Plasma and Fusion Research: Regular Articles

Volume 11, 2402024 (2016)

Anisotropy in Broad Component of Hα Line in the MagnetosphericDevice RT-1∗)
Yohei KAWAZURA, Noriki TAKAHASHI, Zensho YOSHIDA, Masaki NISHIURA,
Tomoaki NOGAMI, Ankur KASHYAP, Yoshihisa YANO, Haruhiko SAITOH,
Miyuri YAMASAKI, Toshiki MUSHIAKE and Masataka NAKATSUKA
Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8561, Japan(Received 30 November 2015 / Accepted 3 February 2016)
Temperature anisotropy in broad component of Hα line was found in the ring trap 1 (RT-1) device by Doppler
spectroscopy. Since hot hydrogen neutrals emitting a broad component are mainly produced by charge exchange
between neutrals and protons, the anisotropy in the broad component is the evidence of proton temperature
anisotropy generated by betatron acceleration.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: magnetosphere, broad component of Hα, temperature anisotropy, inward diﬀusion, betatron acceleration, spectroscopy
DOI: 10.1585/pfr.11.2402024

1. Introduction
Magnetospheres exhibit various intriguing phenomena. A representative example is plasma density profile
with planet-ward gradient [1]. Such a self-organized structure is an apparent counterexample of the entropy principle. The inward diﬀusion theory has successfully explained the self-organization mechanism in a magnetosphere; diﬀusion triggered by violation of a third adiabatic invariant flattens their density on magnetic coordinates rather than Cartesian coordinates [2–4], resulting innonuniform density distribution on Cartesian coordinates.
Such a nonuniform density profile was experimentally observed in the ring trap 1 (RT-1) [5, 6] and Levitated Dipole
Experiment (LDX) [7].
Another interesting self-organization phenomenon inmagnetosphere is plasma heating by inward diﬀusion. Particles are energized along with the inward (i.e., toward
strong magnetic field region) transport keeping first andsecond adiabatic invariants constant. The conservationof the first adiabatic invariant increases the particle’s perpendicular kinetic energy (betatron acceleration), and theconservation of the second results in the increase in parallel energy (Fermi acceleration) [8]. These are the primary generation mechanism for planetary radiation belts.
In point dipole field configuration, betatron accelerationis stronger than Fermi acceleration because the increase
rate of magnetic field strength upon the planet-ward displacement is greater than the decrease rate of longitudinal bounce length. Therefore accelerated particle exhibits
temperature anisotropy (T ⊥ > T || ). Recently we observed
ion temperature anisotropy in RT-1 and proved that theanisotropy was produced by the betatron acceleration [9].
In this study, we found the similar temperature
anisotropy in the broad component of Hα line by Doppler
spectroscopy. Broad component of Hα line has been observed in astrophysical and laboratory plasmas. In astrophysics, Balmer lines emitted around shocks in supernova remnants show clear narrow and broad components.
The broad component is attributed to hot hydrogen particles produced through charge exchange between postshock protons. The narrow component originates in coldhydrogen atoms in pre-shock interstellar medium [10, 11].
The post shock proton temperature is estimated by Dopplerbroadening of the broad component. Broad componentof Balmer lines has also been observed in laboratory
plasma [12–14]. In addition to narrow and broad components, the recent studies identified threefold (cold, warmand hot) Balmer lines [15–17]. The one-dimensional neutral particle transport model clarified that the hot component was generated by charge exchange with protons [17].
Unlike in the case of astrophysical plasma, it is challenging
to infer proton temperature via Doppler temperature of abroad component in laboratory. The preceding studies observed discrepancies between the proton temperature andthe Doppler temperature of the broad component [12, 14].
The discrepancy originates in other processes which generate hot neutrals, such as dissociation and reflection. However, the recent study succeeded to estimate proton temperature from Hα line shape by separating charge exchange
contribution from other competitive processes [18]. Therefore, our observation of the temperature anisotropy in thebroad component has the potential to estimate proton temperature anisotropy.

author’s e-mail: kawazura@ppl.k.u-tokyo.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

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c 2016 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 11, 2402024 (2016)

Fig. 1 (a) The vertical and (b) horizontal cross-sectional views of RT-1 device.

2. Experimental Set-up
The experiment was performed on RT-1 device which
simulates a ‘laboratory magnetosphere’ by a levitatedsuperconducting magnet [19]. Plasma is generated by
the electron cyclotron resonance heating (ECH) using an
8.2 GHz microwave. The discharge duration is about 1 s.
The electron density distribution (measured by three chordinterferometers) has a peak near the dipole magnet and decreases toward outer wall [6, 20]. The electron density at
the peak is on the order of 1017 ∼1018 m−3 . The hot electron temperature is 10∼50 keV and the cold electron temperature is 50∼100 eV. The population of hot electron isapproximately the same as that of cold electrons. The
heating mechanisms of ions are the thermal equilibration
with cold electrons and the betatron acceleration, and thecooling mechanism is charge exchange with neutral particles [9].
We measured visible spectra using a Czerny-Turnerspectrometer (1 m focal length, 2400 grooves/mm grating,and 0.55 nm/mm reciprocal linear dispersion) equippedwith a charge-coupled device (CCD) detector (1024 × 256pixels). The wavelength resolution was 0.01 nm. Figure 1
shows the cross-sectional views of RT-1. The line of sights
for the three sets of Doppler spectroscopies ((a) one vertical chord and (b) two horizontal chords) are shown. The
vertical chord consisted of three fixed channels (548 mm,840 mm, and 925 mm) and one rotating channel whichscanned 375 ≤ r ≤ 925 mm (“vertical chord” in Fig. 1 (a)).
The horizontal chord consisted of three fixed channels(772 mm, 841 mm, and 925 mm) and two rotating channels. One of the horizontally rotating channel scanned
375 ≤ r ≤ 925 mm in the same direction as the three
fixed channels (“horizontal chord 1” in Fig. 1 (b)) and theother rotating channel scanned 375 ≤ r ≤ 743 mm in thesymmetric direction (“horizontal chord 2” in Fig. 1 (b)).
For fixed channels, 10 - 11 shot data were averaged to improve signal-to-noise ratio whereas single shot data wereacquire at each radial point for rotating channels. The ab-Fig. 2 Typical discharge waveforms of (a) line averaged density
at r = 620 mm and (b) diamagnetic signal. The filling
gas pressure and the ECH power for this shot were 5.4 ×
10−3 Pa and 36 kW.

solute value of the Doppler shift was estimated by combination of horizontal chord 1 and 2: for a given radial point,
unshifted central wavelength is obtained by midpoint ofspectra measured by chord 1 and 2 assuming axisymmetric property. Since RT-1 does not have toroidal magnetic
field, the Doppler broadening measured by the horizontal chords gives temperature perpendicular to the ambient
magnetic field (T ⊥ ). On the other hand the vertical chordmeasures the mixture of T ⊥ and the parallel temperature(T || ). Figure 2 illustrates the typical discharge waveforms.
The exposure time of spectrometer was 1.2 - 2.0 s, whichcorresponded to the stable state of the discharge.

3. Results
Figure 3 shows the typical spectra of Hα line in hydrogen discharge measured by the horizontal and verti-2402024-2

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Fig. 3 Spectra of Hα line measured by horizontal and verticalchords at almost the same radial point (r ∼ 600 mm) andtheir dual Gaussian fittings. The vertical spectrum is normalized so that the broad component maximum coincides
with that of horizontal chord. The filling gas pressure andthe ECH power for this shot were 6.0×10−3 Pa and 35 kW.

cal chords at almost the same radial point (r ∼ 600 mm).
The broad component was apparently seen. Dual Gaussian fitting separates the narrow and the broad components.
The Doppler shift was finite only for the broad component
in horizontal chord. The broad component in horizontal
chord was broader than that in vertical chord, showing thetemperature anisotropy (T ⊥ > T || ). In what follows, we callthe hot and cold neutrals as the emission sources of broadand narrow components respectively.
Figure 4 (a) illustrates the spatial profile of line averaged temperature of the hot neutrals. T ⊥ of the hot neutrals had a peak in the middle of the confinement region(r ∼ 600 mm), then decreased towards dipole field magnet
and the outer wall. T || was, on the other hand, spatially homogeneous. Figure 4 (b) shows the temperature profile ofthe cold neutrals. In contrast to the hot neutrals, cold neutral temperature was spatially homogeneous without temperature anisotropy. Therefore, the temperature anisotropy
was maximized at r ∼ 600 mm. Figures 4 (c) and (d) show
the spatial profiles of line averaged toroidal flow velocityof the hot and cold components respectively. The hot neutral flow speed had maximum at the same point of T ⊥ ’smaximum and decreased toward the both boundaries. The
cold neutrals, on the other hand, did not have toroidal flow.
For the comparison of the hot neutrals and ions, wepresent the spatial profiles of the temperatures and thetoloidal flow speed of C2+ impurity ions measured by C III(464.7 nm) line in Figs. 4 (e) and (f), respectively. The ion
temperature had maximum at r ∼ 800 mm. The ion flow
speed had maximum at 600 ≤ r ≤ 700 mm. Thus, spatial profiles are diﬀerent between hot neutrals and impurity ions. The absolute value of C2+ temperature was ten
times higher than that of hot neutrals, and flow speed istwice faster. As we showed in the previous paper [9], He+ions in helium discharge have similar temperature profile
of C2+ ions in Fig. 4 (e). The discrepancy between hot neutrals and ions suggests that hot neutrals are not identical toprotons as proven in the past studies [12, 14].

Fig. 4 Spatial profile of (a) hot and (b) cold neutral temperature,(c) hot and (d) cold neutral flow velocity, (e) temperatureand (f) flow velocity of C2+ ions. All the values are line
averaged. The filling gas pressure and the ECH power forthis shot were 5.4 × 10−3 Pa and 36 kW.

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Therefore, our observation is the evidence of proton temperature anisotropy in RT-1. Although the discrepancy between hot neutrals and impurity ions suggests that hot neutrals are not identical to protons, anisotropy in broad component may be a technique to estimate proton temperature
anisotropy with help from precise analysis of line shape because the preceding study proved that separation of charge
exchange contribution from competitive processes enables
us to evaluate proton temperature [18].
Now, let us discuss the locality of the present observation. We may consider three scenarios for Hα emission
through charge exchange.
H(n=3) + H+ → H+ + H(n=3) ,(1)H(n=2) + H+ → H+ + H(n=2)⇒Fig. 5 Dependence of (a) the line averaged temperature and (b)the anisotropy of hot neutrals on the ECH power (fillinggas pressure was fixed as 5 × 10−3 Pa), and the dependence of (c) the line averaged temperature and (d) theanisotropy on the filling gas pressure (ECH power wasfixed as 35 kW).

Figure 5 shows the dependencies of the line averaged
hot neutral temperatures and anisotropy at r = 583 mm on
ECH power and filling gas pressure. The dipole field magnet was mechanically supported (i.e., without levitation)
for this measurement. From Figs. 5 (a) and 5 (b), both thetemperatures and anisotropy increased as ECH power increased. From Figs. 5 (c) and 5 (d) both the temperatures
and anisotropy decreased as the filling gas pressure increased. These tendencies coincided with those of ions [9].
For ions, the increase in ECH power results in increasein temperature and anisotropy through the relaxation withelectrons because the betatron acceleration heating is proportional to T ⊥ itself. On the other hand the increase in thefilling gas pressure leads decrease in the temperature andanisotropy through the charge exchange loss.

4. Summary and Discussion
We observed the broad component of Hα line emission (hot neutrals). The radial profile of the temperature and the toroidal velocity of hot neutrals were spatially inhomogeneous, whereas the cold component hadflat profiles. Furthermore, the hot neutral temperature had
anisotropy (T ⊥ > T || ). Even though the preceding studies already observed the hot neutral flow [15–17], the observation of the temperature anisotropy in hot neutrals isthe very first. The dependencies of hot neutral temperature and anisotropy were consistent with ions. Since hot
neutrals emitting a broad component are mainly producedby charge exchange between neutrals and protons [17],the flow and the anisotropy were inherited from protons.

H(n=2) + e− → H(n=3) + e− ,(2)

H(n=1) + H+ → H+ + H(n=1)⇒H(n=1) + e− → H(n=3) + e− .

(3)Here, we consider only resonance charge exchange processes because cross section of non-resonance charge exchange is much smaller. The process (1) is resonancecharge exchange of n = 3 level. Then, spontaneous emission of Hα de-excites the charge exchanged atoms to n = 2level. Since the spontaneous emission of n = 3 → 2 occursin ∼ 2.3×10−8 s, the travel distance of hot neutral with 6 eVfrom charge exchange to spontaneous emission is 0.5 mm,implying the suﬃcient locality for this scenario. The process (2) is resonance charge exchange of n = 2 level followed by electron impact excitation to n = 3 level. Then,spontaneous emission of Hα will occur. However, this scenario is not plausible because the electron impact excitation time (calculated as 5 × 10−6 s from ne ∼ 1017 m−3 and10 ≤ T e ≤ 100 eV [22]) is significantly shorter than thetime of the spontaneous emission n = 2 → 1 (∼ 2.1 × 10−9s). Therefore, almost all the charge exchanged atoms inn = 2 level go down to the ground level instantly. Theprocess (3) is charge exchange of ground level followed byelectron impact excitation to n = 3 level. The cross sectionof the resonance charge exchange in n = 1 level is approximately ten times smaller than that of the resonance charge
exchange in n = 3 level [21]. The time of the electron impact excitation n = 1 → 3 is 2 × 10−3 s [22]. The travel
distance of hot neutral with 6 eV is 50 m, which is incomparably larger than the device size. Also, the cross sectionof electron impact ionization (H(n=1) + e− → H+ + 2e− )is approximately ten times larger than that of excitation(H(n=1) + e− → H(n=3) + e− ) [22], and thereby the charge
exchanged neutrals at n = 1 level will be ionized before
excitation to n = 3 level. These estimates suggests that theprocess (3) is rare for a single neutral particle. However,since the number of the ground state neutrals is much larger
than that of n = 3 state neutrals before charge exchange,
the contribution of the process (3) is not negligible. Therefore, both of the scenarios (1) and (3) are feasible, and we

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have yet to determine which of the processes is governing.
Because the spatial resolution of the process (3) is equivalent to the device size, the contribution of this process is, atmost, only “uniform background” temperature in Fig. 4 (a).
The peaks of Figs. 4 (a) and (c) are attributed to the process(1), and the spatial resolution is ∼ 0.5 mm near the peaks.
Precise numerical simulation of neutral particle transport
is necessary for further discussions.
In closing, we remark the comparison with tokamak configuration. The preceding experiment in tokamak
configuration measured the Doppler temperature of broad
component from multiple directions which are tangential
and perpendicular to toroidal magnetic field [12] in thesimilar manner as the present study. The observed broad
component in Hα was precisely isotropic. Therefore, ananisotropy in broad component is distinctive to magnetospheric configuration.

Acknowledgments
This work was supported by JSPS KAKENHI Grant
No. 23224014.
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