Electron cooling at the Recycler: Update and Cooling force characterization October 19 th, 2006 L. Prost, Recycler Dpt. personnel Fermi National Accelerator Laboratory

2 Outline  Overview of electron cooling at the Recycler  Mission, goal  Setup  Electron cooler status  Electron cooling in operation  Cooling characterization  Issues related to electron cooling  Emittance growth  Lifetime  Cure to emittance growth  New working point  Outcome(s)  Current Recycler performance  Conclusion

3 Beam Cooling in the Recycler The missions for cooling systems in the Recycler are:  The multiple Coulomb scattering (IBS and residual gas) needs to be neutralized  The emittances of stacked antiprotons need to be reduced between transfers from the Accumulator to the Recycler  All sources of heating (damper noise, MI ramping,…) need to be neutralized

4 Antiprotons flow (Recycler only shot) Accumulator Recycler Tevatron Transfer from Accumulator to Recycler Shot to TeV 2600e9 400 e mA 100 mA ~30 hours

5 Tevatron collider Run II: Achievements (prior to using electron cooling and Recycler only shots) and targets July 05 GoalRatio Peak LuminosityE31 cm -2 s Pbars in RecylerE Pbar bunches36 1 Total PbarsE MI 95% long. 150 GeV eV s Proton 95% norm. trans. collision mm mrad20 1 Pbar 95% norm. trans. collision mm mrad ~0.75

6 Final goal for Recycler cooling: Prepare 9 (6 eV-s each) bunches for extraction

7 Fermilab cooler – 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

8 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 (continued focus at this time) 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 Today’s topics

9 Pelletron reliability and High voltage stability  During the Spring 2006 shutdown:  Rebuilt shaft  New pulleys (sheaves) for the chain and increase its tension Chain was slipping → Black deposits all over Chain  No required accesses from May 27 to October 11  Last week access for ‘routine’ maintenance only  Longest running time between openings ~1900 hours  Only two full discharges since May 27 (none since end of June)  No conditioning necessary  41 % beam up time since May 27  < 1 interruption/hour Difficult to extract the really meaningful information

10 Beam quality: Longitudinal temperature  The cooling process is determined by an effective energy spread consisting primarily of two components, the electron energy spread at a fixed time and the Pelletron voltage ripple  The energy spread is determined by IBS (the main contributor) and by density fluctuations at the cathode. According to simulations, at currents 0.1 – 0.5 A the energy spread is 70 – 150 eV.  The Pelletron voltage ripple is V r.m.s. (probably, fluctuates from day to day). The main frequency is 1.8 Hz ( ≡ the chain revolution frequency). Estimated with the beam motion in high dispersion area Position Before 180  bend Positions After 180  bend 0.25 mm MI momentum 6 seconds 0.5 mm

11 Beam quality: Electron angles in the cooling section *Angles are added in quadrature † Recent measurements indicate that we might have underestimated this contribution

12 Electron cooling in operation  In the present scheme, electron cooling is typically not used for stacks < 200e10  Allows for periods of electron beam/cooling force studies  Over 200e10 stored  Electron cooling used to ‘help’ stochastic cooling maintain a certain longitudinal emittance (i.e. low cooling from electron beam) between transfers or shot to the TeV  ~1 hour before setup for incoming transfer or shot to the TeV, electron beam adjusted to provide strong cooling (progressively) This procedure is intended to maximize lifetime  In addition, electron beam intensity is kept at 100 mA  Improves beam stability  Higher currents do not cool faster/deeper  May help lifetime too

13 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 5 mm offset 2 mm offset Area of good cooling pbars electrons pbars

14 Typical longitudinal cooling time (100 mA, on-axis) e-folding cooling time: 20 minutes 111×10 10 pbars 5.2  s bunch

15 Strong transverse cooling is now routinely observed e-folding cooling time (FW): 25 minutes 100 mA, on axis Stochastic cooling off 135×10 10 pbars 6.5  s bunch

16 Transverse (horizontal) profile evolution under electron cooling 100 mA, on axis for 60 min Deviation from Gaussian Flying wire data

17 Summary of measured cooling rates  4 sets of measurements  e-beam: 100 mA, on axis  Stochastic cooling off  Fixed bunch length Bunch length such that average current ~the same  Cooling rates  Initial pbar velocities (beam frame) ~same in all directions  Transverse emittances are from flying wire measurements  Cooling rates are the initial derivative of the emittance vs time curves Initial conditionsCooling rates

18 Issues related to electron cooling and large stacks  Since we started to use the electron beam for cooling, we have dealt with two main problems  Transverse emittance growth miningDuring mining  Lifetime degradation When the beam is turned on and/or moved towards the axis (i.e. strong cooling) MINING

19 Emittance growth during mining [Nov ‘05 measurements] pbars intensity for all measurements = 97e10

20 Summary of November 05 study  All cooling systems contribute to the growth rate  ~80% electron beam  ~20% stochastic cooling  Vertical damper was always on  Growth rate depends on the phase density (and likely on the peak current) First consequence: Turn off stochastic cooling before mining (and left off during the whole extraction process)

21 Lifetime degradation throughout a store Pbar intensity Lifetime (1 hour running average) 500 hours 60 × hours Lifetime at injection/extraction

22 Model to explain emittance growth (and lifetime degradation)  Emittance growth could be due to a coherent electron antiproton instability (see A. Burov, for instance: ICFA- HB2006 in Tsukuba, Japan; April ’06 AD Seminar)  Growth rate   xy I e I p, (  xy coupling parameter)  Why is the coupling important ? Electrons drift in a direction, orthogonal to the pbars offset. Thus, for planar (uncoupled) modes, the force, acting back on the pbars, is orthogonal to the pbars velocity. The resulted work is zero, and thus the growth rate is zero too (at the lowest order over small phases). e- p

23 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  xy  More room at higher tunes Recycler sensitive to 0.41 and  Although it worked… a coherent electron- antiproton instability is not the primary cause for the emittance growth during mining  See following slides  New conjecture: Single-particle resonances, washing the resonant pbars out of the beam, while the e-cooling process continues to bring fresh pbars into these holes

24 Changed working point – First trial (before shutdown) (A)0.414/0.418 (H/V) (B)0.451/0.468 (H/V) (A) (B) Initial growth rate: (A) 36  mm mrad per hour (B) 3  mm mrad per hour / ~10

25 No change in growth rate while changing the tunes Mining

26 Emittances of individual parcels during extraction 300×10 10 in both cases Max emittance growth Flying wire data

27 Maximum emittance growth during extraction vs tune separation

28 Changing the operating point also helped the lifetime Lifetime (1 hour running average) Pbars intensity 500 hours Lifetime at injection/extraction ? 75 × hours

29 Lifetime before mining

30 Lifetime under strong cooling (100 mA, on axis)  54×10 10 (lower tunes); 134×10 10 (higher tunes)  Stochastic cooling off  Fixed bunch length Bunch length such that average current ~the same 30 minutes Lower tunes Upper tunes Horizontal emittance Vertical emittance 0.5  mm mrad/div 0.5  mm mrad/div Pbar intensity 0.5e10/div e-beam 100 mA Schottky * FW * ≡ Emittances measured with 1.75 GHz Schottky monitor

31 Consequence of having a ‘good’ lifetime under strong cooling  Changed cooling procedure slightly  Beam is brought on axis before mining and stays on axis throughout the extraction process Also added delay (90s) after mining is done –Reduces longitudinal emittance of individual bunches 15 min. Initial 30 min. STUDY Scope traces of the resistive wall monitor 6.1  s

32 Longitudinal emittance delivered to MI since 06 shutdown That’s the important change

33 Coalescing efficiency as a function of the longitudinal emittance since 06 shutdown That’s the important change

34 The ‘bottom line’… from the Recycler point of view  Electron cooling (and new working point) allowed for:  Larger stashes Ability to cool and extract more  Lower delivered emittances Improved efficiencies downstream See next slides…

35 Evolution of the number of antiprotons available from the Recycler Mixed mode operation Ecool implementation Recycler only shots

36 Present Recycler performance with electron cooling MAX GOAL

37 Luminosity density by source

38 Tevatron collider Run II: Latest achievements and targets Typical Best PL Goal Peak Luminosity (PL)E31 cm -2 s Pbars in RecylerE Pbar bunches36 Unstacked PbarsE MI 95% long. 150 GeV eV s Proton 95% norm. trans. collision mm mrad16 * Pbar 95% norm. trans. collision mm mrad * Horizontal emittance only (vertical flying wire broken)

39 Conclusion  The electron cooler reliability has been improved and is believed to be adequate for the remaining of Run II  Electron cooling rates are sufficient for the present mode of operation of the accelerator complex  Fast transfer scheme and/or storing and extracting 600e10 may require some changes/improvements  Changing our operating point (tune space) improved the Recycler’s performance  Emittance growth during the mining process has been almost completely eliminated  Lifetime of large number of particles has improved significantly  Delivered longitudinal emittance is smaller Better coalescing efficiency  Longitudinal cooling force (drag rate) agrees to within a factor of 2 with a non-magnetized model  Not shown in this report (see ICFA-HB2006, EPAC’06)