notch Filter Cooling • Planar loop pickups and kickers • Transverse (H & V): • 2-4 GHz • Planar loop pickups and kickers • 1/6th of ring from Pickup to Kicker • Signals travel on laser light link BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  11. Recycler Electron Cooler (REC) – Main features • Electrostatic accelerator (Pelletron) working in the energy recovery mode • DC electron beam • 100 G longitudinal magnetic field in the cooling section • Lumped focusing outside the cooling section BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  12. Electron cooling system setup at MI-30/31 Pelletron (MI-31 building) Cooling section solenoids (MI-30 straight section) BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  13. Electron cooling status: From installation to operation • Bringing electron cooling into operations consisted of three distinct parts • Commissioning of the electron beam line • Troubleshoot beam line components • Check safety systems • Ensure the integrity of the Recycler beam line at all times • Establish recirculation of an electron beam • Cooling demonstration • Energy alignment • Interaction of the electron beam with anti protons • Cooling demonstration • Reduction of the longitudinal phase space • Cooling optimization • Optimization of the electron beam quality • Stability over long period of times • Minimize electron beam transverse angles • Define best procedure for cooling anti protons • Maximize anti protons lifetime • Understand and model the cooling force BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  14. Electron cooling in operation • Electron cooling is used when needed • Electron cooling used for 6D cooling (i.e. both longitudinally and transversely) • Transverse stochastic cooling most efficient just after transfers into the Recycler (large transverse emittance), but limited effectiveness when the stack is large and the antiproton beam is compressed • Electron beam adjusted to provide stronger cooling as needed (progressively) This procedure is intended to maximize lifetime Similarly to low energy coolers, cooling seem to induce secondary beam-beam effects. In our case, it affects the lifetime of the cooled beam BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  15. Adjusting the cooling rate • Change electron beam position (vertical shift) • Adjustments to the cooling rate are obtained by bringing the pbar bunch in an area of the beam where the angles are low and electron beam current density the highest Area of good cooling electrons electrons pbars pbars 5 mm offset 2 mm offset This procedure can be regarded as ‘painting’ and, in fact, is almost equivalent to the ‘hollow beam’ concept for low energy coolers. BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  16. Cooling sequence (15-20 hours) Transverse emittance (95%, n) (1.5 p mm mrad/div) Electron beam position (2 mm/div) Longitudinal emittance (15 eV s/div) Number of antiprotons (90×1010/div) Transverse emittance (95%, n) (1 p mm mrad/div) Electron beam position (1 mm/div) Longitudinal emittance (10 eV s/div) Number of antiprotons (1.5×1010/div) When strong cooling is applied, the antiproton distribution becomes peaky (i.e. high density of the core) and the lifetime deteriorates 1.5 hours 17 hours BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  17. Accumulation performance • Use of electron cooling allowed the storage and extraction of more than 450 × 1010 antiprotons • > ~3 times what would be possible with stochastic cooling alone • Much faster too • Delivers very consistent bunches (i.e. same emittances shot to shot) • Plays a major role in increasing the initial and integrated luminosities in the Tevatron • Record initial luminosity > 300 × 1030 cm-2 s-1 08/22/08 Before extraction to the TeV: N = 375 × 1010 eL (95%) = 68 eV s et (95%, n) = 3.1 p mm mrad (Schottky) Longitudinal Schottky distribution BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  18. Electron cooling performance • Evaluated two ways: • Drag rate measurements • As a function of various parameters • Characterizes the intrinsic performance of the electron beam • ‘Standard’ cooling rate measurements • Operation driven measurement • i.e. how fast are we really cooling the antiprotons ? BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  19. Drag rate and cooling force – First approach • Pencil-likeantiproton beam with smallmomentum spread • Drag rate equal to cooling force • Used same assumption in our case and fitted drag rate data to non-magnetized force model to estimate electron beam properties Lab frame quantities ae≡ electron beam anglesdW≡ electron beam energy spreadje≡ electron beam current density BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  20. Drag Rate as a function of the antiproton momentum deviation100 mA, nominal cooling settings (before October 2007) Fits: Non-magnetized force model Drag rate very sensitive to antiprotons transverse emittance Transverse stochastic cooling applied at all times for sets 2-4 BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  21. Interpretation of the dependence of drag rate measurements on the transverse emittance (I) • Given some simplified assumptions, drag rate for finite emittance beams can be written as: • Cooling force dependence on radius comes from • Radial distribution of the current density • Radial distribution of electron beam angles from gun simulations Linear dependence from envelope scalloping Thermal velocities and dipole perturbations from magnetic field BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  22. Interpretation of the dependence of drag rate measurements on the transverse emittance (II) • For the distributions given previously, the first terms of the Taylor expansion of the cooling force on axis are with • For typical parameters, we recoverif sp < 0.4 MeV/c and b > 2 mm (from angle distribution) Easy to fulfill during drag rate measurements Estimate from the radial dependence of the drag rate BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  23. Drag rate as a function of a parallel offset of the electron beam w.r.t. the pbars beam For the present parameters of the antiproton and electron beams, the drag rate significantly differs from the cooling force experienced by an antiproton on axis. The area of the electron beam where cooling is effective is significantly smaller than the physical size of the electron beam, b 1 mm. Voltage jump was 2 kV, Ie = 0.1 A, Np = 4×1010.Set 1: the antiproton beam was scraped to the radius in the cooling section of 1.1 mm, 25 min prior to the measurement.Set 2: negative offsets measured the same day 2 hours after the scrape. During both measurements, eFW = 0.3-0.7 (sr ~ 0.5 mm).Set 3: data of Feb. 2006, taken several hours after scraping. eSch = 1.5-3. Point 4: drag measurement immediately after scraping to 1.1 mm. eFW ~ 0.1-0.2 (sr ~ 0.3 mm) BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  24. Sensitivity of drag rate to antiprotons transverse emittance is carried over to cooling rates Red data: Bunched antiproton beam (with arbitrary fit) Green data: Un-bunched antiproton beam Rates ‘normalized’ to sp = 3.6 MeV/c Emittance from flying wire measurements BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  25. Preliminary measurements with scintillator • The electron beam appears to be very elliptical at the exit of the cooling section • Indicative of quadrupole envelope oscillations • Also presence of a large halo • Could explain various observations & inconsistencies Beam Image from YAG at cooling section exit (~100 mA) • Large beam size measured with scrapers • Small effective electron beam radius • Shallow sensitivity of the drag force/cooling rates on the matching solenoids settings • Large sensitivity of the drag force/cooling rates on antiprotons transverse emittance YAG screen BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  26. Beam rounding procedure (A. Burov) • For two distinct values of SPQ01I • Record initial image • Change upstream quads successively(6 quads) • Record associated images • Calculate ellipticities • Fit ellipse to threshold (binary) image • Extract semi-major and semi-minor • e = 2 (a –b)/(a+b) • Include effect of camera angle (i.e. distortion k) • Compute MULT (i.e. transfer matrix) • SVD algorithm • Use MULT to make the beam more round • Repeat… • If e = 0 for two values of SPQ01I, then the beam is perfectly round YAG screen This was done automatically with a Java application written by T. Boshakov BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  27. Results (after multiple iterations) 120 118 119 Nominal First visible image + 1.5 ms New nominal 121 123 122 BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  28. Envelope analysis (from A. Burov) • Fit of SPQ01A scan for the ‘new’ nominal file: • Give r = 2.4 mm, a = 0.01, b = 2.0 m for initial conditions at the entrance of the cooling section • Show that 10-15% envelope oscillation remain SPQ01 scanw/ fit Beam radius [cm] Corresponding envelope in cooling section SPQ01I [A] r = 2.4 mm ~0.6 mm BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  29. Drag rate performance after rounding Expected improvements from beam rounding did not materialize Effective beam size after rounding is unchanged BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  30. Interpretation and Consequences • Rounding of the beam has failed • Possible reason: Pulse beam measurements (for YAG) vs DC beam measurements (for drag and cooling rates) • Ions capture (neutralization) effect • But YAG measurements proved the need for some quadrupole correction • Empirical optimization based on drag rate measurements with the electron beam offset while changing quadrupole settings BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  31. Current cooling performance Red data: Bunched antiproton beam (with arbitrary fit) Green data: Un-bunched antiproton beam Rates ‘normalized’ to sp = 3.6 MeV/c Emittance from flying wire measurements BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  32. Strong cooling is applied before extraction • Beam is brought on axis just before the final manipulations before extraction and stays on axis throughout the extraction process • Reduces longitudinal emittance of individual bunches STUDY 30 min. Scope traces of the resistive wall monitor 15 min. Initial 6.1 ms BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  33. Pelletron reliability • 77% beam up time since February 07 • Several interruption/day • Conditioning of the accelerating/decelerating structures~once every 2 months • Following series of full discharges (2-3) • Conditioning only takes several hours • Typically done when the electron beam is not absolutely needed • Routine maintenance every 5-6 months (opening of the Pelletron) • Longest running time between openings: 3509 hours • Clean/refurbish charging circuitry CHAIN Black deposit from the chain slipping on the pulleys BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  34. Cooling performance stability • We have 3 major performance-limiting stability issues • Degradation of the cooling section magnetic field (more inL. Prost & A. Shemyakin, Poster Session COOL’07) • Likely due to ground motion in the tunnel • Beam based alignment procedure was developed and tested • Large uncertainties but it worked • Drift of the antiproton trajectories • May be due to ground motion too • Needs to be re-align (3-bump) every 1-2 months • High voltage stability (and calibration) • Next slides ± 0.5 mm ± 0.5 mm BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  35. High voltage/Electron beam energy stability • Average energy drifts by up to 1-2 keV (over several weeks) • Cooling efficiency is greatly reduced • Temperature dependent • ~ -300 V/C • Mostly an issue at turn on (~6-10 hours to reach equilibrium) • Equilibrium temperature stable to within 1C • But not only… Longitudinal Schottky profiles after cooling with the electron beam on axis for ~2h at 100 mA Flatness of the distribution attributed to the electron beam energy to be offset Np = 200 × 1010 Np = 280 × 1010 BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  36. Last CS solenoid Beam position as an energy monitor • Use the 180 bend magnet and beam position monitor downstream as an energy analyzer • Absolute energy calibration done with antiprotons (debunched) • Defines an absolute position • Needs to be recalibrated ~once a month • Defines a relative position • Very stable HV monitor Beam position Calibration: 0.31 mm/kV • Bx/y ≡ BPM x- / y- direction • DY ≡ Dipole y-plane • QN ≡ Quadrupole (normal) • BV ≡ Beam valve BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  37. Issues related to electron cooling and large stacks • Since started to use the electron beam for cooling, we have dealt with three main problems • Transverse emittance growth • Duringmining • Lifetime degradation • Under strong electron cooling • Fast beam loss • Instabilities caused by high phase density and/or high peak current MINING BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  38. Cure to emittance growth (and lifetime degradation) • Changed working point from 0.414/0.418 (H/V) to 0.451/0.468 (H/V) • Increase tune separation to reduce coupling • More room at higher tunes • Recycler sensitive to 0.41 and 0.428 • Althoughit worked… a coherent electron-antiproton instability is not the primary cause for the emittance growth during mining • It was thought to be the mechanism by which the emittance grew but experimental measurements where coupling was large showed that it was not the case BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  39. Emittances of individual parcels during extraction 300×1010 in both cases Max emittance growth Max emittance growth Flying wire data BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  40. Lifetime before mining BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  41. Avoiding fast beam loss (i.e. instabilities) • Dampers to increase the threshold to resistive wall instabilities • Working on increasing the bandwidth • High frequency lines may cause problems as the number of antiprotons increases • Changed mining RF waveform • Wider buckets to decrease the peak current seen by the dampers (so-called ‘soft’ mining) • Avoid saturating dampers electronics ‘Hard’ mining ‘Hard’ mining ‘Soft’ mining ‘Soft’ mining BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  42. Typical longitudinal cooling time (100 mA, on-axis) e-folding cooling time: 20 minutes 111×1010 pbars 5.2 ms bunch BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  43. Strong transverse cooling is now routinely observed 100 mA, on axis Stochastic cooling off 135×1010 pbars 6.5 ms bunch e-folding cooling time (FW): 25 minutes BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  44. Transverse (horizontal) profile evolution under electron cooling Flying wire data 100 mA, on axis for 60 min Deviation from Gaussian BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  45. Beam quality: Electron angles in the cooling section • New measurements and refined analysis indicate that we had over estimated the quality of the electron beam *Angles are added in quadrature BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  46. Conclusion - Recycler • Recycler is an essential component of Fermilab Tevatron Collider Complex • significant contributions to the doubling of the peak and integrated luminosity over the last two years taking advantage of • improved performance of the Antiproton source stacking • Tevatron ability to handle higher intensities • Through mix of stochastic and electron cooling • Prepare intense, bright antiproton beams • doubled peak intensities while maintaining emittance properties • Future: • Collider program through 2009: Antiproton storage ring • Neutrino program: Proton stacker for Main Injector • single turn fill to maximize proton flux BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

  47. Conclusion – Electron cooling • The electron cooler reliability has been exceptional under an increased demand for electron cooling and is adequate for the remaining of the collider operation • Full discharges are sparse and conditioning is only required ~2-3 months • General maintenance of the Pelletron required ~5-6 months • Electron cooling rates are sufficient for the present mode of operation of the accelerator complex • We found them (and drag rates too) to depend greatly on the antiprotons transverse emittance • Preliminary YAG measurements indicate that the electron beam distribution may be the culprit • Good possibility that we can improve the cooling rates by fixing the electron beam distribution • Perhaps will improve lifetime too • It is the remaining most challenging issue related to electron cooling from an operational point of view BNL – Accelerator Physics Seminar November 2008 L. PROST, et al.

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