Managed by UT-Battelle for the Department of Energy SNS MEBT : Beam Dynamics, Diagnostics, Performance. Alexander Aleksandrov Oak Ridge National Laboratory

2Managed by UT-Battelle for the Department of Energy Introduction: the SNS Project Parameters : P beam on target 1.44MW I beam aver.1.44mA Beam energy1 GeV Duty factor6% Rep. rate60Hz Pulse width1ms

3Managed by UT-Battelle for the Department of Energy Introduction: the SNS Front End Systems IS/LEBT RFQ MEBT 65kV; 48mA;.2 π mm mrad 2.5MeV; 38mA;.21 π mm mrad 2.5MeV; 38mA;.27 π mm mrad 3.8m 3.6m 12 cm No beam diagnostics 2 BCMs 6 BPM 5 WS Slit/collect Designed and build by Berkeley N L 1ms 60Hz

4Managed by UT-Battelle for the Department of Energy MEBT functions  Matching beam from RFQ to DTL –3 quads and 1 buncher would be sufficient  Place for chopper –Requires relatively long drifts –Increases compexity: match RFQ to chopper – chopper – match to DTL  Place for diagnostics –Try utilizing free space as much as possible.

5Managed by UT-Battelle for the Department of Energy SNS chopper functions Ion source transient Chopper window single mini-pulse  Providing gaps for ring extraction kicker rise time –700ns ON, 300ns OFF pattern at ~1MHz  Macro-pulse shaping – cut off beam during the ion source transient – create single mini-pulse for turn-by-turn measurements in the ring – decimate mini-pulses (N-on-M-off ) to reduce beam current – ramp up current at the beginning of the macro-pulse to help LLRF system in compensating beam loading effects

6Managed by UT-Battelle for the Department of Energy SNS Front End chopping system time MEBT chopper (10ns, 1e-4) current LEBT chopper (50ns, 1e-2) ChopperLEBTMEBT Ion energy~25 keV2.5 MeV  = v/c ~ Max Voltage  3 kV  2.5 kV gap~ 14 mm18 mm Effective length ~ 27 mm~350 mm Max deflection14 o 1.07 o Time of flight~ 12 ns~ 17 ns Two stage: LEBT + MEBT

7Managed by UT-Battelle for the Department of Energy 2.5MeV MEBT Schematic MEBT Layout RMS beam envelope in MEBT. Simulation (solid) and wire scanner measurements (dots).

8Managed by UT-Battelle for the Department of Energy SNS MEBT chopper/anti-chopper arrangement  Theoretically can eliminate partially chopped beam  Requires identical choppers, power supplies and anti- symmetric transverse optic

9Managed by UT-Battelle for the Department of Energy SNS MEBT Layout 3.7 m of 14 quadrupole magnets; 4 buncher cavities; chopper system; diagnostics Design beam envelope in the MEBT

10Managed by UT-Battelle for the Department of Energy Front End Systems in the FE building

11Managed by UT-Battelle for the Department of Energy MEBT BPM and Beam Box

12Managed by UT-Battelle for the Department of Energy Wire Scanner and MEBT Beam Box

13Managed by UT-Battelle for the Department of Energy New diagnostics  Anti-chopper proved to be unnecessary  Additional diagnostics is added into MEBT “d- box” (former “anti-chopper” box): –In-line beam stop/Faraday cup –Horizontal and vertical in-line emittance scanners –Pencil beam apertures (2mm,1.5mm, 1mm) –Laser based longitudinal profile measurement (temporary decommissioned) –View screen (never was used) –In line Fast Faraday (tested prototype only) –Several ports for future developments

14Managed by UT-Battelle for the Department of Energy Beam diagnostics box replaced the anti-chopper

15Managed by UT-Battelle for the Department of Energy MEBT D-box Slits Harps Laser Ports

16Managed by UT-Battelle for the Department of Energy MEBT diagnostics scrapers (low power) collimator position collimator design Manually actuated Observe collimation effect on: 1.MEBT emiitance device 2.Linac wire scanners 3.Loss monitors

17Managed by UT-Battelle for the Department of Energy MEBT Performance  MEBT was commissioned with 6% duty factor beam  50mA peak current transport with no measurable losses demonstrated  Anti-chopper has been removed as unnecessary (based on simulations)  X-ray radiation from the rebunchers exceeds the expected levels –Established Radiation Area around the MEBT. No access allowed during operation  RF amplifiers do not provide design power for Rebunchers #1 and #4 –Considering new design based on IOT or tapping RF power from the RFQ klystron (more in T. Hardek’s talk)  MEBT chopper doesn’t perform to specifications yet

18Managed by UT-Battelle for the Department of Energy How we tune MEBT  RFQ tuning  MEBT rebunchers tuning  MEBT quads tuning

19Managed by UT-Battelle for the Department of Energy RFQ tuning  No diagnostics to measure absolute transmission  No diagnostics to know what is going on inside  Can characterize resulting beam in the MEBT  422 RF cells, strongest space charge  Defines longitudinal and,in large part, transverse emittances  Only one parameter to set : RF amplitude –Fit output current vs. RF power curve to PARMTEQ model to find the set point RF field amplitude [%] Transmission [%]

20Managed by UT-Battelle for the Department of Energy Characterizing RFQ output beam using MEBT wire scanners Measure transverse profiles using 5 wire scanners Search for input Twiss parameters to best fit model to measured data Repeat several times with different quad settings Twiss parameters have been stable since 2003

21Managed by UT-Battelle for the Department of Energy MEBT tuning  Set strengths of 14 quadrupole magnets –Establishing proper beam profile for chopper operation –Matching beam to DTL –Use design quad values usually (~5% PS calibration accuracy) –Verify with wire profile measurements  Set strengths of 6 hor. and 6 ver. dipole steerers –for minimum beam centroid offset at 6 BPM locations  Set amplitude and phase of 4 rebuncher cavities –Set amplitude to design or max available (~80% of nominal) –Set phase to non-accelerating bunching phase using phase scan –Verified once in 2004 with laser longitudinal profile scanner

22Managed by UT-Battelle for the Department of Energy MEBT transmission is always good Beam current after RFQ (red), MEBT (blue)

23Managed by UT-Battelle for the Department of Energy MEBT rebuncher phase scan

24Managed by UT-Battelle for the Department of Energy Ambiguities during the set up  MEBT rebunchers phase –Resulting set point depends on BPMs selected for scan –(5º - 10º uncertainty) BPM05 BPM10 Example of MEBT rebuncher scan producing different results with different BPMs

25Managed by UT-Battelle for the Department of Energy Beam envelope measurements First wire scanner in the MEBT Last wire scanner in the MEBT Gaussian core + ‘tail’ dependent on MEBT tuning

26Managed by UT-Battelle for the Department of Energy MEBT to DTL transverse matching Vertical Beam profile measured at DTL3 exit for different matching quadrupole settings Approaches Gaussian shape when best match is achieved FPAE057, TPAT030

27Managed by UT-Battelle for the Department of Energy Beam profile measurements after DTL tank3 for various Q13,Q14 settings

28Managed by UT-Battelle for the Department of Energy Measurement of MEBT chopper kick strength  Kick strength is measured by measuring beam deflection at chopper target location  Kick strength of the prototype is ~15% below design value  New design will meet design specs with ~15% margin

29Managed by UT-Battelle for the Department of Energy MEBT chopper efficiency  Simple strip line of solid copper (~16 ns TOF)

30Managed by UT-Battelle for the Department of Energy Vertical beam position along the pulse. Chopper is OFF Vertical beam position along the pulse. Chopper is ON Chopper OFF Chopper ON LEBT chopper performance (II) Measured effective beam size increase due to large fraction of partially chopped beam. With 750 Ohm resistor. Much better now with 50Ohm resistor

31Managed by UT-Battelle for the Department of Energy Beam emittance after MEBT. Vertical (top) and horizontal (bottom) Measured emittance after MEBT Dependance of RMS emittance upon peak beam current Design value =.3  mm mrad (RMS normalized)

32Managed by UT-Battelle for the Department of Energy Emittance along the pulse Emittance at the MEBT exit is not sensitive to beam current

33Managed by UT-Battelle for the Department of Energy mode-locked laser diagnostics for longitudinal profile measurement Longitudinal bunch profile in MEBT Longitudinal bunch profile (blue dots) and Gaussian fit (red line). Bottom right: rms bunch length vs. the rebuncher phase (measurements – squares, simulation – stars, solid line – quadratic fit).

34Managed by UT-Battelle for the Department of Energy MEBT Collimation Experiments Scrapers downstream BCM upstream BCM Emittance scan without scraper Emittance scan with scraper

35Managed by UT-Battelle for the Department of Energy Transverse tails in DTL horizontal a = 8.6 sigma Asymmetric tail in horizontal plane Clear effect of MEBT scraper Do not see effect in vertical plane from horizontal scraping – weak coupling DTL wire scanners usable to level (~ 4 sigma) Significant portion of transverse tail originate in Front End vertical a = 16 sigma

36Managed by UT-Battelle for the Department of Energy Symmetric tails in both plane No measurable effect of MEBT scraper CCL wire scanners usable to level (~ 3 sigma) Significant portion of transverse tail in CCL does not originate in Front End or wire scanner measurements not reliable yet Transverse tails in CCL horizontal a = 13 sigma vertical a = 12 sigma

37Managed by UT-Battelle for the Department of Energy PARMILA vs measurements

38Managed by UT-Battelle for the Department of Energy Do we understand beam dynamics in our linac well enough?  good model requires –Accurate computation algorithm –Accurate representation of beam line elements (strength, position, edge fields, etc. )  More complicated in longitudinal plane as element parameters (RF settings) are not given by found from the model itself –Reliable verification  Good beam diagnostics tools  Good model should be able –To provide good agreement with measured beam parameters –To predict change of beam parameters in response to changes in beam line elements –To provide above capabilities from end to end, not only piecewise –Accuracy of agreement should be well within RMS size  We use different codes for centroid motion and RMS simulations –Online Model, PARMILA, IMPACT

39Managed by UT-Battelle for the Department of Energy Accuracy of beam modeling in different parts of the linac Transv. centroid Transv. RMS Long. centroid Long. RMS RFQ? not so good ?? MEBTgood not so good DTLgood not so good ?? CCLvery good not so good very good not so good SCL not so good ?good not so good

40Managed by UT-Battelle for the Department of Energy Do we understand halo?  Expected in transverse plane –Much studied during design process –Observe non-Gaussian transverse profiles  Do not have diagnostics to measure halo extent below ~ 1% –Does not seem to be a source of losses so far  DTL serves as a collimator ? –Do not have reliable information on losses inside DTL  Probably responsible for some ring injection losses  Did not expect in longitudinal plane –Very low level –Have only limited direct diagnostics tools ( CCL BSMs )  Problem of initial distribution for simulations –“reference” distribution used at design stage seems not adequate