Proposed injection of polarized He3+ ions into EBIS trap with slanted electrostatic mirror*

A.Pikin, A. Zelenski, A. Kponou, J. Alessi, E. Beebe, K. Prelec, D. Raparia

Brookhaven National Laboratory

*Work supported under the auspices of the U.S. Department of Energy

The goal: injection of polarized He3+ into the EBIS trap

The problems:

• Depolarization in magnetic field during injection

• Injection into EBIS (low charge state multiplication)

1. Depolarization:

using He3+ with atomic polarization parallel to nuclear (Murnick, Mei 1985)

Method: 3S1 3P1(F=3/2, 1/2) (circularly polarized narrowband laser)

3S1 – as a result of charge exchange of He3+ on alkali atoms

Ionization – preferably by resonant ionization using UV laser, or charge exchange in a vapor.

1.08μ680 MHz

Optical pumping of 3He1 metastable atoms in collinear

beams technique.

Optical pumping of 3He1 metastable atoms in collinear

beams technique.

1083 nm

Direct optical pumping of the “fast” 3He(2S) beam (proposal).

Direct optical pumping of the “fast” 3He(2S) beam (proposal).

• After Na-neutralizer cell almost 100% of He-atoms are in (23S1) state. Energy defect-0.38 ev.

• Direct optical pumping can produce near 100% nuclear polarization in He(2S) states. P( He++) ~80-90%.

He+ source

Na-vapor cell

He(2S)EBIS

ionizer

EBIS

ionizer

4He-gas

Ionizer cell

He+

He++

He++

100 mA of a 0.75 keV energy He+ ion beam

Optical pumping

at 1083 nm

~3 kG field

He(2S)

Layout of EBIS with external ion beam line

2. Injection into EBIS“Fast” potential trapping of traversing ions. Requires pulsed primary ion beam to fill the trap “on a fly” with limited maximum trapping time (traversing time). For 50 eV He3+ to fill an ion trap of a 10 A electron beam IHe3+~1.5 mA with emittance εRMS norm≈0.02 mm*mrad

“Slow” trapping of ions traversing through the trap. Only ions, which reduced their axial energy per charge between two axial barriers get trapped. The known mechanisms – ionization and energy exchange with ions and molecules during traversing through the trap. The injection time can be longer than the ionization time.

"Fast injection" "Over-barrier injection"

The idea of continuous ion injection – to transfer part of longitudinal ion energy into transverse when the ion is reflected by an electric field which is not parallel to the axis of ion motion (without changing the total energy) so that longitudinal energy reduces and the ion got trapped. The existing radial potential well allows us to do this without losing ions within certain longitudinal energy spread of ions.

Mirror

The process of ion trapping with such mirror has been simulated with 2-dimensional program TRAK and 3-dimensional program KOBRA3-INP on simplified models of EBIS/EBIT.

Parameters of these models are: Electron beam current – 5.0 A, zero current of ion beam, Electron energy – 25 kV Electron beam diameter – 6.0 mm Inner diameter of drift tubes – 20.0 mm Magnetic field – 3 kGs

2-D simulations has been done with cylindrical symmetry, so that the plane mirror was substituted with conical mirror with gap of 1.7 mm.Mesh size: 0.2-0.3 mm, number of electrons – 100, number of ions - 100

2-D model and field distributions:

Axial magnet and potential distributions in a trap

-5.00E+03

0.00E+00

5.00E+03

1.00E+04

1.50E+04

2.00E+04

2.50E+04

3.00E+04

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 240.0

Z (mm)

U (

V)

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

B (

T)

U_no trapU_trapB_axis

Electron beam transmission:

Ion beam transmission with no mirror engaged:

Axial potential distribution in a trap model

2.25E+04

2.30E+04

2.35E+04

2.40E+04

2.45E+04

2.50E+04

2.55E+04

2.60E+04

2.65E+04

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0

Z (mm)

U (

V)

Axial potential distribution in the trap with mirror engaged:

Ion trajectories with mirror engaged:

Simulated trajectory of a single trapped ion (out of 100 other ions):

Z-oscillations of trapped ion

0.00E+00

5.00E-02

1.00E-01

1.50E-01

2.00E-01

2.50E-01

0.0E+00 1.0E-05 2.0E-05 3.0E-05 4.0E-05 5.0E-05

T (s)

Z (

m)

For mirror angle 450 the trapping statistics:

Ion statistics in a trap as a function of ion longitudinal energy (45 degrees, U_DT3 =25300 V)

0

50

100

150

200

250

300

350

27500 27600 27700 27800 27900 28000 28100 28200 28300 28400

E_ion (eV)

Nu

mb

er

of

ion

s

N_Return

N_lost_DT3

N_rev>1*N_part

N>3

Similar calculations has been done for mirror angle 200, 450 and 600

and for different voltages on a mirror drift tube on a trap side (drift tube No.3)

Maximum parameters for different mirror angles

0

50

100

150

200

250

300

350

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0

Angle (degree)

Valu

e

N>3

N_returned

N_rev>1*N_part

Dependence of maximum efficiency on U_DT3

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

24800.0 25000.0 25200.0 25400.0 25600.0 25800.0 26000.0

U_DT3 (V)

Effi

cien

cy (%

)

3D model:Mesh: Z: 1mm, X and Y: 0.5 mmNumber of electrons: 500Number of ions: 1000Hovi Kponou

Electric field in 3D model (a – without el. beam, b- with el. beam):

a

b

Axial electric field distribution with ion trap:

Electron trajectories:

Ion trajectories (600):

Results of 3D simulations:

Fate of Ions vs Ion Energy(for 60 degrees slanted mirror)

0

10

20

30

40

50

60

70

80

23.9 24.0 24.1 24.2 24.3 24.4 24.5 24.6 24.7

E_ion - keV

Fra

ctio

n o

f Ion

s -

%

1 bounce

2 bounces

3 ..

4 ..

5 ..

>2 bounces

Angular dependence of the trapping efficiency:

Fraction (%) with 2 or more bounces in trap for different mirror angles

0

5

10

15

20

25

30

35

23.7 23.9 24.1 24.3 24.5 24.7 24.9

Ion Energy - keV

Fra

ctio

n T

rapp

ed (

2+ b

ounc

es)

% 60 deg % trapped20 deg % trapped40 deg % trapped90 deg % trapped75 deg % trapped

Possible solutions for the mirror angle adjustment:

1. Modifying the mirror voltage by adjusting the potential on a part of the mirror, which is opposite to the injection side while keeping the potential on a mirror tube on a trap side fixed. Presumably this can change the position and angle of the reflecting equipotential with respect to the axis.

2. Using extra wedge-shaped tube(s) with controllable voltage.

0

5

10

15

20

25

30

35

40

45

2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

U_mirror (kV)

% w

ith 2

or

mo

re b

ou

nce

s

Slanted mirrorDrift tube No. 2 Drift tube No. 3

Springs

Plan for mirror test on BNL Test EBIS:

Simulation of electron beam transmission in Test EBIS with slanted mirror:

Resume:

1. Injection of polarized He3 into EBIS trap in a form of He3+ ions from

outside polarizer/ion source should be greatly simplified with a slanted electrostatic mirror on a side of EBIS ion trap opposite the injection end. Such mirror transfers part of longitudinal  energy of ions traversing  the trap into transverse component. Reduction of longitudinal component of ion energy in a space between two axial potential barriers prevents the traversing ions from escaping the trap on a way out and therefore this is a mechanism of continuous ion accumulation. Without limitation for a minimum ion current (which for a pulsed ion injection can be a problem) this method has all other advantages of pulsed ion injection and can be more feasible than the “fast” injection method.

2. With all quantitative differences in simulations with 2D and 3D programs it was demonstrated that at certain conditions the continuous trapping of ions traversing the EBIS trap by transferring part of longitudinal energy into transverse is possible and the optimum efficiency exceeds 40%.