Investigation of ion capture in an Electron Beam Ion Trap charge-breeder for rare isotopes

Transcription

1 Investigation of ion capture in an Electron Beam Ion Trap charge-breeder for rare isotopes Kritsada Kittimanapun ATD seminar August 26, 2014

2 Outline Electron beam ion source/trap principle EBIT charge breeder for ReA Simulation of acceptance and capture efficiency of NSCL EBIT Experimental results Conclusion and outlook K. Kittimanapun Slide 2

3 Electron beam ion source/trap principle EBIT : device to create highly charged ions by electron impact ionization Electron beam: electron impact ionization, radial confinement Magnetic field: electron beam compression Trap electrode: axial confinement Breeding time : e = elementary charge σ EI = electron impact ionization cross section j e = electron beam current density Continuous injection: (no buncher needed) Pulsed extraction: Inner barrier Outer barrier 1+ Q+ Trapping condition: Charge state changes in trap Extraction process: Outer barrier pulsed to trap potential K. Kittimanapun Slide 3

4 Other EBIS/T around the World Flash-EBIT TITAN-EBIT Stockholm Dubna Belfast Kielce Frankfurt Dresden Heidelberg Geneva Shanghai Tokyo Vancouver LLNL NSCL ANL Brookhaven Applications: Atomic spectroscopy Surface interaction Charge breeder Charge breeder (post-accelerator for rare isotope beam) CERN BNL-EBIS TITAN / TRIUMF S-EBIT REXEBIS at CERN was first to successfully use EBIS charge-breeder for rare isotopes NSCL: in commissioning, ANL/TRIUMF: under construction / in planning Super Users Meeting 8/11 4

5 What is ReA: ReApost accelerator at NSCL Post-accelerator to reaccelerate rare isotopes to an energy of a few hundred kev/u several MeV/u Purpose: ostudy key reactions in nuclear astrophysics at near-stellar energies. o Nuclear structure studies near/above the Coulomb barrier. Low energy beam Coupled cyclotron facility (> 80 MeV/u) High energy beam Stopped beam area ReA post-accelerator Intermediate energy beam A1900 separator K. Kittimanapun Slide 5

6 Reacceleratorconcept for rare isotope beam Reacceleration of highly charge ions: compact, cost effective charge breeder Reacceleration of highly charge ions: compact, cost effective charge breeder EBIT charge breeder: Short breeding time, Clean beam, High efficiency (injection, ejection, narrow charge state distribution) Linear accelerator SRF cryomodules MeV/u for 238 U MeV/u for light elements Charge-over-mass separator K. Kittimanapun Slide 6

7 Requirements for a rare-isotope charge breeder Fast (breeding time < 50 ms) Short half-lives High electron beam current density (large electron beam current + strong magnetic field) Large capacity (10 10 positives charges) High intensity (FRIB) Large trapping region + high electron beam current High efficiency (20 50%) Rare isotope K. Kittimanapun Slide 7

8 EBIT charge breeder efficiency Efficiency of EBIT charge breeder depends on Injection and extraction efficiency Good transport Narrow charge-state distributions : Proper electron beam energy and breeding time Capture efficiency Fast charge breeding into charge state 2 + Good overlap between ion and electron beam Importance of overlap of electron and ion beam for capture of ion beam x y e- beam Full overlap, -ideal! Ion trajectory Partial overlap -nice, but bad! No overlap -no capture K. Kittimanapun Slide 8

9 NSCL electron beam ion trap EBIT design parameters: High beam current < 2.5 A Magnetic field up to 6 T High current density A/cm 2 Electron gun Superconducting magnet Helmholtz coils Solenoid 0.8 m Electron collector Solenoid : low compression, long trap, large acceptance Helmholtz coil: high compression, short breeding time Electron gun Magnet Electron collector K. Kittimanapun Slide 9

10 My EBIT Research Study ion capture in the ReAEBIT : simulation Develop a code to study physics of EBIT Optimize EBIT acceptance, support commissioning Benchmark with reliable tools to validate code Study capture efficiency EBIT parameters: electron beam current, magnetic field, trap size, ion beam energy etc. Optimize ion transport optics K. Kittimanapun Slide 10

11 My EBIT Research Study ion capture in the ReAEBIT : experiment Develop new technique to optimize ion injection and determine space charge potential Investigate capture efficiency for different EBIT parameters Study charge breeding process and measure effective electron beam current density Compare simulation against experimental results and use as guidance for EBIT operation K. KittimanapunSlide 11

12 Numerical Simulations Develop NSCL EBIT Simulation Code (NEBIT) NEBIT: Optimize acceptance and study ion behavior in EBIT How:Calculate ion trajectory with Monte Carlo electron impact ionization (EI) Use: Electric field: SIMION Magnetic field: Analytic solution for coil set Space charge potential: analytical model Ion dynamics : Runge-Kutta integrator EI cross section: Lotzformula * *W. Lotz, Z. Phys 206:205, 1967 K. Kittimanapun Slide 12

13 Physics forebit simulation Sample trajectory Ion beam e beam Collector Barrier Trap center Electric field: Electrode voltages + space charge (electron beam) Magnetic field: 1T (solenoid) -6T (Helmholtz) configuration Axial magnetic field (T) Potential (kv) Potential (kv) Bz (T) E-beam current 0.8 A e-beam radius Axial B-field Drift tube potential Space charge potential z (mm) Total potential -1.2 kv mm Axial coordinate z (mm) (mm) 6T Space charge potential (kv) Space charge potential (kv) Electron beam radius (mm) Space charge potential (kv) K. Kittimanapun Slide 13

14 Physics forebit simulation Sample trajectory Ion beam e beam Collector Barrier Trap center Monte Carlo Ionization process : Breeding process Captured ion K. Kittimanapun Slide 14

15 From acceptance to capture probability Acceptance = phase space of captured ions Capture probability = overlap of ion beam emittance and acceptance Acceptance Capture probability a (mrad) emittance x (mm) Capture probability kevion beam 0.8 A electron beam ε (π mm mrad) K. Kittimanapun Slide 15

16 NEBIT Code Benchmarking Test of energy conservation along ion trajectory Comparison of capture efficiency between NEBIT and analytic formula charge evolution between NEBIT and CBSIM capture efficiency between current and earlier versions of NEBIT acceptance from NEBIT and analytical formula K. KittimanapunSlide 16

17 NEBIT Code Benchmarking Comparison of acceptance from NEBIT with analytical formula Analytical formula of EBIT acceptance * : electron beam, radius, and magnetic field Determine maximum number of ions fit into electron beam (exclude EI process) Ion trajectory Electron beam Acceptance phase space a x, a y (mrad) x-direction y-direction Analytical value Acceptance (E e 12.5 kev, I e 1 A, B-field 6 T): Analytical value = 2.22 πmm mrad NEBIT value = 2.20 πmm mrad(0.9% error) x, y (mm) Both results are consistent and code is ready to be used * F. Wenander, CERN-OPEN, K. Kittimanapun Slide 17

18 Measurement of emittance of beam from test ion source Preparation of ion injection Experimental Studies Measure axial energy spread of ion beam Optimize injection with new technique Investigation of capture process Capture efficiency vs. EBIT parameters (electron beam current, trap size, and trap potential ) Study of charge state evolution Determine optimum charge breeding time and calculate effective current density K. Kittimanapun Slide 18

19 New approach to optimize ion injection Study ion reflection with time-of-flight spectra Intuitively determine ion reflection region Maximize the transport efficiency into the EBIT trap Ion reflection occurs due to axial kinetic energy < electric potential MCP Q/A separator Ion source Recording of time-of-flight signal starts when the deflector voltage changes from injection to extraction voltage K + BOB1 Deflector K + EBIT K. Kittimanapun Slide 19

20 New approach to optimize ion injection K + MCP signal vstime-of flight MCP signal (mv) Trap entrance LTE1 Inner barrier (LTE4) With this technique: Ions mostly reflect off inner barrier and trap entrance By monitoring ion current with FC, more than 95 % of detected beam reached the EBIT trap center K. Kittimanapun Slide 20

21 Investigation of Capture Efficiency Q/A spectrum of highly charged K in EBIT Current@BOB4 (pa) K 18+ K 18+ K 17+ K 17+ K 16+ K 16+ K K 14+ K 14+ K 13+ K 13+ K 12+ K 12+ Q/A W ith K+ injection Electron beam current 126 ma Electron beam energy 19.5 kev Continuous injection with 5 Hz repitition rate K 11+ K 11+ KK 10+ K K 8+ K 8+ Total efficiency : K. Kittimanapun Slide 21

22 Investigation of Capture Efficiency Capture efficiency vs electron beam current 1.4 π mm mrad Maximum efficiency 2.3% 5.5 π mm mrad Experimental and simulated efficiencies follow the same trend but differ significantly Larger electron beam current leads to higher electron beam current density faster process for charge state higher capture efficiency K. Kittimanapun Slide 22

23 Investigation of Capture Efficiency Capture efficiency vs trap potential depth Trap potential needs to be optimized : Shallow trap potential small axial kinetic energy Deep trap potential efficiently trap ions of 2+ charge state 6 Capture efficiency (%) Upper bound simulated efficiency / 7 Lower bound simulated efficiency / 7 experiment 1.4 π mm mrad π mm mrad Optimized trap potential is at -30 V Trap depth (V) K. KittimanapunSlide 23

24 Investigation of My Capture Efficiency Why is capture efficiency overpredicted by factor 7? Possible reasons : Experimental emittance > expected emittance? With large emittance, simulation overpredicts by a factor 3 Ion beam misalignment with electron beam? NEBIT expects factors of 1.2, 2 for 0.5, 1 mm misalignment Limitation of trap capacity? EBIT trap overfilled with 1.4 na injected beam Including this factor, overprediction drops factor of 1.5 Electron beam not uniformly distributed? K. Kittimanapun Slide 24

25 Study of charge state evolution of K ions Determination of effective current density Charge evolution of potassium A/cm Q/A 0.2 Effective electron beam current density for K 12+ = 157 A/cm 2 and K 16+ = 243 A/cm 2 K. Kittimanapun Slide 25

26 Study of charge state evolution of K ions Distribution of electron beam current density Space charge potential Simulation of electron beam current density K 12+ K 12+ K 16+ K 16+ K 1+ High charge state small radius high current density Overpredictionof capture efficiency can be explained if K 1+ ions travel in a region of low electron beam current density K. Kittimanapun Slide 26

27 Conclusion and my outlook Transport efficiency of 95% has been achieved with a new technique to optimize ion injection Simulation overpredictedthe experimental capture efficiency of 2.3% by a factor 7 Effective electron beam current density was determined Distribution of electron beam current density is an important factor for overprediction NEBIT can be improved by importing the electric field of space charge from SIMION and including different electron beam current density distribution K. Kittimanapun Slide 27

28 Facility of Rare Isotope Beam (FRIB) Project completion : June 2022 K. KittimanapunSlide 28

29 Acknowledgement Georg Bollen (Advisor) Oliver Kester EBIT Team: Stefan Schwarz Alain Lapierre Thomas M. Baumann Committee members: Daniela Leitner Norman Birge Vladimir Zelevinsky ReA people and many more Thank you for attention! K. Kittimanapun Slide 29

30 K. Kittimanapun Slide 30

31 Electron potential and Herrmann radius Electron potential Herrmann radius K. KittimanapunSlide 31

32 Charge evolution (1) Electron impact ionization Radiation combination K. KittimanapunSlide 32

33 Charge exchange Charge evolution (2) Ion heating by electron beam K. KittimanapunSlide 33

34 Charge evolution (3) Ion-ion energy exchange K. KittimanapunSlide 34

35 Which breeder Requirements Breeder requirements High efficiency, breed into 1 charge state Breeding times ~ 10 ms Beam intensity ~ 10 9 ions/s EBIS/EBIT charge breeding is the method of choice over ECRs Continuous injection High acceptance, low emittance Fast and slow extraction Expected performance of ECR and EBIS/T: Mini workshops at NSCL with external experts, January and June 2006 ε (A<40) ε (A=100) ε (A=200) Breeding times Beam limit Risk ECR <20% (1 CS) <20% (1 CS) <20% (1 CS) 50 ms >> 10 9 /s present performance: 20% of values EBIT/EBIS > 60% (1 CS) > 50% (1 CS) > 40% (1 CS) 10 ms > 10 9 /s present performance : 25-50% of values 1+ scheme 40% (1-2 CS) 16% (3 CS) 12% (4 CS) for reacceleration of beams with rates as expected for ISF and similar facilities EBIS + post-accelerator concept already successfully in use at REX-ISOLDE at CERN % >> 10 9 /s no ε(ebit)/ε(1+) Single charge state

36 Space charge potential Electron beam energy Drift tube potential Space charge potential Iterative solution for Electron beam energy Electron energy (kev) E 0 E 1 E n Electron current 2.4 A Magnetic field 6 T Initial electron beam energy 12.5 kev, Space charge potential (kv) z ( m ) U 0 U n U z ( m m ) SC potential kv (without correction) SC potential kv (with correction) U = 0.75 kv K. Kittimanapun Slide 36

37 Test of energy conservation Aim : To check numerical error from calculation if NEBIT handles forces correctly Parameters : Fe-56, 60 kev, 1T6T magnetic field configuration Deviation of total energy (ev) Energy (kev) x = 2.31 mm, y = 0.12 mm ax = m/s, ay = 1192 m/s I e = 1A Energy of both on- and off-axis Off-axis On-axis Total energy Potential energy Kinetic energy Deviation of total energy (ev) 25 x = 2.31 mm, y = 0.12 mm ax= m/s, ay= 3946 m/s I e = 2.5A On-axis Off-axis z (m) z (m) Energy is conserved with negligible deviation < 1% space charge potential K. KittimanapunSlide 37

38 Ion trajectory validation Ion trajectories compared against SIMION (both existence and absense of space charge) Ion beam parameters -> Fe-56, 60 kev, x ini = y ini = 0.5 mm, ax ini = ay ini = 0 mrad EBIT parameters -> 1T6T magnetic field configuration Without space charge With space charge (Ie =1A) Trajectory deviation 1. 0 N E B I T S I M I O N w i t h p o i s s o n s o l v e r 0. 5 y (mm) z ( m ) Trajectories are identical as the deviation is negligible K. KittimanapunSlide 38

39 Test of geometrical acceptance with NSCL EBIT Aim : To confirm NEBIT can provide geometrical acceptance of complicated system and consistent with analytical formula Parameters : Electron beam energy 12.5 kev, 0.1 A, 1T6T magnetic field T6T-1T 1T6T-6T 6T6T 2 ax (mrad) x (mm) K. KittimanapunSlide 39

40 Test of geometrical acceptance with NSCL EBIT Aim : To confirm NEBIT can provide geometrical acceptance of complicated system and consistent with analytical formula Parameters : Electron beam energy 12.5 kev, 0.1 A, 1T6T magnetic field 8 Calculation acceptance (pi mmmrad) NEBIT FW formula %error 0.1 A 1T6T-1T T6T-6T T6T ax (mrad) T6T-1T 1T6T-6T 6T6T x (mm) NEBIT calculates the geometrical acceptance consistent with analytical formula K. KittimanapunSlide 40

41 Check of capture probability Aim : Compare the capture probability from NEBIT and a combination of geometrical acceptance and ionization cross section Condition : Ion is flying through constant magnetic and space charge fields. Capture probability vs emittance Geometrical acceptance Ionization probability NEBIT provides πmm mrad 14% are ionized before the first barrier ~8.5% lost in the trap K. KittimanapunSlide 41

42 Reaccelerator concept for rare isotope beam Q/A-separator EBIT RIB RFQ K. KittimanapunSlide 42

43 Charge evolution and ion dynamics + Single electron impact ionization ( 10 µs for ) - Radiative recombination ( 10 ms) - Charge exchange between ions-neutral atoms ( 100 ms) Ion heating by electron beam (10 ms 10 s) Ion-ion energy exchange ( 1 ms) ; σ( E e, I A ) ; σ( E e, q A ) ; σ( q A, I B ) ; R( M j, M i ) ; R( E e, M i ) For the acceptance calculation, only the electron impact ionization is considered Time scale for a current density 4x10 4 A/cm 2 K. Kittimanapun Slide 43

44 Energy spread and radial energy Aim : Minimize the radial energy to increase overlap fraction 400 BOB1 Collector Faraday cup Electron gun Current (pa) Potential barrier Trap potential Potential barrier Faraday cup 1st derivative of current (pa/kv) nd potential barrier (kv) Beam energy kevwith FWHM 2 ev K. KittimanapunSlide 44

45 Electron impact ionization ; σ( E e, I A ) x10-17 K cross section of potassium Breeding time : EI Cross section (cm 2 ) E-3 1E-4 1E-5 Electron energy > K 9+ Ionization energy K Electron beam energy (kev) For j e = 667 A/cm 2, E e = 19.5 kev, σ= 9.24 x cm 2 t 12 = 16.2 µs

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47 Charge state and breeding time K. KittimanapunSlide 47

48 New approach to optimize ion injection Reproduction of TOF spectrum of ion reflection Ideal expectation with 2 reflection regions Voltage (mv) Time (µs) Bottom deflector electrode Injection voltages Extraction voltage Upper deflector electrode Time Problem of electronic device figured out by TOF spectra Trap entrance Inner barrier K. Kittimanapun Slide 48

49 Comparison of NEBIT Predictions with CBSIM Charge evolution calculated from two different approaches; CBSIM : Rate equation with semi-empirical EI cross section NEBIT : Monte-Carlo based ion trajectory calculation Parameters : Electron beam energy 12.5 kev, 1A electron beam currents, Fe-beam, 6T6T magnetic field Optimal breeding time for Fe 15+ : CBSIM = 0.4 ms, NEBIT = 0.6 ms Charge state evolution NEBIT provides charge state evolution consistent with CBSIM K. Kittimanapun Slide 49

50 Emittancemeasurement 10 Intensity Row Numbers K Triplet Row Numbers Method: Capture beam images with different potential applied to a quadrupole Extract beam sizes (1σ) from Gaussian fit Obtain transfer matrix from SIMION Extract emittance from fitting beam size with transfer matrix elements K. Kittimanapun Slide 50

51 Emittancemeasurement Emittance fit 8 Quadrupole A ε x = 1.4 πmm mrad ε y = 4.5 ±0.4 πmm mrad 1.4 Quadrupole B x 2, y 2 (mm 2 ) 4 x 2 (mm 2 ) y 2 (mm 2 ) 2 Vertical axis Horizontal axis 0.6 x 2,y 2 (mm 2 ) Voltage (V) Quadrupole C ε x = 5.5 ±0.1 πmm mrad ε y = 3.7 ±0.4 πmm mrad Voltage (V) Emittancecannot be fitted for quadrupole B Beam diameter is large at quadrupoleb Emittanceranges πmm mrad Quadtrupole Voltage (V) K. Kittimanapun Slide 51

52 Investigation of Capture Efficiency Capture efficiency vs trap size 5.5 π mm mrad Different trap sizes obtained by adjusting trap potential 1.4 π mm mrad Larger trap size leads to Longer traveling time Higher ionization probability for charge state higher capture efficiency K. Kittimanapun Slide 52

53 Experimental setup Test ion source Q/A separator Produce K + beam of 20 kevvia surface ionization process Diagnostic devices at BOBs MCP Faraday cup M. Portillo et al., Proceeding of PAC09 ion and charge state selection 2 electrostatic benders and 1 bending magnet Q/A acceptance Acceptance ~120 πmm mradfor a beam of 12 kev/n Image a beam and detect the TOF signal Monitor ion beam electric current K. Kittimanapun Slide 53

54 New approach to optimize ion injection MCP signal vstime-of flight 1+ With this technique, Two returning locations : inner barrier and trap entrance By monitoring ion current with FC, more than 95 % of detected beam reached the EBIT trap center Trap entrance (LTC11 ) LTRAP LTE1 Inner barrier (LTE4) K. Kittimanapun Slide 54

55 Determination of Trap Capacity Beam current of K 16+ (pa) K 16+ current vsinjected beam current Charge breeding efficiency of K 16+ vsinjected current Charge breeding breeing efficiency into K (%) x10 X Injected beam current (pa) Injected beam current (pa) Efficiency depends on incoming beam current With I e =135 ma, E e = 19.5 kev, L trap = m Charge capacity 1nC 1 na Efficiency of K 16+ = 2.4 x 10-3 for 1.4 naincoming beam Capture efficiency drop by a factor of 1.5 K. Kittimanapun Slide 55

56 Experimental setup 2T2T magnetic field configuration 19.5 kev electron beam energy K. Kittimanapun Slide 56

57 New approach to determine effective space charge potential Space charge affects to ion trajectory 1+ Without e-beam Voltage (mv) Without e-beam With e-beam With electron beam of 90 ma and 19.5 kev: Total potential becomes lower ions travel faster Change in TOF signal allows determination of space-charge potential affecting to K + Electron beam is not uniformly distributed over its cross section (more details later) Effective space charge potential on K + is ~20 V Voltage (mv) Voltage (mv) With 90 ma e-beam With 25 ma e-beam 1: trap entrance, 3: trap end 2: area between trap entrance and end K. Kittimanapun Slide 57

58 K. KittimanapunSlide 58

59 Present status and EBIT outlook Electron gun has been modified and is able to provide 800 ma Electron beam radius was measured and larger than expected EBIT has reached a 30% capture efficiency EBIT is a suitable charge breeder for ReA Working towards higher current and current density K. Kittimanapun Slide 59