The PROTO-SPHERA experiment is an unconventional magnetic confinement scheme,
which aims at producing a Spherical Torus (with Ie≤300 kA) around a Plasma Centerpost (with Ie=70 kA) fed by electrodes of annular shape, in contrast with the metal centerpost of conventional Tokamaks. The Phase-1 of PROTO-SPHERA, aimed of obtaining a stable Hydrogen Plasma Centerpost, lasting 1 sec at Ie=8.5 kA...
PROTO-SPHERA is an axi-symmetric magnetic confinement experiment, just like a spherical Tokamak: toroidal and poloidal magnetic fields (i.e. a plasma toroidal current IST) are required inside a spherical torus to contain the plasma. PROTO-SPHERA produces both fields while removing the two central metal conductors: the central toroidal rod and the central ohmic transformer; they are replaced by...
The PROTO-SPHERA experiment can produce plasma spherical tori with a simply connected configuration (the centerpost is practically implemented by the plasma itself) and a high confinement efficiency (β→1). Such configurations are obtained from the self-organization of a plasma arc (screw pinch) in a magnetostatic field. This approach leads to several technical and economic advantages, that...
Plasma tori formed around a 10 kA centerpost screw-pinch plasma discharge have been observed in phase-1 of PROTO-SPHERA experiments.
A simple model of plasma equilibrium has been developed, which reproduces the morphological features observed so far, in particular the bumpy shape of the centerpost discharge and the slim torus surrounding it.
The model is based on a generalization of the...
The first stage in the experimental programme of MAST-U is planned for late 2019. A modelling framework, using time-dependent vacuum field calculations, has identified viable direct induction scenarios on MAST-U that achieve suitable null quality for rapid breakdown combined with the requirements for plasma equilibrium and passive vertical stability during the early plasma current ramp up [1]....
Presentation on the features and status of MAST Upgrade
LTX-β, the upgrade to the Lithium Tokamak Experiment, has now been operated with full lithium coatings on all plasma-facing surfaces, and at increased toroidal fields >0.3 T. Plasma current has so far been limited to <100 kA. The upgrade includes a neutral beam injector provided by Tri-Alpha Energy Technologies. The beam is designed to operate at 20 kV, with 35A of ion current. So far >600 kW...
The Pegasus Toroidal Experiment has focused on exploring the unique features of extremely low aspect ratio operation to study plasma stability at A ~ 1 and near-unity beta, including access to ohmic H-mode with associated ELM behavior and plasmas with an absolute minimum-B topology. For such studies, the Local Helicity Injection (LHI) technique was developed to initiate and drive toroidal...
Identifying attractive means of initiating current without using induction from a central solenoid remains a critical challenge facing the spherical tokamak (ST) concept, and is desirable for tokamaks in general. An electron Bernstein wave (EBW) heating and current drive system is being designed for use in overdense ST plasmas on the upgraded Pegasus Toroidal Experiment. The research program...
The spherical tokamak (ST) may require and the advanced tokamak would considerably benefit from the elimination of the central solenoid. PEGASUS-3 is a ST non-solenoidal startup development station under design and fabrication dedicated to solving the startup problem. On PEGASUS-3, Transient and Sustained coaxial helicity injection (T- and S-CHI) will be explored, as well as possible synergies...
Described are first results obtained on the Globus-M2 tokamak – upgraded version of Globus-M machine in spring-summer 2019 operating campaign. Operation was performed at toroidal magnetic field strength 0.7 T and plasma current 0.3 MA. Results of the campaign may be tentatively joined into a few groups. First group is related to initial stage of the discharge. Plasma breakdown conditions were...
The presentation is devoted to the thermal energy confinement study at the compact spherical tokamaks Globus-M and Globus-M2. Experiments were performed under auxiliary heating using neutral beam injection (NBI) in plasma with lower null magnetic configuration (major radius R = 0.35 m, minor radius a = 0.21-0.22, elongation ~1.9, triangularity δ~0.35) for the ranges of plasma current and...
Tokamak Energy is a privately funded company based in the UK with a mission to deliver a faster route to fusion. Founded in 2009, Tokamak Energy is developing compact fusion power plants based on two promising technologies: Spherical Tokamaks (STs) and magnets made from High Temperature Superconductors (HTS). The inherent compactness and improved efficiency of the spherical tokamak, coupled...
ST40, engineering, commissioning, first results
LATE (Low Aspect ratio Torus Experiment) is a small device with the toroidal magnetic field up to 0.16 T at R = 0.25 m. It has no center solenoid and the plasma current is initiated and maintained by ECH/ECCD alone. There are three launchers for 2.45 GHz microwave and one launcher for 5 GHz microwave, each of which is installed on the radial port and injects microwave in the left-handed...
Y. S. Hwang and VEST team
Department of Nuclear Engineering, Seoul National University, Korea
yhwang@snu.ac.kr
The high-power reconnection heating of merging tokamak plasma has been developed mainly in TS-3, TS-4 and MAST experiments. This unique method is caused by the promising scaling of ion heating energy that increases with squire of reconnecting magnetic field Brec. We studied mechanisms for this scaling of reconnection (ion) heating up to 2.3keV mainly using TS-3 and TS-6 experiments and PIC...
Ion heating/transport process of CS-free merging plasma startup through magnetic reconnection has been investigated in the TS-3U (TS-6) spherical tokamak using ultra-high resolution 96CH/320CH 2D ion Doppler tomography diagnostics. In addition to the previously reported high-temperature plasma startup up to $\sim$250eV in TS-3 and $\sim$1.2keV in MAST, this research focused on the detailed...
A new extensive validation study performed for a modest beta NSTX NBI-heated H-mode predicts that electron thermal transport can be entirely explained by short-wavelength electron-scale turbulence fluctuations driven by the electron temperature gradient mode (ETG), both in conditions of strong and weak ETG turbulence drive. For the first time, local, nonlinear gyrokinetic simulations carried...
Machine Learning approaches to predict the ideal stability properties of spherical tokamak plasmas
A.Piccione 1, J.W. Berkery 2, S.A. Sabbagh 2, Y. Andreopoulos 1
1 Department of Electronic and Electrical Engineering, University College London, London, WC1E 7JE, UK
2 Department of Applied Physics and Applied...
Utilizing high-Z solid walls such as tungsten is challenging for next-step/Fusion Nuclear Science Facility (FNSF)/Pilot Plant applications due to a range of issues. These issues include Plasma-Facing Component (PFC) material damage from erosion and re-deposition and neutrons, high-Z impurity accumulation and associated core plasma radiative collapse, relatively low heat flux limits at the...
Guiding Centre theory has been developed to describe the dynamics of charged particles subject to the Lorentz force. In the context of fusion theory, it is used in numerical simulations to find approximate trajectories of particles in an external field which can be locally approximated by a straight uniform one, but also the collective effect of an ensemble of such particles as a part of a...
The largest normalized energy confinement attained over a confinement time on NSTX (H98 > 1.5) was achieved in the ELM-free Enhanced Pedestal (EP) H-mode regime [1-3] that features a wide pedestal with a significant increase in the edge carbon temperature and rotation gradients. These discharges achieve improved energy and momentum confinement with a beneficial decrease in the impurity...
The PROTO-SPHERA experiment is an unconventional magnetic confinement scheme,
which aims at producing
The neutral beam injection (NBI) system commissioning and first beam heating experiments in Versatile Experiment Spherical Torus (VEST) have been carried out. The ion beam power of ~0.6MW was successfully extracted from the ion source through commissioning. The ion source consists of arc plasma source and multi-aperture accelerator assembly which is developed by Korea Atomic Energy Research...
The low aspect ratio spherical tokamak MEDUSA (Madison Educational Small Aspect Ratio Tokamak) built by the University of Wisconsin-Madison (USA) and donated to $\textit{Instituto Tecnológico de Costa Rica}$ is currently being re-commissioned at $\textit{PlasmaTEC}$. The main characteristics of this device (renamed MEDUSA-CR) are: plasma major radius $R_0$ < 0.14 m, plasma minor radius a <...
The electron Bernstein wave (EBW) is a promising method for heating and current drive in overdense plasmas. To evaluate spontaneous conditions for EBW heating on SUNIST spherical tokamak, EBW emissions in the frequency of 3 to 6 GHz, corresponding to the fundamental and second harmonic in low toroidal field (0.1 T) discharge, were measured and analyzed. Emissions were collected by an in-vacuum...
Recent research on MHD instabilities in VEST (Versatile Experiment Spherical Torus) has focused on the tearing mode during current ramp up phase and the so-called IRE (Internal Reconnection Event) known to occur frequently in spherical torus. The tearing mode at the current ramp-up phase, known to be related to the hollow current density profile with fast ramp-up rate, seems to be suppressed...
The QUEST ECH solenoid-free start-up experiment utilizing the 28 GHz gyrotron at 2nd harmonic frequency has demonstrated remarkable efficiency and achieved record start-up current values [1]. The experiment provides rich opportunities to understand and optimize ECH-based tokamak/ST current start-up and ramp-up concept. Another potentially noteworthy aspect of the QUEST 28 GHz experiment is its...
Plasma current (Ip) start-up and ramp-up are being studied on the TST-2 spherical tokamak (R = 0.36 m, a = 0.23 m, Bt = 0.3 T, Ip = 0.1 MA). Both inductive and non-inductive methods are used. Up to 400 kW of RF power at 200 MHz, in the lower hybrid (LH) frequency range, is available. An innovative antenna called the capacitively-coupled combline (CCC) antenna was developed to excite a...
The eight major scope items in the NSTX-U Recovery Project include: (1) six redesigned inner PF coils, (2) redesigned upper and lower polar region structures, (3) redesigned select plasma facing components, (4) improved bake-out, (5) additional component stress/strain trending instrumentation, (6) enhanced test cell shielding, (7) implementation of the accelerator safety order, and (8)...
Helicity injection is a non-inductive current drive method used in the startup of Spherical Torus and Spheromak. The mechanism of current drive is explained by Taylor relaxation by which the plasma will relax into the Taylor state. The most important feature of Taylor state is that $\lambda\equiv{j\cdot B}/{\vert B^2\vert}$ is constant all over the plasma. However, the research of how the...
