A3 significant cooperative activities

When a discharge is longer, the handling of high heat load over the divertor and the first wall surrounding high-temperature plasma becomes a vital issue because it creates an entirely different situation from what has been studied in normal conducting toroidal devices with the discharge length strictly limited. The issue is a common and challenging subject among the three different devices in the three countries of CJK. The study of critical physics for the steady state operation of high-performance plasma is made possible only by superconducting devices and will produce important results for the first time with the three superconducting devices of C-J-K. Then, as the study focusing only on the critical physics that will be revealed for the first time by the steady state discharge of high-performance plasmas, the following three critical physics issues are listed up for the joint research among C-J-K as shown in Fig.1.

  1. Steady state sustainment of magnetic configuration
  2. Edge and divertor plasma control
  3. Confinement of alpha particles

The three issues still remain as open questions for the steady state discharge of high-performance plasma, i.e., 'Critical physics issues for steady state operation' in the steady state operation as shown in Fig.1. The issues involved three unresolved subjects, which have to be studied through the present Joint Research. Individual studies should be along with the planned 'Collaboration programs' (also see Fig.1), which consider the characteristics of each of the devices. The details of each study are described in the following.

(I) Steady state sustainment of magnetic configuration

The characteristic time scale of energy and particle transports in high-temperature plasmas is the order of energy confinement time, which is in the range of 0.1 to 1 second in existing large tokamaks and LHD. On the other hand, the magnetic configuration formed by external magnetic field and plasma itself evolves along with the time scale of magnetic diffusion time, which is up to 50 seconds in these existing devices and will reach around 200 seconds in ITER. That is, the MHD equilibrium evolution takes much longer than the plasma does in the existing tokamaks. Accordingly, high performance plasmas produced in these tokamaks are sustained only transiently. For the realization of an attractive fusion reactor, it is crucial whether or not the high performance plasma can be sustained for a longer period of time than the magnetic diffusion time. For a tokamak-type fusion reactor, the toroidal plasma current has to be steadily sustained using non-inductive current drives and self-generated current called "bootstrap current", while for a helical-type fusion reactor the plasma can be sustained without such help, as seen in LHD (see Fig.2). The following topics are thought to be an exceedingly important target of collaboration with the three superconducting devices of EAST, LHD and KSTAR.

I-1. Steady state sustainment of disruption free plasma (LHD)
I-2. Current drive and current profile control for steady state operation (EAST)
I-3. Long pulse operation with safety factor (q-) profile control and pressure profile optimization (KSTAR)

In EAST, the long divertor plasma discharge up to 100 seconds is demonstrated by lower hybrid waves (LHWs) which are traveling waves in the toroidal direction, in relatively low density and high electron temperature plasmas. Recently, a long pulse H-mode (High confinement mode) accompanying with edge transport barrier (ETB), shown in Fig.3, has been produced by only LHWs during a period more than 100 times longer than the energy confinement time. It is important that H-mode is sustained without any strong ion heating and momentum input, of which condition is surely reactor-relevant. At present, the LHW system is being further upgraded for longer pulse operation of high performance plasma such as the H-mode. The other type of high performance plasma called "Internal transport barrier (ITB) mode (see Fig.3)" is also an important research target, because it has high confinement and high bootstrap current fraction which reduces the requirements for non-inductive current drive. A new challenge on plasma current drive and control of the safety factor (q-) profile is to maximize the plasma performance using optimized launching of LHWs. The long sustainment of high performance plasma can be achieved by cooperating with edge and divertor plasma control.

In KSTAR, the long pulse operation of high performance plasma is a main research target. A possible candidate of high performance plasma is an ITB plasma produced in a reversed magnetic shear configuration through q-profile control and fine tuning of plasma pressure profile, where the high bootstrap current fraction as well as the high plasma confinement are expected to be valuable with high power neutral beam injection (NBI), ion cyclotron range of frequency (ICRF) heating and electron cyclotron heating (ECH) and current drive (ECCD). These heating systems are being upgraded year by year. The H-mode with ETB is another operational target in KSTAR, and has been successfully achieved. This achievement clearly provides an opportunity to study high performance plasmas with confinement improvement in the near future. Improvement on MHD stability of these high performance plasmas at high beta regime is critical for achieving stable and long sustainment. Here, the value of beta is defined as the ratio of plasma pressure to magnetic pressure. This is planned to perform with active feedback control of the external resonant magnetic fields produced by in-vessel control coil sets.

The Large Helical Device (LHD) has the capability of sustaining high performance plasma in the steady state condition without plasma current drive. The plasma can be sustained in stably-generated magnetic bottle without plasma disruptions. The long pulse operation up to one hour has been achieved using ICRF heating by sweeping the heat deposition area on the divertor plates while controlling magnetic configuration. The sweeping leads to uniforming heat load distribution on the divertor plates. On the other hand, high confinement plasmas such as H-mode are transiently obtained by controlling the edge condition with stochastic magnetic fields, whereas the plasma collisionality is still high compared with reactor relevant conditions. Similar to the tokamak H-mode, large amplitude ELMs are observed in such H-mode discharges. This observation gives us a different viewpoint to study ELM mitigation in KSAR and EAST. ITB-like plasmas are also obtained in both regimes of high electron and ion temperatures at relatively low density. Effects of rotation, radial electric field and zonal flow have been extensively studied, both experimentally and theoretically, for these LHD plasmas confined with the magnetic field characterized by a different rotational transform profile to that of tokamak. Here, it is noted that both of the rotational transform ( ) and the safety factor (q) express a twist in the toroidal direction. The rotational transform, which is generated by external helical coils, is usually used in HELICAL research, while the q-profile is usually used in TOKAMAK research, which is generated by plasma current. The two parameters can be expressed by an opposite relation of 1/q. The Production of these high performance plasmas and the long sustainment of such plasmas are being sought through optimizing a heating scenario and magnetic configurations.

The three devices with different magnetic configurations adopt different experimental approaches for the attempt towards the steady state operation with high-performance plasma. The collaborations on current drive and profile control are within the same network, so it will certainly bring fruitful results on plasma physics in both fields of experiment and theory.

(II) Edge and divertor plasma control

In order to mitigate the heat load over divertor (see Fig.4) in the steady state discharge, several methods based on different physics backgrounds have been considered in the three devices as follows.

II-1. Stability of edge MHD with RMP field(LHD), 
 Mitigation of large-amplitude ELM by edge perturbation (EAST)
 Mitigation of large-amplitude ELM by RMP field (KSTAR)
II-2. Plasma detachment in three-dimensional (3D) divertor (LHD)
II-3. Divertor plasma control with high-Z material.(EAST)
II-4. Wall conditioning and edge plasma control (EAST, KSTAR)

The intermittent burst of heat flux onto divertor plates caused by large amplitude ELMs induced with the H-mode has to be mitigated, otherwise around 10% of the total plasma stored energy will be lost, which leads to the heat flux exceeding the tolerance of divertor materials. The stochastic magnetic filed can reduce the pressure gradient in the ETB region. If the large amplitude ELM can be resultantly replaced by several small ELMs, the mitigation of the heat load to the divertor is expected. Here, it is very important to estimate the impact of the island chain and/or stochastic magnetic field layer generated by Resonant Magnetic Perturbation (RMP) coils. It is also important to calculate the stochastic magnetic field generated by RMP coils while taking into account the plasma response. Understanding of the effects of the islands and/or stochastic magnetic field region on particle and energy transports in such region is crucial because it will clarify the mechanism of mitigating the ELMs. Besides the RMP coils, in KSTAR, several other attempts are made based on hydrogen pellet injection and super sonic gas puff injection to tailor the edge pressure gradient in order to suppress the ELMs while simultaneously maintaining the core plasma performance. In LHD, on the other hand, the structure of the edge magnetic field is intrinsically stochastic due to the presence of higher orders of Fourier components in the magnetic field generated by helical coils, forming the characteristic topology called ergodic layer. This inherent stochastic layer could reduce the heat flux caused by ELM-like activities, if the collisionality of the plasma in such region is low enough and the plasma feels stochastic field region in such a collisionality. So far, large-amplitude ELMs are still induced in LHD H-mode plasmas because the edge plasma is still collisional. In LHD, the structure of stochastic magnetic field can be exactly calculated under a vacuum condition, and the MHD equilibrium in the magnetic configuration with the stochastic field region is also calculated by a simulation code named HINT, while taking account of the plasma response. Therefore, the comparative study between KSTAR and LHD will lead to a new discovery of plasma physics on heat and particle transports in the stochastic magnetic filed. The phenomenon of intermittent burst at the plasma edge called "blob" is also an important issue to be addressed in such plasma physics. Although the blob induces a perpendicular particle flux with a velocity of a few km/s in tokamaks, the direction is not uni-directional in LHD due to the presence of helical ripple. Theoretical work is also planned to study the RMP effects. Moreover, the HINT code is prepared to calculate the structure of stochastic magnetic fields generated by RMP coils in tokamaks as well as in LHD.

Impurity radiation can change the edge heat flux into thermal radiation. This leads to the mitigation of heat flux coming to divertor plates since the radiation to all directions is emitted. Noble gases such as neon and argon are injected for the steady discharge through the gas puffing method. The edge plasma cooling through impurity radiation is easily examined at high-density operation in LHD because there exists no current-driven instability. Studies are also important on the momentum loss along the magnetic filed caused by collisions with neutral gas. The interaction of fluid plasma with neutral gas is frequently expressed by the Navier Stokes neutral fluid model. In this study the edge impurity transport becomes very important in terms of parallel and cross-field transports. Since in LHD the connection length of field lines in the edge ergodic layer is long enough, the effect of cross-field transport is important in addition to the parallel transport, while in tokamaks the parallel transport becomes dominant in the scrape-off layer. Therefore, special attention has to be paid to the effect of density and temperature gradients along the magnetic filed, which give driving forces to transfer the impurity ions downstream and upstream depending on the collisionality, respectively. The two driving forces along the magnetic filed line are based on the momentum loss of impurity ions due to collisions with protons and the ion temperature gradient force called thermal force, which dominantly determine the edge impurity transport, in addition to the cross-field transport. In KSTAR, an appreciable effect of stochastic magnetic field by RMP on the edge impurity transport is also inferred in addition to the effect of edge current profile affecting the stability. In order to conduct a quantitative analysis of impurity radiation, the atomic physics study in plasmas is also important. Three-dimensional theoretical modeling using EMC3-EIRENE code is prepared for understanding the edge plasma and impurity transports, especially those in the ergodic layer and scrap-off layer.

In existing toroidal devices a carbon material is used for the plasma-facing components. In the future machine with D-T burning plasma, however, the use of carbon is recently considered to bring unavoidable problems on tritium absorption and chemical and physical erosions. A new material has to be tested in the steady state operation of high-performance plasmas. In EAST, a new experimental attempt has been started with the materials of first wall and divertor replaced by high-Z elements, such as molybdenum (Z=42) and tungsten (Z=74), which have less erosion and high melting point. In order to improve the plasma performance, the profiles of edge temperature and density have an extremely important meaning because the core plasma performance is always linked to the edge plasma performance. Since the elements show fairly different characteristics from carbon in terms of impurity transport, hydrogen recycling and hydrogen retention as well as radiation losses, it is very important to control the edge plasma with edge transport barrier (ETB) and to sustain the steady state discharge. In other words, the effort in EAST can give a really good opportunity to see the transport of the edge plasma facing the high-Z material in steady state discharge. The study on the relation between the influx of the high-Z impurities and edge stochastic magnetic fields becomes also an important issue in terms of impurity screening. It is predicted, according to the neoclassical theory, that the high-Z elements will have a tendency to accumulate in the central column of the plasma. As the high-Z impurities have a large radiation cooling rate, the impurity transport in the core plasma is also a target of the collaboration. Spectroscopy and atomic physics on the high-Z elements are also consequently important for a reliable diagnostics and investigation into the transport of high-Z impurities.

As mentioned above, the plasma performance is strongly linked to the edge condition including the first wall. Up to now, a variety of wall-conditioning techniques has been tested in many toroidal devices. In EAST, recently, the plasma performance has been much improved after coating the first wall with lithium, which has finally led to the H-mode transition. Therefore, it becomes very important to study the physics mechanism between the core plasma performance and the wall conditioning including lithium coating, in particular, in the steady state operation of high-performance plasmas. In both of the edge and core plasmas, especially, it has to be made clear what kinds of turbulence can be controlled by the wall conditioning. Furthermore, a study on plasma-wall interactions is also important to understand the physics mechanism between the edge plasma and the surface condition of plasma-facing components. Theoretical modeling is attempted for understanding the physics mechanism, and theoretical analysis using a simulation code named ERO (Erosion and Redeposition) is also underway to challenge the complexity in physics.

(III) Production and confinement of alpha particle

Production and confinement of alpha particles is an extremely important subject in the future burning plasma because alpha particles not only heat and maintain the steady state plasma but also give a serious damage to the plasma-facing components when alpha particles are not well confined. Therefore, it becomes very important to simulate the alpha particle transport using the energetic particles existing in the present devices. In particular, controlling of energetic particles in the steady state discharge is important in addition to understanding of the physics of energetic particle transport. In order to predict the performance of burning plasma heated by alpha particles and to explore a possible discharge scenario to avoid serious damages caused by the alpha particles, the transport of energetic ions has to be intensively studied in the existing devices. In relation to the physics the following three subjects are considered.

III-1. Transport of energetic particles in 3D geometry (LHD)
III-2. Confinement of energetic particles (EAST)
III-3. Interaction between MHD instabilities and energetic particles (KSTAR)

LHD and KSTAR are equipped with the heating systems of neutral beam injection (NBI) and ion cyclotron resonance frequency (ICRF). EAST is equipped with ICRF and the installation of NBI is scheduled on 2013. The NBI brings energetic ions to plasmas, e.g., 200keV for the LHD NBI, and ICRF accelerates ions to a higher energy range, e.g., several MeV. On the other hand, the study on production and confinement of alpha particle is the largest issue for the next-generation fusion device with D-T burning. The transport of energetic particle has been studied in many devices and behaviors of the alpha particles have been simulated based on the experimental results obtained from short pulse discharges of normal conductor tokamaks. The confinement of energetic particles is very sensitive to small changes in various plasma parameters. Therefore, it is important to study the transport of energetic particles in longer time scales and to find how to control energetic particles in the steady state discharges. The transport of alpha particles in the steady state discharge will be then studied using such energetic ions generated in the three devices. It will focus passing and trapped ions, ions with precessional drift and ions with a relatively large orbit deviated from magnetic flux surfaces existing in high-performance plasmas with low collisionality. The physics issues on the orbit topology including ripple-induced transports and energetic-particle-driven magnetohydrodynamic (MHD) instabilities become particularly important for the transport of energetic particles. The ripple-induced transport of energetic ions has been studied for many years in the fusion research. Recently, however, the ripple transport has drawn a lot of interests again because of the necessity of re-investigating the enhanced loss of alpha particles due to the insertion of test blanket module (TBM) in ITER. Since the insertion produces unfavorable magnetic field ripples, it can enhance the alpha particle loss. It should be noted here that the magnetic field structures of LHD and KSTAR/EAST tokamaks are quite different, giving different characteristics to the energetic-ion transport. This means that in the three-dimensional LHD structure, the non-axisymmetric helical ripple is dominant, while in the axisymmetric KSTAR and EAST, small toroidal field (TF) ripples appear between two TF coils and 3D field perturbations by ELM control coils are seen. Therefore, the joint collaboration among LHD and KSTAR/EAST will be able to create an interesting field to help general understanding of the impact of magnetic field ripples on the energetic ion confinement in toroidal fusion plasmas. At present, energetic ion loss based on the energetic-particle-driven MHD instabilities is of great concern. The energetic ions can potentially behave as a free energy source to destabilize the energetic-particle-driven MHD instabilities such as toroidicity-induced Alfv?n eigenmode (TAE), energetic-particle continuum mode (EPM) and so on. A great deal of attention should be paid to redistribution or loss of energetic ions induced by these instabilities because the interplay between energetic ions and those instabilities has not been fully understood yet. Significant differences between LHD and KSTAR/EAST are seen in the rotational transform profile, providing different shear Alfv?n continuum, e.g., different TAE gap structure. Collaborative research between LHD and KSTAR/EAST can contribute to a comprehensive understanding of the interplays between energetic ions and energetic-particle-driven MHD instabilities in magnetically confined fusion plasmas. Recent theories predict that energetic electrons are also capable of destabilizing these instabilities since the excitation of these modes depends on the precessional drift or the circulating frequency of a charged particle, but not on its mass. On the other hand, energetic electrons are characterized by small dimensionless orbits, similar to alpha particles in reactor-relevant plasmas. In this point of view, experiments on energetic-particle-driven MHD instabilities based on the energetic electrons are also valuable as it will enhance the understanding of concerned physics. Energetic electrons can be produced by electron cyclotron resonance heating (ECRH) or lower hybrid wave current drive (LHCD) systems. Joint experiments on the transport of energetic electrons are of course feasible among LHD, KSTAR and EAST.

National Institute for Fusion Science