Status of the KSTAR superconducting magnet system development

Kim, K.,Park, H.K.,Park, K.R.,Lim, B.S.,Lee, S.I.,Chu, Y.,Chung, W.H.,Oh, Y.K.,Baek, S.H.,Lee, S.J.,Yonekawa, H.,Kim, J.S.,Kim, C.S.,Choi, J.Y.,Chang, Y.B.,Park, S.H.,Kim, D.J.,Song, N.H.,Kim, K.P.,So International Atomic Energy Agency 2005 Nuclear fusion Vol.45 No.8

<P>The aim of the Korea superconducting tokamak advanced research (KSTAR) project is to develop a steady-state-capable advanced superconducting tokamak for establishing a scientific and technological basis for an attractive fusion reactor. Since the KSTAR mission includes the achievement of a steady-state-capable operation, the use of superconducting coils is an obvious choice for the magnet system. The KSTAR superconducting magnet system consists of 16 toroidal field (TF) and 14 poloidal field (PF) coils which include 8 central solenoid coils. Both the TF and PF coil systems use internally-cooled cable-in-conduit conductors (CICC). The TF coil system provides a magnetic field of 3.5 T at the plasma centre and the PF coil system provide a flux swing of 17 V s. The major achievement in the KSTAR magnet system development includes the development of CICC, a full size TF model coil, a background magnetic field generation coil system and the construction of a large scale superconducting magnet and the CICC test facility. TF and PF coils are at the stage of fabrication for the KSTAR completion in the year 2007.</P>

Effects of finite gradient <i>B</i> drift on collisional boundary layer analysis for neoclassical toroidal plasma viscosity in tokamaks

Shaing, K.C.,Seol, JaeChun,Sun, Y.W.,Chu, M.S.,Sabbagh, S.A. International Atomic Energy Agency 2010 Nuclear fusion Vol.50 No.12

<P>The effects of finite gradient B drift are included in the collisional boundary layer analysis to improve the accuracy of the neoclassical toroidal plasma viscosity in tokamaks that have error fields or magnetohydrodynamic activities present. Depending on the sign of the electric charge of the species and that of the radial electric field, the effects of finite gradient B drift can either reduce, if the E x B drift is in the same direction of the gradient B drift, or enhance, if these two drifts are in the opposite direction, the magnitude of the neoclassical toroidal plasma viscosity. Here, E is the electric field and B is the magnetic field. However, because the gradient B drift depends on the effective pitch angle, the net effects have to be properly weighted by integrating over the particle energy.</P>

3D field induced collisionless zonal flow decay in tokamak plasmas

Choi, G.J.,Hahm, T.S. International Atomic Energy Agency 2018 Nuclear fusion Vol.58 No.2

<P>We perform a gyrokinetic study of 3D resonant magnetic perturbation (RMP) field effect on zonal flows in tokamak plasmas. The result indicates that energy-dependent secular radial drift of particles due to the tangential component (<img ALIGN='MIDDLE' ALT='$\delta B_\Vert $ ' SRC='http://ej.iop.org/images/0029-5515/58/2/026001/nfaa9ac1ieqn001.gif'/>) of the RMP field induces a further long term algebraic decay of the Rosenbluth–Hinton (1998 <I>Phys. Rev. Lett</I>. <B>80</B> 724) residual flow expected in an axisymmetric tokamak. This mechanism could be related to an increase of H-mode transition power threshold in the presence of RMP observed in many tokamak experiments.</P>

Simulation of the hybrid and steady state advanced operating modes in ITER

Kessel, C.E.,Giruzzi, G.,Sips, A.C.C.,Budny, R.V.,Artaud, J.F.,Basiuk, V.,Imbeaux, F.,Joffrin, E.,Schneider, M.,Murakami, M.,Luce, T.,St John, Holger,Oikawa, T.,Hayashi, N.,Takizuka, T.,Ozeki, T.,Na, International Atomic Energy Agency 2007 Nuclear fusion Vol.47 No.9

<P>Integrated simulations are performed to establish a physics basis, in conjunction with present tokamak experiments, for the operating modes in the International Thermonuclear Experimental Reactor (ITER). Simulations of the hybrid mode are done using both fixed and free-boundary 1.5D transport evolution codes including CRONOS, ONETWO, TSC/TRANSP, TOPICS and ASTRA. The hybrid operating mode is simulated using the GLF23 and CDBM05 energy transport models. The injected powers are limited to the negative ion neutral beam, ion cyclotron and electron cyclotron heating systems. Several plasma parameters and source parameters are specified for the hybrid cases to provide a comparison of 1.5D core transport modelling assumptions, source physics modelling assumptions, as well as numerous peripheral physics modelling. Initial results indicate that very strict guidelines will need to be imposed on the application of GLF23, for example, to make useful comparisons. Some of the variations among the simulations are due to source models which vary widely among the codes used. In addition, there are a number of peripheral physics models that should be examined, some of which include fusion power production, bootstrap current, treatment of fast particles and treatment of impurities. The hybrid simulations project to fusion gains of 5.6–8.3, β<SUB>N</SUB> values of 2.1–2.6 and fusion powers ranging from 350 to 500 MW, under the assumptions outlined in section 3. Simulations of the steady state operating mode are done with the same 1.5D transport evolution codes cited above, except the ASTRA code. In these cases the energy transport model is more difficult to prescribe, so that energy confinement models will range from theory based to empirically based. The injected powers include the same sources as used for the hybrid with the possible addition of lower hybrid. The simulations of the steady state mode project to fusion gains of 3.5–7, β<SUB>N</SUB> values of 2.3–3.0 and fusion powers of 290 to 415 MW, under the assumptions described in section 4. These simulations will be presented and compared with particular focus on the resulting temperature profiles, source profiles and peripheral physics profiles. The steady state simulations are at an early stage and are focused on developing a range of safety factor profiles with 100% non-inductive current.</P>

Summary of the 2nd International Workshop on Environmental, Safety and Economic Aspects of Fusion Power

Wu, Y.,Chen, Z.,Meng, Z.,Hu, L.,Gonzalez de Vicente, S.M.,Merrill, B.,Panayotov, D.,Zucchetti, M.,Kolbasov, B.,van Houtte, D.,Bustreo, C.,Kim, Y.,Sakamoto, Y.,Kim, K.,Maisonnier, D.,Clark, D.,Kalashni International Atomic Energy Agency 2018 Nuclear fusion Vol.58 No.9

<P>The 2nd International Workshop on Environmental, Safety and Economic Aspects of Fusion Power (ESEFP) was held on 23 September 2017 in Kyoto, Japan. The workshop was initiated by the International Energy Agency Technology Collaboration Program on ESEFP. The workshop was well attended with approximately forty participants representing fifteen institutions in eight countries. The presentations covered safety issues and environmental impacts, availability improvement and risk control and socio-economic aspects of fusion power. Quantitative safety assessment of fusion reactors was discussed in depth with the consensus output presented as a plenary presentation at the 13th International Symposium on Fusion Nuclear Technology (ISFNT-13). The next workshop is planned to be held in conjunction with the ISFNT-14 in 2019.</P>

Electron cyclotron heating during ECRH-assisted pre-ionization in a tokamak

Seol, JaeChun,Park, B.H.,Kim, S.S.,Kim, J.Y.,Na, Yong-Su International Atomic Energy Agency 2010 Nuclear fusion Vol.50 No.10

<P>In this research, we investigate electron cyclotron resonance heating (ECRH) for low-energy electrons analytically and numerically, which is applicable to ECRH-assisted plasma start-up in a tokamak. From the previous experimental studies, it is well known that the first harmonic ECRH is more effective than the second harmonic ECRH. In this work, we develop an analytic model of ECRH in the start-up process and present comparisons of efficiency between the first harmonic and the second harmonic. It is found that electrons gain a large amount of energy from the waves at the first harmonic resonance up to several hundred electronvolts. However, electrons gain a large amount of energy from the waves only up to similar to 10 eV and the energy gain starts decreasing afterwards at the second harmonic resonance. When seed electrons are heated from room temperature to a temperature far above the ionization energy, the seed electrons can bring about an avalanche of electrons. Thus, pre-ionization with the second harmonic can be delayed since electrons need more time to be heated up to the breakdown temperature due to the slow heating speed compared with the first harmonic ECRH.</P>

Extended MHD simulation of resonant magnetic perturbations

Strauss, H.R.,Sugiyama, L.,Park, G.Y.,Chang, C.S.,Ku, S.,Joseph, I. International Atomic Energy Agency 2009 Nuclear fusion Vol.49 No.5

<P>Resonant magnetic perturbations (RMPs) have been found effective in suppressing edge localized modes (ELMs) in the DIII-D experiment (Evans <I>et al</I> 2006 <I>Phys. Plasmas</I> <B>13</B> 056121, Moyer <I>et al</I> 2005 <I>Phys. Plasmas</I> <B>12</B> 056119). Simulations with the M3D initial value code indicate that plasma rotation, due to an MHD toroidal rotation or to two-fluid drifts, has an essential effect on the RMP. When the flow is below a threshold, the RMP field can couple to a resistive mode with a helical structure, different from the usual ELM, that amplifies the non-axisymmetric field. The magnetic field becomes stochastic in the outer part of the plasma, causing density and temperature loss. At higher rotation speed, the resistive mode is stabilized and the applied RMP is screened from the plasma, so that the stochastic magnetic layer is thinner and the temperature remains similar to the initial unperturbed state. The rotational flow effects, along with the remnants of the screened RMP, cause a density loss which extends into the plasma core. The two-fluid model contains intrinsic drift motion and axisymmetric toroidal rotation may not be needed to screen the RMP nor stabilize the resistive mode.</P>

Transport simulation of ELM pacing by pellet injection in tokamak plasmas

Kim, Ki Min,Na, Yong-Su,Hong, Sang Hee,Lang, P.T.,Alper, B. International Atomic Energy Agency 2010 Nuclear fusion Vol.50 No.5

<P>This paper deals mainly with the numerical simulation on edge localized mode (ELM) pacing by pellet injection that is useful for fuelling and control of plasma profiles to achieve enhanced tokamak operations. The fuelling and pellet-induced ELMs are simulated with a 1.5-dimensional core transport code, which includes a neutral gas shielding model and a grad-B drift model for pellet deposition in H-mode tokamak plasmas. Fuelling and ELM pacing experiments by pellet injections at JET are introduced as a current experimental approach. For the description of ELM triggering by pellet injection based on ideal ballooning mode criteria, three possible models are suggested and discussed on their ELM characteristics, respectively: (i) the density enhanced ELMs in the post-pellet phase, (ii) the modification of the surface averaged pressure profiles in a transport time scale and (iii) the local increase in the pressure (density and/or temperature) gradients perturbed by pellets. Among them, the pellet-induced density perturbation model is adopted, in practice, to carry out an ELM pacing simulation in preparation for future experiments in KSTAR. The numerical simulation shows that the artificially induced ELM by pellets releases the reduced energy bursts, compared with spontaneous ELMs. The energy loss per burst by the pellet-induced ELM turns out to be much smaller than that by the spontaneous ELM as the pellet injection frequency becomes higher in ELM pacing. Based on the simulation results showing good agreement with the general ELM characteristics observed in pellet pacing experiments, the ELM pacing by pellet injection is very promising for mitigating the ELM energy bursts to the divertor by controlling the injection frequency.</P>

Generation of plasma flow in noise-driven Hasegawa–Mima model

International Atomic Energy Agency 2010 Nuclear fusion Vol.50 No.4

<P>The presence of macroscopic flows in hot plasmas is of great importance in nuclear fusion experiments. For pedagogical purposes, the spontaneous appearance of the flow is studied using the Hasegawa–Mima equation (HME) driven by external noise. The noise is assumed to model a bath of microscopic-scale turbulence. If the noise has a parity-nonconserving element, it is shown that an advective term associated with a uniform flow is inherently generated in the HME. A renormalization-group approach up to the lowest order is taken in the analysis of the asymptotic state. Parity-conserving noise is shown to lead to the renormalizations of field and viscosity and the critical dimensions are obtained.</P>

Unbiased and non-supervised learning methods for disruption prediction at JET

Murari, A.,Vega, J.,Rattá,, G.A.,Vagliasindi, G.,Johnson, M.F.,Hong, S.H. International Atomic Energy Agency 2009 Nuclear fusion Vol.49 No.5

<P>The importance of predicting the occurrence of disruptions is going to increase significantly in the next generation of tokamak devices. The expected energy content of ITER plasmas, for example, is such that disruptions could have a significant detrimental impact on various parts of the device, ranging from erosion of plasma facing components to structural damage. Early detection of disruptions is therefore needed with evermore increasing urgency. In this paper, the results of a series of methods to predict disruptions at JET are reported. The main objective of the investigation consists of trying to determine how early before a disruption it is possible to perform acceptable predictions on the basis of the raw data, keeping to a minimum the number of ‘ad hoc’ hypotheses. Therefore, the chosen learning techniques have the common characteristic of requiring a minimum number of assumptions. Classification and Regression Trees (CART) is a supervised but, on the other hand, a completely unbiased and nonlinear method, since it simply constructs the best classification tree by working directly on the input data. A series of unsupervised techniques, mainly K-means and hierarchical, have also been tested, to investigate to what extent they can autonomously distinguish between disruptive and non-disruptive groups of discharges. All these independent methods indicate that, in general, prediction with a success rate above 80% can be achieved not earlier than 180 ms before the disruption. The agreement between various completely independent methods increases the confidence in the results, which are also confirmed by a visual inspection of the data performed with pseudo Grand Tour algorithms.</P>