Abstract Velocity and spatial distributions of calcium (Ca*

Aramaki Mitsutoshi

Hayasaka Kazuhirol

Sakawa Youichi

Shoji Tatsuo

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J. Plasma Fusion Res. SERIES, Vol.4 (2001) 309-3t2
Velocity Distributions and Density Profiles of Buffer-Gas
Gooled One Component Plasma in Toroidal RF lon Trap
ARAMAKI Mitsutoshi*, SAKAWA Youichi, SHOJI Tatsuo and HAYASAKA Kazuhirol
Department of Energy Engineering and Science, Nagoya university, Nagoya 464-8603, Japan
I Communications Re search Laboratory, Kobe 6 5 l -2492, J apan
(Received: 5 December 2000 / Accepted: 3 September 2001)
Abstract
Velocity and spatial distributions of calcium (Ca*) ions confined in a toroidal RF trap have been
measured by using laser-induced fluorescence method. Cat ion temperature in the toroidal direction
decreases from over 2000 K to about 500 K by collisions with He atoms when He pressure is increased
from 5 x 10-7 Torr to 5 x 10-5 Torr. The ion density, estimated from the fluorescence intensity of the
trapped ions, is roughly estimated as 105 cm-3. The Coulomb coupling parameter f, defined by the ratio
of Coulomb energy to kinetic energy of ions, is about 10-4. Ca* ion temperature in poroidal cross-section
is estimated from the measured density profile. The velocity distributions and density profiles of the
cooled ions are not deviated from those obtained from Boltzmann statistics.
Keywords:
toroidal RF ion trap, buffer-gas cooling, velocity distribution, density profile
1. Introduction
Several types of ion trap have been used for precise
spectroscopical diagnostics ofions [] because they can
confine ions in a small space for long time. They are
also used to study one-component plasma physics such
as crystallization 12,31, nonlinear phenomena [4], and
statistical property [5] ofion or electron cloud. The aim
of our experiment is to study the statistical property of
ion cloud confined in a toroidal RF ion trap [6]. In this
paper, Ca* ion cloud is cooled by He buffer-gas cooling
method, and He gas pressure Ps" dependence ofDoppler
spectra and density profile are measured by using laser-
induced fluorescence (LIF) method. Since the toroidal
ion cloud are irradiated tangentially by excitation lasers
(397 nm and 866 nm) of Ca*, ion temperatures Z, in
toroidal direction and Zo in the poroidal cross-section
are estimated from Doppler spectra and density profile,
respectively. We also measure the Ps" dependence of
* Corresponding author's e-mail: aramaki @ nuee.nagoya-u. ac.j p
ion confinement time r from the decav curve of the LIF
signal.
2. Experimental
Figure I shows the top view of the experimental
setup. The toroidal RF ion trap, placed in a high-vacuum
chamber, is used to confine Ca* ions. Table 1 shows
dimensions of the toroidal RF ion trap and experimental
parameters. Ca vapor is introduced into the ion trap
from the oven placed under the ion trap, and ionized by
electron-beam bombardment. Velocity distributions and
density profiles of the Ca* ion cloud are measured by
using LIF method. Electronic energy levels of Ca' are
shown in Fig. 2. To pump up the energy levels of Ca* to
2Pr,r,397 nm (2S172 - 2P,r, transition) and 866 nm (2Dr,,
-'Pr,, transition) light obtained by diode lasers are
collinearly overlapped and focused tangentially to the
@2001 by The Japan Society of Plasma
Science and Nuclear Fusion Research
309
Fabry-Perot
lnterferometer
Beam Postion
Controller
Aramaki M. et al., Velocity Distributions and Density Profiles of Buffer-gas Cooled One Component Plasma in Toroidal RF Ion Trap
'P,,,
866nm
>\tio
51cg
OC(Jf
c
8€ nrU)ro-.'
b
=IIFig. 1 Experimental setup of the laser-induced
fluorescence measurement svstem.
Table 1 Dimensions of the toroidal RF ion
trap and experimental parameters.
100 200 300
Time [s]
Main radius
Minor radius
RF voltage
RF frequency
DC voltage
Base pressure
He pressure
40 mm
4.5 mm
250V
2MHz
OV
4 x 10-e Ton
l0'i - lO5 Ton
R
ym
O=
Vrc
PB
Pfu
toroidal ion cloud. The 397 nm UV light is obtained by
frequency doubling of 794 nm light of the diode laser.
The emission of 25r,, -2Puz(397 nm) transition is used
for the detection of Ca* ion fluorescence. To measure
the radial Ca'ion density profile, the beam focusing
positionis swept radially in a poroidal cross section with
a fixed laser frequency. The Doppler broadening of 2S172
-zPll2radiation is observed by detuning the UV laser
frequency around the resonance frequency at the center
of the ion cloud. Temperature of Ca* ion is estimated
from the density profile and the Doppler broadening.
,D,,,
's,,,
Fig. 2 Electronic energy levels of Ca..
0.01
Fig.3 He gas pressure dependence of the
fluorescence intensity of Ca..
400
decay of
The 397 nm radiation from Ca' ion is collected by
optical lens, and detected by photomultiplier tube
(PMT). The number of photon pulse-signal from the
PMT is counted by the multi channel analyzer (MCA)
and stored in the computer (PC).
3. Results and Discussion
He buffer-gas is introduced into the vacuum
chamber at room temperature. Figure 3 shows Pg"
dependence of 397 nm fluorescence decay after the
electron beam is turned off. The ion confinement time t
deduced from Fig. 3 is 70, ll0, and 150 s for Ps.= 6x
lO-7, 7 x 10-7, and I x 10-6 Torr, respectively. These
confinement time are more than two-orders of
magnitude longer than that of non-cooled Ar* ion [6].
The buffer-gas cooling of Ca* ions elongates t
Figure 4 shows the P11" dependence of Doppler
spectra of Ca* ion cloud. The frequency of the UV laser
Pue=1 x1
r=150 s
Pue=7x l
t=1 10 s
310
Aramaki M. et al., Velocity Distributions and Density Profiles of Buffer-gas Cooled One Component Plasma in Toroidal RF Ion Trap
is swept +3 GHz from the '5r,, - 2P,r, resonance
frequency. These Doppler broadening spectra agree with
Gaussian profiles. The ion temperature d in the toroidal
direction is obtained from the Doppler spectrum since
the probing lasers are focused tangentially to the
toroidal ion cloud. d is estimated as 2600, 1200, and
550 K for Pn"= 6 x 10-7, 2x10-6, and 3 x 10-s Torr,
respectively. We roughly estimate the ion density n as
lOs cm-3 from the measured number of photon by using
rate equation [7]. Here, we considered only the photons
of 2S,,, - 2Pu, transition, since the high-intensity 866 nm
laser repumps the ions in the metastable 2D32 state to
the2PtD state. Table 2 shows parameters of the photon
counting system. Here, V is observation volume, Q is
product of total transmittancy of optical system and
quantum efficiency of PMT, dQ is solid angle of optical
system, and A is Einstein A coefficient. The Coulomb
coupling parameter f , estimated from these
experimental results, is about l0'4. In order to obtain the
larger value of f for strongly-coupled plasma studies, it
is necessaryto cool the ions by the laser-cooling
technique.
Figure 5 shows the Ps" dependence of density
profile of the Cat ion cloud in the poroidal cross-
section. The full width at half maximum of UV laser
spectrum is much narrower than that of the Doppler
spectrum of Ca* ion. The increase in the fluorescence
intensity by increasing Pr" is mainly caused by the
decrease in d by He buffer-gas cooling. The ion density
profile in the RF potential can be calculated from the
differential equation derived from Poisson equation and
Boltzmann distribution of ions [8,9],
t t\2
n,_lr=J *4_ "?.r,_ + lr!!=_=0. (l)'. n 'r €okBTp,mrfrerk"T,-".
where e is elementary charge, es is permittivity in
vacuum, ft" is Boltzmann constant, n is Ca* ion mass,
and 7o is ion temperature in the cross section of ion
cloud. The measured density profiles (Fig. 5) are
Gaussian type, because the fourth term on the left-hand
side of Eq. (l) is much smaller than the fifth term in this
experiment. Zo and full width at half maximum D of the
density profile are estimated by curve-fitting the density
profile with Gaussian distribution as (1800 K, 0.9 mm),
(1600 K, 0.8 mm), and (1400 K, 0.7 mm) for P"" - 5 x
l0'7,2 x 10-6, and 4 x 10-6 Torr, respectively. Both Zo
and D decrease by increasing Pn" due to He buffer-gas
cooling. Note that Zo has weaker dependence on PHs
than that of 7,. This weaker Pr1" dependence of Io may
be explained in terms of the ion heating by the radial RF
8 104
-2-1012
Laser Detuning at 755 THz [GHz]
Fig. 4 Helium gas pressure dependence of the Doppler
spectrum ofthe ca* ion cloud.
Table2 Parameters of the photon counting system.
V = 0.65x10"m1
O=0'1
dO = 0.13 sr
A = 1.4 x 108 s-l
104
104
0
-1.5 -1 -0.5 0 0.5 1 1.5
Beam Position [mm]
Fig.5 Helium gas pressure dependence of density
profile of the Ca' ion cloud in the poroidal cross-
section.
field in the poroidal cross-section. In order to evaluate
the dependences of Z, and Zo on Pg" quantitatively,
precise treatment of non-adiabatic process of ion heating
inactual RF trap configuration is needed.
4. Conclusion
Ca' ions are trapped in a toroidal RF trap. The
velocity and spatial distributions of the ion cloud have
been measured by using the LIF method. The Ca* ions
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Aramaki M. et aI., Velocity Distributions and Density Profiles of Buffer-gas Cooled One Component Plasma in Toroidal RF Ion Trap
are cooled by collisions with He atoms of the buffer gas.
We have estimated the ion density as 105 cm-3 from the
measured number of photon by using rate equation.
The estimated f from these experimental results is about
10-4. The velocity distribution and density profile of ions
cooled by He-buffer gas agree with those of
Maxwellian. The deviation from the Boltzmann statistics
is hardly observed in this temperature region.
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Velocity Distributions and Density Profiles of Buffer-Gas Gooled One Component Plasma in Toroidal RF lon Trap

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Velocity Distributions and Density Profiles of Buffer-Gas Gooled One Component Plasma in Toroidal RF lon Trap

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