Beam optics analysis of large-acceptance superconducting in-flight separator BigRIPS at RIKEN RI Beam Factory (RIBF)

Transcription

1 Beam optics analysis of large-acceptance superconducting in-flight separator BigRIPS at RIKEN RI Beam Factory (RIBF) Magnetic spectrometer used for the production of radioactive isotope (RI) beams based on in-flight scheme Objective: study of exotic nuclei far from the stability using a variety of RI beams Presented by Hiroshi Suzuki RIKEN Nishina Center Toshiyuki Kubo, Hiroyuki Takeda, Kensuke Kusaka, Naohito Inabe, Naoki Fukuda, & Daisuke Kameda Germany, Aug. 2, 212

2 Outline of My Talk Introduction Overview of BigRIPS separator, emphasizing its ionoptics issues Optics Calculation Optics Calculation for BigRIPS, which is a large acceptance and large-aperture ion-optical system Field map measurements The procedure to deduce b n, (z) from magnetic field vector The procedure to fit the Enge function Optics calculation using with COSY INFINITY Comparison with Measurement Matrix terms A/Q resolution from New-isotope Search Exp. Summary

3 Introduction

4 RIBF Layout New-generation in-flight RI beam facility, Energy : 345 MeV/u up to 238 U ions Reaction mechanism of RI production Projectile fragmentation In-flight fission 238 U Very powerful for neutron-rich RIs in mid-heavy region

5 Features of BigRIPS Separator 1) Large acceptances Comparable with angular / momentum spreads of in-flight fission at RIBF energy (+/-5 mrad, +/-5%) 2) Superconducting quads with a large aperture Pole tip radius: 17 cm Max. pole tip field: 2.4 T 3) Two-stage separator scheme 1 st stage : 2 bend, p/δp=126 2 nd stage : 4 bend, mirror F5, p/δp= 342 Better resolution at 2 nd stage for particle ID From SRC Production & separation Wedge 1 st stage 2 nd stage Matching section Wedge Particle identification & two-stage separation STQ (Superferric Q) Parameters: Δa = +/-4 mrad Δb = +/-5 mrad Δp/p = +/-3 % Bρ = 9 Tm L ~ 77 m STQ1-14: Superconducting quad. triplets D1-6: Room temp. dipoles (3 deg) F1-F7: focuses

6 Ion Optics of BigRIPS 1 st stage Matching section 2 nd stage (Mirror symmetry at F5) X Y F F1 F2 F3 F4 F5 F6 F7 FF1 F3F4 (x x)=-1.7, (x a)=, (y y)=-5., (y b)=, (x δ)=-21.4 mm/%, (a δ)= (x x)=2., (x a)=, (y y)=1.6, (y b)=, (x δ)= mm/%, (a δ)= FF2 (x x)=-1.8, (y y)=-1.18, (x a)=(y b)=(a x)=(b y)= (x x)=-1.4, (x a)=-.791, (y y)=-3.45, (y b)=-.272, (x δ)=-22.1 mm/%, (a δ)= F2F3 F3F5 (x x)=.92, (x a)=, (y y)=1.6, (y b)=, (x δ)=31.7 mm/%, (a δ)= F7F5 F7F6 P/ΔP = 126 (1 st stage) 342 (2 nd stage) Mirror symmetry of F3-F5

7 Large-Aperture, Short-Length Superconducting Quadrupole Superferric (STQ2-26) : iron dominated 2 current (A) 1 Superferric Q5,5 Bcenter 1 1,5 (T) 2 b2,[mt] Superferric Q Position (mm) Large fringe field regions and saturation effects! Superferric Q5 current 1 (A) 2 Excitation function Field distribution Effective length Leff (mm) 2 6 Air-core (STQ1) 3 Air-core STQ1-P2 b2, (T) position (mm) Field distribution

8 Calculation for Optical Setting Our goal: precise ion-optical setting, in which tuning is not needed. Quadurpoles have large fringe field region and strong saturation effects. The field distribution varies very much with the magnet excitation. The effect of the varying distribution should be included in the simulation. Procedure of the field & optics analysis Measure detailed field-map as a function of magnet current. Deduce b n, (z,i) from the magnetic field map. Fit b n, distribution by Enge function. Its Enge coefficients are the function of magnet current. Make detailed ion-optical calculation using the deduced Enge coefficients and COSY INFINITY code. Search magnet current setting, which satisfies the ion optical setting. 1 F( z) = 5 1+ exp a1 + a2( z/ D) + + a6( z / D) Field Distribution 2, 1,, -1, Q5 165A position (mm)

9 Optics Calculation

10 Field Map Measurement Quadrupole & Sextupole Dipole Beam axis STQ magnet Br Bz Bθ r rotation 14 probes 3 R6, 3deg. Radius : r = 81,94,17 mm Step : Δz = 1 mm Δθ = 9 degree (for quadrupole) 3 degree (for sextupole) Range : outside +/-5 mm Step : 2 mm 4 (center : 1 mm) Plane : mid-plane, +/-1, 2, 3, 4 mm (Gap : +/-7 mm)

11 Magnetic Field in θ Direction Q5, z=223mm, I Q =1A, I SX =A Q5, z=198mm, I Q =1A, I SX =A Quadrupole B (mt) Br Btheta Bz B (mt) Br Btheta Bz -15 θ (deg) -15 θ (deg) SX, z=223mm, I Q =A, I SX =46A SX, z=198mm, I Q =A, I SX =46A 3 3 Sextupole B (mt) Br Btheta Bz B (mt) Br Btheta Bz θ (deg) magnet center -3 θ (deg) field boundary region (edge of mag.)

12 Multipole Analysis of 3D Mag. Field Magnetic field vector (B r, B θ, B z (r,θ,z )) is expressed by a scalar b n, (z). measurement 1 st step (Fourier Analysis) (w/o skew components) n=1: dipole n=2: quadrupole n=3: sextupole 2 nd step (Deducing b n, )

13 Fourier Transform of Differential Eq. (m>) Fourier transform z derivative can be translated into simple algebraic calculation by FT

14 Using the procedure of Fourier tr. and inverted Fourier tr., b n, (z) is obtained from B r,n (r,z), without solving the high differential equation. Procedure to Deduce b n, from B r,n (diff. eq.) Fourier tr. (diff. eq.) decomposed from measured data Inv. Fourier tr. b n, from B θ,n

15 2 15 b2, (mt) 1 5 b 2, (z) Distribution along the Axis Position (mm) 2 1 b2,(mt) -1 Q5 z 2A 3A 4A 5A 6A 7A 8A Q1 b2,(,mt) z position (mm) Q A A 15 A 1 14 A 5 13 A 12 z A A 1 2A 4A 5A 6A 7A 8A 9A 1A 11A 12A 13A 14A 15A 16A Position(mm) The fringe region is very large. The shape of the distribution varies much with the Deduced from B r (r=17 mm) A

16 Enge Function for Fringe of b n, 2 1,5 Dipole 11A 3 STQ1M (air-core) F b2, (T) b2, (T) ( z) 1,5 -,5 2, 1,5 1,,5, -, position (mm) Q5 165A = 1+ exp position (mm) 1 ( a + a ( z / D) + + a ( z D) ) / D : Pole tip diameter a 1 -a 6 : parameter z z F ( z) b2, (T) 2,5 2 1,5 1,5 -,5 = 1+ exp position (mm) Overshooting & undershooting part Second term is introduced (a 7 -a 11 ) 1 ( a + a ( z / D) + + a ( z D) ) / (( z / D) a ) + + a ( ( )) 7 tanh a8 + a9 z / D exp 2 a11 z 1 2

17 ,2 -,2 -,4 -,6 -,8 -,1 -,12 -,14 -,16 1,5 1,4 1,3 1,2 1,1 1,9,8,7 y = -1,3479x 6 + 7,848x 5-17,559x ,27x 3-1,515x 2 + 2,743x -,3295,5 1 1,5 2 a1out 26 Current (x 1 A) y = 6,3437x 6-38,749x ,942x 4-11,23x ,923x 2-19,753x + 3,387 a4out 26,5 1 1,5 2 Enge Coefficients As a function of magnet current (Q5, inner side) a 1 (I) a 2 (I) a 3 (I) 4,65 4,6 4,55 4,5 4,45 4,4 4,35 -,2 -,3 -,4 -,5 -,6 -,7 y = 3,178x 6-18,834x ,58x 4-49,38x ,727x 2-7,5177x + a2out 5, ,5 1 1,5 2,5 1 1,5 a5out 2 26 y = -,1145x 4 +,7414x 3-1,334x 2 +,618x -,4933,6,4,2 -,2 1 a3out ,4 y = -12,67x ,362x 5 - -,6 181,7x ,83x 3-128,27x ,53x - -,8 4,433 Current (x 1 A) Current (x 1 A) a 4 (I) a 5 (I) a 6 (I),5 y =,6736x 4-2,7726x 3 +,4 3,9692x 2-2,616x +,517,3,2,1 a6out Current (x 1 A) Current (x 1 A) Current (x 1 A) Enge coefficients are fitted with polynominal function. Fitted Enge coefficients are used in our optics calculation.

18 Optics Search using COSY INFINITY e.g. a 2 for Q5 outer side Optics search using symplectic transfer maps that allow symplectic scaling is made. 4,65 4,6 4,55 y = 3,178x 6-18,834x ,58x 4-49,38x ,727x 2-7,5177x + 5,3738 a2out 26 Symplectic transfer maps are calculated beforehand using the fitted Enge coefficients for discrete values of magnet current (see the lower plot). During the search, symplectic transfer maps whose magnet current is closest are chosen and used for optics calculation applying the symplectic scaling. This scheme allows fast search, saving computational time ,5 4,45 4,4 4,35,5 1 1,5 2 Fitted Enge coefficients Symplectic transfer maps are calculated for the points below C urrent (x1 A ) Current (x1 A)

19 Comparison with measurements

20 Determination of matrix terms from 2ndary beam (x x) F5x vs F3x correlation 1st order matrix elements from F3 to F5 (a x) F5a vs F3x correlation (x a) F5x vs F3a correlation (a a) F5a vs F3a correlation (x δ) F5x vs TOF correlation 9 Br is selected (produced by in-fight fission). (a δ) F5a vs TOF correlation F3x: ±1mm, F3a: ±1mrad gates are applied. δ is gated with TOF37: ±1ns.

21 Comparison for the matrix terms (x x) (a x) measured (x a) (a a) Focusing term Not sufficient (x δ) (a δ)

22 Particle Identification in 2 nd Stage TOF-Bρ-ΔE method with track reconstruction Improve the Bρ and TOF resolution Measure β, Bρ, 2 nd stage + isomeric Z de / dx = f γ-rays Bρ Z, A/Q A/ Q = cβγ ( Z, β ) Bρ by track reconstruction. x 3, a 3, x 5, a 5 Bρ 35 x 7, a 7, x 5, a 5 Bρ 57 (ion optics) β by the couple equations. TOF 37 = L 35 /β 35 c + L 57 /β 57 c A/Q = Bρ 35 / cβ 35 γ 35 A/Q = Bρ 57 / cβ 57 γ 57 a: θ(angle in horizontal) β 35, Bρ 35 x 5, a 5 β 57, Bρ 57 Wedge x 3, a 3 Wedge (energy loss) x 5, a 5, ΔE 1 st stage 2 nd stage TOF 37

23 PID Power for Fission Fragments High enough to well identify charge states. thanks to the track reconstruction! Z vs. A/Q plot A/Q spectrum for Zr isotopes (Z=4) 16 Zr Zr Zr Zr Zr Zr Zr 39+ ΔP/P =+- 3% U-beam+2.9-mm Be, Bρ1 = 7.99 Tm F1 deg: 2.18-mm Al, Δp/p: +/-3%, G3 setting (Z~4) J. Phys. Soc. Jpn. 79 (21) r.m.s. A/Q resolution:.35 % 112 Zr 4+ A/Q

24 Improvement of PID Power Sn isotopes produced by in-flight fission of 238U Up to 1st order (calc.) Sn 131Sn49+ Reconstructed δ and F3a vs A/Q 134Sn5+ σa/q =.58% for 132Sn5+ Up to 3rd order (calc.) F3a σa/q =.5% Up to 3rd order (deduced from exp.) σa/q =.41% A/Q δ35

25 Issues COSY predictability improvement Improvement of magnetic field measurement Magnetic field distribution Cross talk between Q and SX (not only Q SX but also SX Q) Better analysis of measured magnetic field-maps B-I curve quality Fitting b n, distribution with Enge function (the function of z) Fitting Enge coefficient (the function of I) allowing us to achieve our goal: precise optics setting, in which any tuning is not needed. allowing us to achieve excellent track reconstruction without using experimentally-determined transfer maps.

26 Summary Introduction BigRIPS has large acceptance and large aperture for the fission fragments of 238 U beam. For this feature, Superconducting quadrupoles are used. The field distribution of STQ varies very much with the magnet excitation. Optics Calculation Goal: precise ion-optical setting is calculated, in which tuning is not needed. To achieve this goal, the varying field distribution should be included in the optics calculation. The procedure of the magnetic field analysis is shown. For deducing b n,, a new approach using Fourier Transform is shown. Comparison with measurement Matrix term: the agreement of (x a) term is not sufficient. A/Q resolution: there is room for improvement.

27 Thank you for your attention!