Proceedings of ITC/ISHW2007

Ion Heating Experiments Using Perpendicular Neutral Beam
Injection in the Large Helical Device
K. Nagaoka, M. Yokoyama, Y. Takeiri, K. Ida, M. Yoshinuma, S. Matsuoka1), H. Funaba,
S. Morita, T. Mutoh, T. Seki, K. Ikeda, M. Osakabe, K. Tsumori, Y. Oka, O. Kaneko,
and the LHD Experimental Group
National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japan1)
Dep. Nuclear Engineering, Kyoto Univ., Kyoto 606-8501, Japan(Received day month year / Accepted day month year should be centered 10-point type)
Perpendicular neutral beam (P-NB) injector was installed in the large helical device (LHD), and utilized forhigh-power ion heating and measurement of ion temperature profile by charge exchange spectroscopy. The ion
heating experiments have a significant progress using P-NB, and the high-ion-temperature of 5.2keV was achievedat plasma center. The peaked profile of ion temperature with steep gradient of ion temperature was observed. The
enhancement of toroidal flow was observed with the ion temperature rise in the core region. The transport analysis
shows the improvement of anomalous transport in the core region, when the high ion temperature is realized.
Keywords: perpendicular NBI, ion heating, charge exchange spectroscopy, beam fueling, toroidal
rotation, ion thermal transport

1. Introduction
Ion temperature is one of the most importantparameters to realize burning plasmas in magneticallyconfined fusion plasmas.
Before 9th experimental campaigns of the large helicaldevice (LHD), ion heating experiments were performed bytangential neutral beam (NB) injection. There are three
beam lines of negative-ion-based NB with very high beamenergy of 180keV, and total port-through beam power was
beyond 13MW. However, such high energy beam over thecritical energy (Ec~120keV for plasmas with Te~4keV)
mainly heats electrons, thus ion heating power was
relatively low. In order to increase the net ion-heatingpower, the ion heating experiments were performed in thehigh-Z discharge, and the high-ion temperature ofTi=13keV was realized [1-2], showing high potential
high-temperature plasma confinement in helical devices.
For ion heating experiment in low Z plasmas, lowenergy and high current NB was required, andperpendicular NB (P-NB) with low beam energy of 40keV
and port-through power of 3MW was installed and started
the beam injection in 9th experiment campaign of LHD,
and the power was upgraded to 6MW at 10th experimental
campaign [3]. The NB has a high ion-heating efficiency of80% for plasmas with Te=4keV, and the ion heating power
was significantly increased in LHD.
The P-NB can be utilized for profile measurements ofauthor’s e-mail: nagaoka@LHD.nifs.ac,jpion temperature by charge exchange spectroscopy (CXS)[4], thus ion heat transport can be experimentally estimated.
Moreover, the P-NB has a high particle fueling rate due tothe high beam current of 150A, and is also utilized forparticle fueling and control of density profile. In this paper,
recent results of high-ion-temperature experiment using
P-NB in LHD are presented, and the density profile
dependence of ion temperature, ion transport property,
toroidal flow associated with high-ion-temperature arediscussed.

2. Experimental
The LHD is a world-largest helical device with major
radius of 3.9m and averaged minor plasma radius of 0.6m.
The toroidal and poloidal periods are n=10 and m=2,respectively. The plasmas are heated by electron cyclotron
resonance heating (ECH) of 2MW, ion cyclotron heating(ICH) of 2MW and three tangential negative-ion-based
NBIs with total port-through power of 14MW. A
perpendicular NBI with low energy of 40keV was installedin 5-O port of LHD in 2005. Four positive ion sourceswere mounted on the beam line and the total port-throughpower is 6MW. Two power supply systems (#4A and #4B)
can independently control the beam operation such as pulse
timing, beam energy, beam duration and so on. The
nominal pulse duration is 10 sec, and the beam species ishydrogen. The beam injection with total port-throughProceedings of ITC/ISHW2007

Fig. 1. (a) The response of line averaged electron density with P-NB injections. (b) The density profiles
with P-NB injection of 0A, 40A, 82A and 156A. The profile of density increase is shown in upper region.
The beam fueling rate calculated by FIT code is also shown in the bottom region, which is consistent
with the observed density increase.
power of 7MW was achieved in 10th experimental
campaign.
Two systems of active CXS were installed for theP-NB as a probe beam. One is toroidal system having
toroidal line of sights and can measure profiles of ion
temperature and toroidal rotation of the plasmas. The otheris poloidal system having poloidal line of sights and canmeasure profiles of ion temperature and poloidal rotationof the plasmas. The P-NB is modulated (100msec ON and
100msec OFF) for CXS measurement to acquire thebackground signal.

3. Results and Discussions
A low beam energy and high current beam has a large

Fig. 2. The ion temperature as a function ofion heating power.

particle fueling rate. In order to investigate beam fueling,
the beam current of P-NB was performed by changing thenumber of operating ion source with the same gas
condition. The electron density increases with the beam
current, which is shown in Fig. 1(a) and the increase rate of
3x1016m-3s-1A-1 or 2.3x1020s-1MW-1 was obtained. Thechange of the density profile and the profile of density
increase rate are shown in Fig. 1(b). The density increase
rate has a flat profile. The deposition profile is considered
to be broad profile because of low density of the target
plasma. The beam deposition calculated by FIT code,
which is also shown in Fig. 1(b), is consistent with theexperimental results.
The P-NB power dependence of ion temperature wasinvestigated in the plasmas heated by only P-NB. The
maximum ion temperatures of each discharge are plotted inFig. 2 as a function of ion heating power normalized by theion density. In this figure, it is clearly seen that the ion
temperature increases with ion heating power. It is also
experimentally confirmed that the ion temperature
increases with P-NB power in the other heating condition
such as full power heating with tangential NBs.
The high ion temperature experiments using P-NB
were performed and central ion temperature of 5.2keV wasrealized with the central electron density of 1.2x1019m-3,
which is shown in Fig. 3. The high ion temperature regime
was extended toward high density plasmas, and the central
ion temperature of 3keV was achieved with the central
electron density of 3.2x1019m-3. The central electron
temperature is lower than ion temperature in low density
region, indicating the strong ion heating. The electron andion temperatures are almost same in high density region.
This is considered to be attributable to short equi-partition
time. These high ion temperature were realized in theProceedings of ITC/ISHW2007

Fig. 3. The central ion temperature as afunction of central electron density. Thecentral electron temperature at the time withmaximum ion temperature is also shown.
density decay phase after superimpose of tangential NBI
on the P-NB heated plasmas. The time trace of ion
temperature, line averaged electron density and the central
density divided by line averaged density as a density
peaking factor are shown in Fig. 4. After superimpose oftangential NBs heating (t=1.1sec), the electron density
decreases, and the ion temperature increase and reaches themaximum values at t=1.35sec. Then the ion temperaturestarts to decrease gradually while the electron density
continues to decrease. The peaking factor increases in theP-NB heating phase and decreases after superimpose oftangential NBs. This is a general tendency in LHD
plasmas; the density profile becomes peaked one in P-NB
heating plasmas and flat or hollow one in tangential NBs.
The peaking factor of the electron density keeps still highvalue when ion temperature reaches the maximum, while
the electron density decreases quickly. Therefore, it seems
that the peaking factor of electron density is important forion temperature rise and the peaked density profile ispreferable to realization of high ion temperature.
Figure 5(a) shows the profiles of ion temperature at
P-NB phase (t=0.95sec), maximum ion temperature phase(t=1.35sec) and after the decrease of ion temperature(t=1.75sec). The ion temperature has a peaked profile and asteep gradient is formed when ion temperature becomes
high. The electron temperature is 4keV at the center and isalmost same at R>4.2m when t=1.35sec. The large toroidal
flow with velocity of 60km/sec was generated in the coreregion, when the high ion temperature was realized, whichis shown in Fig. 5(b). The flow direction is consistent withFig. 4. The ion temperature (upper), line averaged
electron density (middle) and a peaking factor(bottom) given by the central density normalized
by the line averaged density are shown,respectively.
that of tangential NB injection. These observations indicate
strong correlation between ion transport and toroidalrotation. The poloidal rotation in core region can not beobserved when ion temperature becomes high, because thecarbon impurity has a strong hollow profile associated with
ion temperature rise. This “impurity hole” also shows
strong correlation with ion temperature rise. The outward
flow of carbon impurity against a negative gradient ofcarbon density was also observed. The large error bars ofion temperature in core region with t=1.35sec in Fig 5(a)are attributable to formation of impurity hole. The
neoclassical calculation shows negative radial electric field(ion root), while the impurity pumping-out can be expectedby electron root in neoclassical theory. Therefore theunderstanding of impurity hole formation in the
neoclassical ion root is a new subject related to thehigh-ion temperature experiments.
The neoclassical ion thermal diffusivity does not change
significantly in core region [5], which is shown in Fig. 5(c),while ion temperature increases more than twice. In such alow collisional regime, the neoclassical ion flux without
electric field strongly depends on ion temperature as Ti7/2.

Proceedings of ITC/ISHW2007

(a)

(b)

(d)
(c)

Fig. 5. (a) The profiles of ion temperature of the plasma heated by only P-NB (t=0.95sec), just after
superimpose of N-NB (t=1.35sec) and mainly heated by N-NB (t=1.75sec). (b) The toroidal rotation
profile at the same time with (a). (c) The neoclassical ion-thermal diffusivity w/ and w/o neoclassicalambipolar radial electric field. (d) The ion thermal diffusivity obtained by power balance analysis.
The degradation of ion transport is considered to besignificantly suppressed by negative radial electric field.
The neoclassical electron diffusivity also remains
unchanged in high ion temperature phase. The ion thermal
diffusivity normalized by gyro-Bohm factor, which isestimated by power balance analysis, is shown in Fig 5(d),and the significant reduction of thermal diffusivity wasclearly observed in core region, when ion temperature
increases. Therefore, it is considered that the reduction of
anomalous transport occurs when ion temperature
increases.

3. Conclusion
The installation of P-NB injection was significantly
progressed high ion temperature experiments on LHD; oneis upgrade of ion heating power, and the other is profile
measurements of ion temperature, toroidal and poloidal
flow, impurity density. The high ion temperature over

5keV was achieved and extended toward high density
plasmas. The large toroidal flow and impurity hole wereobserved in core region associated with ion temperaturerise. The negative radial electric field (neoclassical ionroot) significantly suppresses the degradation of ion
thermal diffusivity, and reduction of anomalous transport
was observed in core region. These results are still
preliminary and not fully understood yet. The systematic
experimental study and neoclassical viscosity analysis arein progress.

References[1]
[2]
[3]
[4]
[5]

Y. Takeiri, et al., Nucl. Fusion 47, 1078 (2007).
S. Morita et al., 19th IAEA Fusion Energy Conference,Lyon, 2002, EX/P2-18.
M. Osakabe, et al., this conference.
M. Yoshinuma, et al., this conference.
S. Matsuoka, et al., this conference.