Investigation of ion capture in an Electron Beam Ion Trap charge-breeder for rare isotopes Kritsada Kittimanapun ATD seminar August 26, 2014
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
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 1+ 2+ in trap Extraction process: Outer barrier pulsed to trap potential K. Kittimanapun Slide 3
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) REX-EBIS @ 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
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
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 0.3-3 MeV/u for 238 U 0.3-6 MeV/u for light elements Charge-over-mass separator K. Kittimanapun Slide 6
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
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
NSCL electron beam ion trap EBIT design parameters: High beam current < 2.5 A Magnetic field up to 6 T High current density 10 4-10 5 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
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
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
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
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) 60 58 56 54 52 50 8 6 4 2 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 0 0.04 mm 0.01 1000 1500 2000 2500 Axial coordinate z (mm) (mm) 6T 0.0-0.2-0.4-0.6-0.8-1.0-1.2 100 10 1 0.1 Space charge potential (kv) Space charge potential (kv) Electron beam radius (mm) Space charge potential (kv) K. Kittimanapun Slide 13
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
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) 40 30 20 10 0-10 -20-30 -40 emittance -4-3 -2-1 0 1 2 3 4 x (mm) Capture probability 1.2 1.0 0.8 0.6 0.4 0.2 0.0 60 kevion beam 0.8 A electron beam 0 10 20 30 40 50 ε (π mm mrad) K. Kittimanapun Slide 15
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
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) 10 5 0 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) -5-10 -0.4-0.2 0.0 0.2 0.4 x, y (mm) Both results are consistent and code is ready to be used * F. Wenander, CERN-OPEN, 2000-320 K. Kittimanapun Slide 17
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
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
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
Investigation of Capture Efficiency Q/A spectrum of highly charged K in EBIT Current@BOB4 (pa) 120 100 80 60 40 20 0 K 18+ K 18+ K 17+ K 17+ K 16+ K 16+ K 15+ 15+ 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 0.33 0.25 K 11+ K 11+ KK 10+ K 9+ 9+ K 8+ K 8+ Total efficiency : K. Kittimanapun Slide 21
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 1+ 2+ charge state higher capture efficiency K. Kittimanapun Slide 22
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 (%) 5 4 3 2 Upper bound simulated efficiency / 7 Lower bound simulated efficiency / 7 experiment 1.4 π mm mrad 1 5.5 π mm mrad Optimized trap potential is at -30 V -70-60 -50-40 -30-20 -10 Trap depth (V) K. KittimanapunSlide 23
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
Study of charge state evolution of K ions Determination of effective current density Charge evolution of potassium A/cm 2 12+ 16+ 0.33 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
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
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
Facility of Rare Isotope Beam (FRIB) Project completion : June 2022 K. KittimanapunSlide 28
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
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Electron potential and Herrmann radius Electron potential Herrmann radius K. KittimanapunSlide 31
Charge evolution (1) Electron impact ionization Radiation combination K. KittimanapunSlide 32
Charge exchange Charge evolution (2) Ion heating by electron beam K. KittimanapunSlide 33
Charge evolution (3) Ion-ion energy exchange K. KittimanapunSlide 34
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+) 1.5 3 3 10 3 12 Single charge state
Space charge potential Electron beam energy Drift tube potential Space charge potential Iterative solution for Electron beam energy Electron energy (kev) 1 6 1 4 1 2 1 0 8 6 4 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) 2 3 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2 1 0-1 - 2-3 - 4 z ( m ) U 0 U n U 1-5 1. 0 1. 2 1. 4 1. 6 1. 8 2. 0 2. 2 2. 4 z ( m m ) SC potential -3.37 kv (without correction) SC potential -4.12 kv (with correction) U = 0.75 kv K. Kittimanapun Slide 36
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) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 60 50 40 30 20 10 0 x = 2.31 mm, y = 0.12 mm ax = 14286 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= 13922 m/s, ay= 3946 m/s 20 15 10 5 0 I e = 2.5A On-axis Off-axis 0.0 0.5 1.0 1.5 2.0 2.5 z (m) 0.0 0.5 1.0 1.5 2.0 2.5 z (m) Energy is conserved with negligible deviation < 1% space charge potential K. KittimanapunSlide 37
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) 0. 0-0. 5-1. 0 0. 0 0. 5 1. 0 1. 5 2. 0 z ( m ) Trajectories are identical as the deviation is negligible K. KittimanapunSlide 38
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 6 4 1T6T-1T 1T6T-6T 6T6T 2 ax (mrad) 0-2 -4-6 -8-0.3-0.2-0.1 0.0 0.1 0.2 0.3 x (mm) K. KittimanapunSlide 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 8 Calculation acceptance (pi mmmrad) NEBIT FW formula %error 0.1 A 1T6T-1T 1.47 1.5-2.0 1T6T-6T 0.65 0.68-4.4 6T6T 0.64 0.68-5.9 ax (mrad) 6 4 2 0-2 -4-6 1T6T-1T 1T6T-6T 6T6T -8-0.3-0.2-0.1 0.0 0.1 0.2 0.3 x (mm) NEBIT calculates the geometrical acceptance consistent with analytical formula K. KittimanapunSlide 40
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 57.49 % @ 3 πmm mrad 14% are ionized before the first barrier ~8.5% lost in the trap K. KittimanapunSlide 41
Reaccelerator concept for rare isotope beam Q/A-separator EBIT RIB RFQ K. KittimanapunSlide 42
Charge evolution and ion dynamics + Single electron impact ionization ( 10 µs for 1 + 2 + ) - 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
Energy spread and radial energy Aim : Minimize the radial energy to increase overlap fraction 400 BOB1 Collector Faraday cup Electron gun Current (pa) 200 0 19.980 19.985 19.990 19.995 20.000 Potential barrier Trap potential Potential barrier Faraday cup 1st derivative of current (pa/kv) 19.980 19.985 19.990 19.995 20.000 2 nd potential barrier (kv) Beam energy 19.99 kevwith FWHM 2 ev K. KittimanapunSlide 44
Electron impact ionization ; σ( E e, I A ) x10-17 K 1+ 100 10 cross section of potassium Breeding time : EI Cross section (cm 2 ) 1 0.1 0.01 1E-3 1E-4 1E-5 Electron energy > K 9+ Ionization energy K 19+ 0.01 0.1 1 10 Electron beam energy (kev) For j e = 667 A/cm 2, E e = 19.5 kev, σ= 9.24 x 10-18 cm 2 t 12 = 16.2 µs
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Charge state and breeding time K. KittimanapunSlide 47
New approach to optimize ion injection Reproduction of TOF spectrum of ion reflection -22-20 Ideal expectation with 2 reflection regions -18-16 Voltage (mv) -14-12 -10-8 -6-4 0 50 100 150 200 250 300 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
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
Emittancemeasurement 10 Intensity 5 0 0 50 100 150 Row Numbers K + 15 10 Triplet 5 0 0 50 100 150 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
Emittancemeasurement Emittance fit 8 Quadrupole A ε x = 1.4 πmm mrad ε y = 4.5 ±0.4 πmm mrad 1.4 Quadrupole B 20 16 6 1.2 x 2, y 2 (mm 2 ) 4 x 2 (mm 2 ) 1.0 12 8 y 2 (mm 2 ) 2 Vertical axis 0.8 4 0 Horizontal axis 0.6 x 2,y 2 (mm 2 ) 25 20 15 10 5 600 900 1200 1500 1800 2100 2400 Voltage (V) Quadrupole C ε x = 5.5 ±0.1 πmm mrad ε y = 3.7 ±0.4 πmm mrad 2500 2600 2700 2800 2900 3000 3100 3200 Voltage (V) Emittancecannot be fitted for quadrupole B Beam diameter is large at quadrupoleb Emittanceranges 1.4 5.5 πmm mrad 0 0 1000 1200 1400 1600 1800 2000 2200 2400 Quadtrupole Voltage (V) K. Kittimanapun Slide 51
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 1+ 2+ charge state higher capture efficiency K. Kittimanapun Slide 52
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 0.2-0.5 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
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
Determination of Trap Capacity Beam current of K 16+ (pa) 80 70 60 50 40 30 20 10 0 K 16+ current vsinjected beam current Charge breeding efficiency of K 16+ vsinjected current Charge breeding breeing efficiency into K 16+ 16+ (%) x10 X 10-3-1 4.5 4.0 3.5 3.0 2.5 0 500 1000 1500 2000 Injected beam current (pa) 2.0 10 100 1000 Injected beam current (pa) Efficiency depends on incoming beam current With I e =135 ma, E e = 19.5 kev, L trap = 0.637 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
Experimental setup 2T2T magnetic field configuration 19.5 kev electron beam energy K. Kittimanapun Slide 56
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
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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