Improvement of core heat transport in NBI plasmas of heliotron J using high-intensity gas puffing

Controlling the heat transport profile is important for high performance in magnetically confined fusion plasmas. In this study, improved electron heat transport was achieved in neutral beam injection plasmas by applying high-intensity gas puffing (HIGP) on a stellarator/heliotron device called Heliotron J. Compared with conventional gas puffing (GP) fueling discharge, a higher and more peaked electron temperature profile was obtained, and the core ion temperature was slightly higher but similarly shaped. Using similar parameters, the electron density profile for HIGP remained similar and differed from the hollow density profile observed in electron cyclotron heating-eIBT plasma. Transport analysis using the FIT3D and TR-snap codes showed a clear reduction in the effective electron heat transport coefficient in the plasma core region. However, more detailed experiments are required to understand the mechanisms underlying this improvement fully.


Introduction
To obtain high performance in magnetically confined fusion plasmas, control of the heat transport profile is important.For example, in heliotron/stellarator devices, core electron root confinement (CERC) was achieved with strongly peaked electron temperature profiles via electron cyclotron heating (ECH) Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Heliotron J is a mid-sized stellarator/heliotron device.CERC was achieved by 2nd harmonic 70 GHz ECH with gas puffing (GP) fueling [4,6].Core electron heat transport was improved with an electron internal transport barrier (eITB).With the formation of the eITB, a hollow density profile is observed; this phenomenon is normally observed in ECHheated eITB plasma in stellarator/heliotron devices [4,6,7].
In recent experiments with Heliotron J, a new phenomenon was observed.Using a newly developed fuelling method called high-intensity GP (HIGP) [16][17][18][19][20][21], a fuelling technique which puffs a much larger amount of gas compared with the traditional GP in a very short period (normally 10-15 ms), a peak electron temperature profile was formed in a plasma heated by neutral beam injection (NBI).As the electron temperature profile was formed, the density profile remained similar to the conventional GP discharge.In this study, the radial profile of the heat transport coefficient of HIGP-fuelled NBI plasma was evaluated, and this result was compared with the GP-fuelled NBI plasma.

Experiment setup
Heliotron J is a medium-sized (R = 1.2 m, a = 0.2 m, B < 1.5 T) L = 1/M = 4 stellarator/heliotron device.R and a represent the major and minor radial of the device, respectively.B is the device's magnetic field strength at the magnetic axis, L is the pole number of the magnetic of the helical coil, and M is the pitch number of the magnetic field in the toroidal direction.Its magnetic configuration could be varied by controlling the current in each magnetic coil.In this study, the device was operated under a low-toroidal magnetic component (low-ε t ) configuration [21].This configuration was expected to help the plasma have better confinement of trapped particles.
The plasma was initiated by the 2nd harmonic 70 GHz ECH and then heated only by the co-going NBI, in which the additional toroidal current produced by NBI injection increases the poloidal magnetic field, with an acceleration voltage of 24 kV.Charge exchange recombination spectroscopy [22,23] (CXRS) was used to obtain the ion temperature profile and parallel flow velocity, and an Nd:YAG Thomson scattering system [24,25] was used to obtain the electron density and electron temperature profiles.

Experimental results
We compared two groups of discharges using different fuelling methods: HIGP fuelling and GP fuelling.The discharge parameters used are shown in figure 1.In the initial phase (170-190 ms), a 245 kW ECH pulse with pre-puffing gas was injected into the device to generate the initial plasma.The plasma was then heated by NBI injection.The NBI injection powers of the HIGP and GP discharge were 172 and 205 kW, respectively.HIGP fuelling was applied at t = 210 ms for 10 ms, whereas the conventional GP continuously puffed the gas from t = 200 ms to 260 ms.The quantity of gas pumped into the plasma can be reflected from the H-alpha signal of port 15.5 near the GP valve.Port 3.5 is far from the valve; therefore, its H-alpha signal indicates the wall-recycling condition of the device.
The two cases were compared at t = 240 ms.The electron density n e profiles, electron temperature T e profiles, and ion temperature T i profiles for the two studied cases are shown in figure 2. As shown in figure 2(a), n e profiles at t = 240 ms  are similar: flat at r/a < 0.5 and a gradient at 0.5 < r/a < 1.As shown in figure 2(b), a higher and more peaked T e profile was formed using HIGP.In the core region of the plasma, the electron temperature with HIGP reaches 350 eV, which is 150 eV higher than that in the GP case, whose core electron temperature is only 200 eV.A similar T e profile was not observed in previous NBI-only heating experiments.As shown in figure 2(c), a higher T i profile was formed by the HIGP.However, there is only a 20 eV difference in the core region, and the profile shapes were similar.Comparing the results of the HIGP case with previous results with ECH heating [4,6], the hollow density profile was not observed.This phenomenon is different from the normal CERC plasma.In the CERC plasmas, the density hollowness becomes high as the T e increases.In our case, the peaked T e is formed without the change.

Heat transport analysis
The heat transport coefficient was analyzed using the TR-snap [26] code, and the effective heat transport coefficient χ j using the code was derived by the following equation: where j is the species of the particles, and ´Pj V ′ dr is the heating source of the particle.´Pj V ′ dr includes the NBI heating power and the equipartition term, which relates to the heat transport between different species of particles.The symbols u j and Γ j in the second and third terms represent the thermal pinch and the particle flux, respectively.In the Heliotron J experiment, the second and the third terms are not large, so these two terms are neglected in the effective/experimental heat transport coefficient evaluation.Note that the NBI heating power in equation ( 1) is the NBI power deposited to the plasma.In this study, the deposited NBI power was estimated using the FIT3D [27] code based on the electron density, electron temperature, and ion temperature profiles shown in figure 2. The NBI deposition profile is shown in figure 3. Due to the different injection power, the profiles of the two cases in the figures look different.The absorption rate of both cases is around 36%, and the shine-through fraction for the particles is very similar.We estimated the absorption rate of the NBI injection using the FIT3D [27] code and then calculated the expected confinement time based on ISS04 [28] scaling at t = 240 ms.The theoretical confinement times of the HIGP and GP cases were 17 ms and 15 ms, respectively, whereas the respective experimental confinement times were 24.5 ms and 21.4 ms.In this case, the state is considered quasi-steady; therefore, the practical heat transport analysis with equation ( 1) can be applied.
The electron and ion heat transport coefficients, χ e and χ i , are shown in figure 4. Because the ion temperature is reliable at 0.15 < r/a < 0.7, only the heat transport coefficients in this region were evaluated.The error bar of the heat-transport coefficient depends on the upper and lower bounds of the equipartition term, which is related to the difference between T e and T i .
As shown in figure 4(a), a significant reduction of χ e is observed at 0.15 < r/a < 0.5, which coincides with the more peaked region of T e obtained with HIGP.In this region, the  reduction is up to 80%; at r/a near 0.2, the χ e improved from approximately 1.0 m 2 s −1 to approximately 0.2 m 2 s −1 .Conversely, the χ e of the two cases is similar at the r/a region near 0.6.
As shown in figure 4(b), the radial profile of χ i has no significant difference between the two cases.

Discussion
We evaluated several plasma parameters to elucidate the mechanism behind the observed phenomenon.
First, we compared the parallel flow velocities of the two cases; the profiles of the two cases were similar, and the parallel flow velocities estimated by the CXRS system were both approximately 10 km s −1 , indicating that the improvement cannot be attributed to the toroidal flow velocity shear.Second, the total plasma current in the plasma of both cases was less than 1.6 kA, and the maximum difference in the current is no more than 0.2 kA; hence, the change in the safety factor or magnetic shear is not the mechanism behind the phenomenon.Third, the radial electric field predicted by the neoclassical transport analysis PENTA [29] is different from that observed in CERC plasmas.In the neoclassical transport analysis, the only solution of the ambipolar condition below 0.02 keV m −1 is identified at the core region for both cases.Although a small positive radial electric field is observed, this value is much lower than that typically observed in electron-root solution in the CERC plasmas.Furthermore, the radial profile of the radial electric field remains consistent across the two cases.This means CERC is not a candidate to explain the confinement improvement of the HIGP plasma.
Some studies suggest that edge plasma control is a 'hidden factor' in achieving high plasma performance [30][31][32].We observed a decrease in the intensity of the H-alpha signal, indicating a decreased neutral density in the edge region.However, there is no direct proof of such a link between wall recycling and an improvement in the core electron temperature.

Summary
In summary, we observed a higher and more peaked electron temperature profile with the HIGP-fueled discharge, which has never been observed with conventional GP-fuelled NBI plasma in Heliotron J. Compared with previous results using ECH heating, although a peaked electron temperature profile was formed, the density profile did not change significantly, which is not observed with ECH-heated eITB plasma.For the HIGP discharge, the electron heat transport in the core region was improved.At r/a near 0.2, the heat transport coefficient χ e improved from 1.0 m 2 s −1 to 0.2 m 2 s −1 .In the HIGPfuelled discharge, a higher ion temperature in the core region with a profile shape similar to that with the GP-fuelled discharge was observed, and there is no significant difference in the ion heat transport.However, more detailed experiments are required to fully understand the mechanisms underlying this improvement.

Figure 1 .
Figure 1.Time evolution of plasma parameters for HIGP-fueled (blue) and conventional gas puffing (red) discharges.

Figure 2 .
Figure 2. The radial profile of (a) electron density, (b) electron temperature, and (c) ion temperature for HIGP and continuous GP-fueled discharges.

Figure 4 .
Figure 4.The radial profile of heat transport coefficient for (a) electron and (b) ion.