Transcript Development of high-resolution halo

Development of a Polarized 6Li3+ Ion Source
at RCNP
A. Tamii
Research Center for Nuclear Physics,
Osaka University, Japan
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Contents
1. Motivation
2. Outline of the ion source
3. Simulations (depolarization)
4. Recent Status (pictures)
5. Summary
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Collaboration of the Polarized 6Li3+ Ion Source Project
RCNP, Osaka University, Japan
K. Hatanaka, A. Tamii, Y. Sakemi, Y. Shimizu, K. Fujita,
Y. Tameshige, and H. Matsubara
CNS, University of Tokyo, Japan
T. Uesaka and T. Wakui
CYRIC, Tohoku University, Japan
H. Okamura
Kyushu University, Japan
T. Wakasa
RIKEN, Japan
T. Nakagawa
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Motivation
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Study of nuclear structures by using polarized 6Li beam at
100MeV/U.
– Study of spin dipole excitations (2-,1-, and 0-), especially 0-, via (6Li,6He)
reaction.
Tensor analyzing power at 0º is sensitive to J of SD excitations.
– Study of isovector spin-flip excitations via (6Li,6Lig) reaction.
6
Study of reaction mechanism of composite particles
– elastic scattering, inelastic scattering, (6Li, 6He) Reaction
– diff. cross section and analyzing power
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
For these purposes, development of a polarized 6Li3+ ion source
is required.
Requirements (or goal):
Injection energy to AVF cyclotron:
Beam intensity:
Beam polarization (ratio to maximum):
57 keV (19 kV)
≿ 10nA on target
≿ 0.7
Reduction of depolarization of 6Li nuclei in the ionization
process is one of the key points of the development.
Feasibility test has been planned.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Outline of the ion source
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Outline of the polarized 6Li ion source
(6Li0+ injection to ECR)
6Li0+:
50 pmA Pol. >90% at Heidelberg and Florida State Univ.
Mean free path of single ionization in ECR plasma is 10-30cm.
6Li0+→6Li1+
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Simulations
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Assumption of the Plasma Condition
The following plasma condition is assumed according to an empirical
study of the laser ablated Al ion intensities from a 14.5 GHz ECR
ionizer (SHIVA).
(M. Imanaka, PhD thesis, Univ. of Tsukuba)
Buffer Gas:
Oxygen
7+
250 W
Neutral Gas Density (ngas):
1.4×1010 cm-3
Electron Density (ne):
2.2×1011 cm-3
Electron Temperature (Te) :
580 eV
Ion Temperature (Ti) :
5 eV
beam intensity [a.u.]
1.5
RF Power:
-3
ne=2.23(±0.06)e11[ cm ]
T e=582(±46)[ eV]
τc=9.1(±0.5)[ ms]
T i =5[ eV]
-3
ngas=1.44e10[ cm ]
6+
3+
1
4+
8+
0.5
9+
0
0
0.01
0.02
0.03
time [s]
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Confinement Time of Ions in the ECR plasma
Form the same study using 14.5GHz SHIVA, confinement time of 27Al3+ was
obtained by fitting the data as (M. Imanaka, PhD thesis, Univ. of Tsukuba)
tc(27Al3+) = 2.3msec
By applying the following relation (Shirkov, CERN/PS 94-13)
t i  i Ai
i: charge state, Ai: mass
confinement time of 6Li ions are
t1+ = 0.3 msec, t2+ = 0.7 msec, t3+ = 1 msec
From our laser ablation experiment by using 18GHz SC-ECR at RIKEN, we obtained
tc(7Li2+) = 0.4 msec
It is more or less consistent with the above values.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Assumption of the Plasma Size
Plasma size is not well
known.
We conservatively assume
that the plasma size is the
same as the volume inside of
the ECR region.
Two times larger size will be
used as an optimistic
assumption.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Critical Magnetic Field
The critical magnetic field for decoupling the hyper-fine interaction
between an electron and a nucleus in 6Li2+ is Bc=3kG.
Our SC-ECR has a minimum magnetic field of B~ 5kG.
Thus
Calc. by H. Okamura
B
x
 1.7
Bc
x  2 .1  2 . 5
dep. on the assumption
of the plasma size.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Depolarization caused by the electron spin resonance (ESR) effect
on 6Li2+ (following the procedure of M. Tanaka et al., NIMA524,46)
If a 250W microwave is fed in a non-resonating cylinder with a diameter of 78mm.
u
W
10

1
.
7

10
J/cm3, B1  m0u  0.15 Gauss
2
r c
The thickness of the ESR region is
R  4.0mm at R  5.0cm (in axial direction)
R  0.9mm at R  1.9cm (in radial direction)
The effective thickness averaged over isotropic ion
motion and averaged length between the ESR regions are
L
Plasma size may be larger
than the ESR region. We
conservative assume this
worst case.
4.0  0.9  2 
R 
5.0  1.9  2
  2  ln
 2.9 cm
  22 mm, R 
3
2

R
3


Spin rotation angle of an electron caused by ESR is, by random-walk approx.
    N  4.5 103 rad 180  6.0 102 rad  3.5
Nuclear depolarization is further caused by the hyper-fine coupling between electron
and nucleus. Hence depolarization caused by ESR effect is negligibly small.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Nuclear depolarization caused by inhomogeneous magnetic field
(6Li1+ and 6Li3+)
The T1 relaxation time is expressed by, Schearer et al., Phys. Rev. 139 (1965) A1398
1 2 v 2  H y


T1 3 g I2t c H z4  y



2
Quantum axis is taken along the
direction of the local magnetic field.
For 6Li1+ and 6Li3+, by putting the following numbers
g I  3.94107 rad / s / T
t c  1.2 106 sec
v  1.3 106 sec
H z  0.70 T
1
Hz
1  H y 

  0.18 rad2cm-2T -2
4 
H z  y 
2
 H y 

  0.25 rad / cm
 y 
T1 = 9.2 msec
If the plasma size is larger by a factor of 2 (in length)
H z  1.1 T
1  H y 
2
-2 -2



0
.
11
rad
cm
T

H


4
1
 y 
y
H

  0.28 rad / cm

z 
H z  y 
7.0cm
T1 = 15 msec
2
10cm
3.8cm
20cm
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Reaction Rates and Depolarization in
(de-)ionization/(de-)excitation processes in the ECR plasma
following the procedure of
M. Tanaka et al., NPA524, 46.
Ddep
 0.967 0.033 0.000


  0.033 0.910 0.056
 0.000 0.056 0.944


when x=2.1
rates in Hz
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Results of the simulation
(confinement time dependence)
The total depolarization (pol~0.75) is expected to be at acceptable
level, while the efficiency (beam intensity) is not high.
The intensity can be improved by increasing the electron density in
the ECR plasma and/or improving the Li oven and Laser system.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Results of the simulation
(confinement time dependence)
optimistic case
The results much depends on the plasma assumption. If an
optimistic assumption is applied, i.e. 2.3 times larger electron
density (5×1011 cm-3) and 2 times larger plasma size, the estimated
beam intensity much increases.
Feasibility test experiment is required.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Present Status (Pictures)
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Top View
18GHz
SC-ECR
CR
injection to AVF
(downward)
6Li Atomic
Beam Source
Wien Filter
for controlling the
polarization axis
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Summary
•
Simulations have been done about the depolarization and
ionization efficiency of a 6Li3+ ion source by using an ECR ionizer.
•
Under an assumption of the plasma condition, the calculated
polarization (0.75) is acceptable. The beam intensity is somewhat
low (~100 nA) and improvements may be needed.
This method looks hopeful.
•
Feasibility test experiment is required for conforming the
simulation, and optimizing plasma parameters by tuning magnetic
field, RF power, gas density, and extraction geometry.
•
Final design and construction is in progress.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Outline of the polarized 6Li3+ ion source (I)
(6Li1+ injection to ECR)
F=3/2
2p1/2
F=3/2
F=1/2
-3/2
2S1/2
-1/2
1/2
3/2
Level Diagram of a6Li atom
6Li1+:
20-30 pmA Pol. 80-90% at Florida State Univ.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
From calculations and simulations
• Emittance of the 6Li1+ beam from the surface ionizer
vertical dir.: 300  mmmr
horizontal dir.: 200  mmmr
•
~70% of the beam is reflected at the deceleration electric field
(19 kV→10 eV) placed at the entrance of ECR.
• Dense plasma with a thickness of ≿50 cm is required to
efficiently decrease the energy of 10 eV 6Li1+ ions and trap
them in the plasma.
Efficient injection of the 6Li1+ beam into ECR plasma is not
expected in the assumed setup.
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Simulation of the Optical Pumping
F=3/2
F=1/2
2p1/2
Retro-reflector
Li Oven
3
5
700 mm
70 mm
2
5
1
15
1
3
1
15
1
3
2
5
8
15
8
15
2
5
1
2
1
6
3
5
2
5
1
2
1
6
2S1/2
2% 95%
-3/2
-1/2
1%
1/2
2%
3/2
F=3/2
F=1/2
MF
Level Diagram of a6Li atom
Laser-2
Cylindrical
Lens
l/4 plates
10mW
10mW
Laser-1
Polarizing Cube
Beamsplitter
l/4 plate
Polarizing Cube
Beamsplitter
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Study of the Confinement Time of Li ions
by the Laser Ablation method
18GHz SC-ECRIS
Lens, Mirror and
LiF rod
t = 0.4 ms
YAG 523nm 5ns
Max 100mJ/pulse
Laser ablation test
in atmosphere
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Study of the Confinement Time of Li ions
by the Laser Ablation method
t = 0.3, 0.4, 0.5 ms
Note: the ECRIS operation has not tuned to 6Li3+
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Magnetic-Substate Transition Matrix (1/2)
(according to the calc. of 3He by M. Tanaka and Y. Plis)
The wave functions Yi(t) of the electron-nucleus system in a magnetic field system are written as a linear
conbination of |IJ> states as
1
1
(1    ) cos   
(1    )
2
2
1
 x
3
 
2
1 x  x2
3
B
x
Bc : criticalmagneticfield
Bc
YI (0)   1
sin   
YII (0)  sin    0  cos    1
YIII (0)  sin    1  cos    0
YIV (0)   1
YV (0)   cos    1  sin    0
YVI (0)   cos    0  sin    1
The time revolution of the |↓+1> state is
 1 t  cos   YII (t )  sin   YIV (t )
 cos   YII (0) exp(iEIIt )  sin   YIV (0) exp(iEIVt )
 cos   sin    0  cos    1 exp(iEIIt )
 sin    cos    0  sin    1 exp(iEIVt )
The probability to find |↓+1> and its time average (after sufficient time) is
P(t )  cos2   exp(iEIIt )  sin 2   exp(iEIVt )
2
 cos4    sin 4    2 cos2   sin 2   cosEII  EIV t 
P  cos4    sin 4   
1
(1   2 )
2
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Magnetic-Substate Transition Matrix (2/2)
By similar calculations we obtain










1

1
 1 '  
1   2

2
0 '  
 
 1 '


 1 ' 
 
0 '  

 1 '  
1
1   2

2














1
2 
1  
2



1
1   2
2

1
1   2
2



1
1   2
2
1


1
1   2
2


1
1   2
2



 1
0
 1
 1
0
 1










We are not interested in the electron spin.
In the case that the orientation of the electron spin is random at t=0, by taking the average for the initial
state and sum for the final state concerning the electron spin, we obtain







1
2
 3  
 1 '  4

1
0 '    1   2

4
 1 '  
0






1
1   2
4

1
2   2   2
4
1
1   2
4




  1 
2 
1   0 


 1 
3   2 

0

1
4
1
4




When x=5/3, the matrix is
Ddep
0 
 0.955 0.045


  0.045 0.871 0.083
 0
0.083 0.917

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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Ionization Rate by Electron Impact
Voronov’s empirical fit
6
Li i
G.S. Voronov, Atom. Data and Nucl. Data Tables 65 (1997)1.
1  PU 1/ 2 K U
 i i 1  ve  A
U e
X U
I
U i
Te
cm3s-1
3.26×10-9 cm3s-1
6Li2+→ 6Li3+: 7.53×10-10 cm3s-1
ii 1  ii 1ne
ne: 2.23×1011 cm-3
li+1,i [cm3s-1]
A, P, X, K: Fitting Parameters
6Li1+→ 6Li2+:
Li i+1
10-6
Te: Electron Temperature
4.52×10-8
6
[cm3s 1 ]
Ii: Ionization Energy
6Li0+→ 6Li1+:
li+1,i
10-7
6Li 0+→6Li 1+
10-8
6Li 1+→6Li 2+
6Li 2+→6Li 3+
-9
10
582 eV
10-10
1
10
100
1000
10000
Te [eV]
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Charge Exchange Reaction Rate with the Neutral Gas
Muller and Saltzborn Empirical Fit
6
A. Muller and E. Saltzborn, Phys. Lett. A62 (1977) 391.
Li i-1
li-1,i
6
Li i
2.76
  1.431012 i1.17 I gas
[cm2 ]
 2.76
 i i 1  vi  3.15106 i1.17 I gas
Ti
Ai
[cm3s 1 ]
Igas: Ionization Energy of the Neutral Gas (Oxygen: 13.6 eV)
Ti: Ion Temperature (5 eV)
Ai: Ion Mass in AMU
6Li1+→ 6Li0+:
2.14×10-9 cm3s-1
6Li2+→ 6Li1+: 4.81×10-9 cm3s-1
6Li3+→ 6Li2+: 7.72×10-9 cm3s-1
ii1   ii1ngas
ngas: 1.44×1010 cm-3
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Atomic Excitation Rate by Electron Impact (1/2)
6Li0+→ 6Li0+* 2s→2p
6
Li i*
(including cascade)
D. Leep and A. Gallagher, Phys. Rev. A 10 (1974) 1082.
 ~ 3.5a02  3.11016 [cm2 ] at Te ~ 600eV
ve  4.5 107 [cm3s1 ] 00*  ve ne
li * i
a factor of ~10 larger than the ionization rate coefficient
6
Li i
6Li1+→ 6Li1+* 1s→2p
assume that a factor of ~5 larger than the ionization rate
coefficient
ve  1.6 108 [cm3s1 ] 11*  ve ne
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii
Atomic Excitation Rate by Electron Impact (2/2)
6Li2+→ 6Li2+* 1s→2p
6
Li i*
Fisher et al., Phys. Rev. A 55 (1997) 329.
Empirical fit of 1s→2p excitation cross sections of hydrogen-like atoms
 ~ 1.0a02 Zi4  1.11018 [cm2 ] at Te ~ 550eV
ve  1.6 109 [cm3s1 ] 22*  ve ne
li * i
Summing up transitions 1s→2,…,6 and taking the Boltzmann distribution
ve  1.82109 [cm3s1 ]
6
Li i
a factor of ~2 larger than the ionization rate coefficient
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Hawaii 2005, September 18-22, 2005 at Kapalua, Hawaii