Beam beam simulation. S.Isaacman, A.Long,E.Pueschel, D.Rubin. October 1,

Transcription

1 Beam beam simulation S.Isaacman, A.Long,E.Pueschel, D.Rubin October 1,

2 Weak-strong simulation - Strong beam is fixed - Weak beam consists of N macro particles. - Track weak beam macro-particles through lattice Guide field includes all magnetic elements Third order map for wigglers RF cavities Synchrotron radiation damping and excitation Beam beam elements at each of parasitic crossing points and at IP Beam beam element is 2-d. Represent longitudinal extent of strong bunch at IP with 2-d slices October 1,

3 Single beam scan Before/after sextupole optimization to Minimize energy dependence of beta October 1,

4 Weak-strong simulation - Rationale In CESR, nonlinearities associated with wigglers, pretzel, multiple parasitic crossings, impose the beambeam current limit Conjecture - Coherent beam beam effects do not contribute because the threshold is even higher October 1,

5 Simulation Initialization Specify Horizontal, vertical, synchrotron tunes Bunch current Bunch pattern Long range beam beam ineractions - Identify locations of parasitic crossings - Split elements at pc s - Reverse separator polarities and compute closed orbit, twiss parameters of strong beam -Insert beambeam element at splits using coordinates of closed orbit to set x,y offset, and twiss parameters and emittance to set beam size Beam beam interaction at IP - Compute orientation and profile of strong beam at IP (depends on beta*, coupling parameters, emittances) - Insert beam beam element and set offsets, and sizes October 1,

6 Collision assurance Strong beam There is no symmetry to ensure that the closed orbit at IP is zero and in general it is not - Imperfect closure of L3 vertical separation bump - lattice/ pretzel asymmetry - field errors Weak beam Closed orbit depends on current in strong beam And bunch pattern - parasitic crossings October 1,

7 Collide Beams Close pretzel - Adjust voltage of separators 8E/8W (pretzing 13) to zero differential horizontal offset at IP For each voltage setting Update closed orbit for strong beam reset offsets for beambeam elements for parasitic crossings Compute closed orbit for weak beam Close vertical - Adjust vertical phase advance between vertical separators and vertical separator voltage asymmetry to zero differential vertical offset and angle at IP Update strong beam closed orbit and beambeam element offsets Set tunes - Qtune (arc quads) to specified horizontal and vertical tunes. - Cavity voltage -> synchrotron tune Repeat 7

8 BEAMBEAM_SCAN: Initially Qx = Qy = Closed orbit E E E E BEAMBEAM_SCAN: After parasitic interactions added 2.0mA/bunch Qx = Qy = Closed orbit E E E E-03 Strong beam: sigma_x = E-03 sigma_y = E-05 sigma_z = E-01 Pitch : x= E-02 y= E-03 Offset : x= E-03 y= E-05 Tilt = E-03 BEAMBEAM_SCAN: After beambeam added 2.0mA/bunch Qx = Qy = Closed orbit E E E E-03 October 1,

9 close_pretzel: 0 8W(mr) = E(mr) = dx,dxp,dy,dyp (mm) = W(mr) = E(mr) = dx,dxp,dy,dyp (mm) = W(mr) = E(mr) = dx,dxp,dy,dyp (mm) = Qx = Qy = Qz = CLOSE_VERT 0 48W(mr) = E(mr) = dx,dxp,dy,dyp (mm) = W(mr) = E(mr) = dx,dxp,dy,dyp (mm) = W(mr) = E(mr) = dx,dxp,dy,dyp (mm) = Qx = Qy = Qz = Closed orbit E E E E-03 BEAMBEAM: turn off beam beam at IP Qtune with pretzel and vert closed but beam beam at IP off: Qx = Qy = Qz = CLOSE_VERT: 0 48W(mr) = E(mr) = dx,dxp,dy,dyp (mm) = W(mr) = E(mr) = dx,dxp,dy,dyp (mm) = qtune to Qx = Qy = Qz = Turn Beambeam on Qx = Qy = dx,dxp,dy,dyp (mm) =

10 Generate weak beam distribution - Calculate equilibrium normal mode emittances - Generate random distribution in horizontal, vertical and longitudinal phase space - Transform to lab coordinates - Shift centroid of distribution to coordinates of closed orbit at IP - Tilt distribution to match angles of closed orbit October 1,

11 October 1,

12 Track weak beam macro particles - Track through guide field that now includes beambeam elements at parasitic crossings and at IP - Particles do not communicate with each other Every N turns (~500) - Luminosity Calculate luminosity - Update size of strong beam Fit gaussians to x-y-z distribution of weak beam Set size of strong beam to match weak beam - Is the distribution of the weak beam gaussian? October 1,

13 Distribution after 200,000 turns I b =1.25mA October 1,

14 χ 2 vs turn For fitted gaussian to weak beam distribution -> weak beam remains gaussian October 1,

15 Convergence in ~ 10 Damping times (200,000 turns) 200k turns 1000k turns October 1,

16 Dependence on number of macro-particles in weak beam October 1,

17 October 1,

18 October 1,

19 Parallel processing After weak beam distribution is generated, track particles on independent nodes Every N (500) turns send particle coordinates back to central processor Linux clusters in use CHESS - Sirius, Feynmann (<100 dual processors) CLEO - lnx (12 dual processors) 200 particles turns/15seconds on 2X9 processors =200X particle-turns/6000 seconds Theory Center? October 1,

20 Tune scan 200 particles 200k turns 1.5mA 9X4 equilibrating strong beam October 1,

21 D sep-04 8X4, 1.89GeV 200 particles 200k turns 9X4 October 1,

22 October 1,

23 5.3GeV 9X4 Red - equilibrating Strong beam size Green - fixed strong beam 0.5% emittance coupling D October 1,

24 1.89GeV 200k turns 200 particles Red -Skew errors from 9-sep-27 characterization Green - No skew errors October 1,

25 Efffect of parasitic crossings 1.89GeV Single bunch 9X4 October 1,

26 Summary - Status -Measured vs calculated tunes Is relevant tune with/without parasitic interactions? How does it depend on current? -Number of macro particles in weak beam More macro particles -> higher luminosity Perhaps 200 is not enough to identify 3 gaussian distributions -How to introduce emittance coupling without prejudice? -Contintue investigation of effect of long range interactions Use tune scan to find operating point and compare with measurements To date weak beam bunch 1. Is there some bunch dependence? -Simulate head on conditions and compare with measurements - Tune simulation - Any/all lattice parameters/groups can be tuned - More computers? October 1,

27 CESR-c Electrostatically separated electron-positron orbits accomodate counterrotating trains Electrons and positrons collide with ±~3 mrad horizontal crossing angle 9 5-bunch trains in each beam (768m circumference) Beam October 1,

28 Open squares are calculation Red points are data October 1,

29 October 1,