Particle Confinement Time Research Articles

Magnetic Fusion Basics

At the core of the sun and stars, light nuclei combine - or fuse - to create heavier nuclei. This process releases a significant quantity of energy and is the source of the heat and light that we receive. Harnessing this type of reaction on earth for the purpose of generating energy would open the way to almost unlimited resources. This is the aim of fusion research undertaken by the leading industrial nations.The main principles for mastering fusion energy on earth will be described in this paper. After a reminder of the main fusion reactions and the conditions to obtain fusion, this document will focus on the magnetic fusion based concepts. The magnetic confinement principles (particle trajectories, confinement time, Q factor, particles and heat transport, confinement methods, stability limits …) will be exposed with a special emphasis on the Tokamak configuration. The role of the main components of such device will be described: magnetic system, plasma facing components including divertor, fuel injection, heating systems, diagnostics …Methods for creating, heating and mastering plasma inside a tokamak will be explained and specifi cities due to long plasma duration capabilities will be pointed out (plasma wall interactions…).

Fast gas injection as a diagnostic technique for particle confinement time measurements

The determination of the effective particle confinement time (τp*), i.e., the particle confinement time normalized to recycling coefficient, is difficult when its value is long compared to the discharge duration in magnetically confined plasmas. Recent experiments on the current drive experiment upgrade (CDX-U) spherical torus have successfully achieved a significant reduction in recycling with large-area liquid lithium plasma-facing surfaces. The low recycling walls result in an increase in particle pumping and make it possible to measure τp* in short duration plasmas. Measurements of τp* are made using a supersonic gas injector which is closely coupled to plasma. A fast gas pulse is emitted from the supersonic gas injector, after which the density decay is measured using a microwave interferometer. The design of the supersonic gas injector and its configuration on CDX-U will be presented. The results of this technique will be shown as applied to the study of the effects of a liquid lithium toroidal limiter and evaporative lithium coatings on overall plasma density and τp*.

Experimental Confirmation of Stable, Small-Debye-Length, Pure-Electron-Plasma Equilibria in a Stellarator

The creation of the first small-Debye length, low temperature pure electron plasmas in a stellarator is reported. A confinement time of 20 ms has been measured. The long confinement time implies the existence of macroscopically stable equilibria and that the single particle orbits are well confined despite the lack of quasisymmetry in the device, the Columbia non-neutral torus. This confirms the beneficial confinement effects of strong electric fields and the resulting rapid E x B rotation of the electrons. The particle confinement time is presently limited by the presence of bulk insulating materials in the plasma, rather than any intrinsic plasma transport processes. A nearly flat temperature profile is seen in the inner part of the plasma.

Study of confinement improvement in presence of a MARFE in the HT-7 superconducting tokamak

A new set of actively cooled toroidal double-ring graphite limiters have been developed on the HT-7 tokamk for long pulse operation. In this paper, a Multifaceted asymmetric radiation from the edge (MARFE) phenomena and improved particle confinement induced by a MARFE, characterized by Ha line emissions drops and the line-averaged density increase have been studied in the HT-7 ohmic discharges (where the plasma current Ip=145 kA, loop voltage Vloop =2-3 V, toroidal field BT =1.7 T, and Zeff =2-8). It is found that the improved particle confinement phase exists for about 90 ms and the particle confinement time tP increased about 1.6 times.

Growth of thin hydrocarbon films and hydrogen production in a cusp plasma device

A plasma device with a cusp magnetic field configuration and operating under steady state condition has been built to investigate surface deposition of hydrocarbon films at low temperature and, simultaneously, the cracking of methane in a plasma environment to hydrogen production. The gas (methane or a methane/argon mixture) has been fed to the plasma source (a cylindrical capacitively coupled rf type). The addition of a static magnetic field has increased the particle confinement time and, consequently, the hydrogen production rate. Here the preliminary results concerning the growth of thin hydrocarbon films and the dissociation of methane in the cusp plasma are presented. In our case, at the line and point cusp, we have a large flux of energetic particles as well as neutral, ions, thermal radicals that participate in the growth process. A fraction of the converted methane has been deposited on the substrates, at the line cusp, as amorphous hydrocarbon (a-C: H) film and amorphous carbon (a-C) film. The surface morphology of the film has been observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) has been used to determine the structure of the film. The results have indicated that the film has a prevalently amorphous structure with a presence of crystalline clusters.

Lower hybrid current drive experiments with graphite limiters in the HT-7 superconducting tokamak

Recent progress of lower hybrid (LH) experiments with new graphite limiters configuration in the HT-7 tokamak is presented. The lower hybrid current drive (LHCD) efficiency can be determined by fitting based on experimental data. Improved particle confinement was observed via LHCD ( P LHW > 300 kW ) characterized by the particle confinement time τ p increased about 1.56 times. It is found that runaways are suppressed during loop voltage is decreasing at the flat-top phase of LH discharges. The main limitations of pulse length are presented in long-pulse experiments with new limiter configuration.

LHCD Properties with a New Lower Hybrid Wave Antenna on HT-7 Tokamak

The structure and the characteristic power spectrum of a new lower hybrid wave antenna on the HT-7 tokamak are briefly described in this paper. The main experimental properties with the new antenna have been demonstrated by showing the dependence of current drive efficiency and particle confinement time on phase difference between adjacent waveguides of the antenna. A few preliminary analyses about the experimental results are also discussed in the paper.

Wobble motion on field-reversed configuration plasmas

A wobble motion on a field-reversed configuration plasma is investigated in detail. It is found from magnetic and optical measurements of the wobble motion that a few magnetic islands appear inside a separatrix at the formation phase and merge with each other at the axial contraction phase. Due to the radial and azimuthal asymmetries of the merging plasmas, the wobble motion is triggered. To assist the symmetrical formation of the plasma, conducting rings are installed near the ends of the theta-pinch coil. Using the effect of the conducting rings, the amplitude of the wobble motion is controlled to approximately one-half that without using the conducting rings and the extreme degradation of particle confinement time is not observed. An analysis of the plasma affected by a restoring force of open field lines well explains an experimental result that the period of wobble motion depends on the square root of mass density.

Matching to ELMy plasmas in the ICRF domain

An ICRF antenna coupled to the plasma presents a load impedance different from what the generator requires for optimal power transfer. The required matching must be able to accommodate changes in coupling impedance due to varying plasma conditions. The changes occur on a timescale varying between particle confinement time and MHD events. In ELMy plasmas, the latter can be as fast as 50 μs. Several methods are in use, but only a few can cope with this most challenging condition: hybrid couplers and conjugate-T. Hybrid couplers have been implemented on ASDEX Upgrade, Double-III-D and will be on JET, while Alcator C-mod, JET, Textor and Tore Supra have been outfitted with conjugate-T.

Particle confinement and density profile behaviour on HL-1M

In this paper the density profile behaviour and the particle confinement operation regime on HL-1M have been studied under the pellet injection (PI), supersonic molecular beam injection (SMBI), gas puffing (GP) and lower hybrid current drive experimental situations. The relationships between density profile, particle confinement time and edge safe factor have been explored. The density profile, which is measured by six-channel far-infrared ray laser interferometer has been analysed by using the peaking coefficient calculation code. Changes of the outward and inward diffusion velocities before and after the peaking of the central density profile have been calculated using the global particle balance equations. The particle confinement operation regimes have been discussed. The peaking density profile can be easily obtained under the condition of efficient fuelling. In ohmic discharges, confinement time increases as the peaking density profile factor rises and is saturated at a critical value related to the fuelling efficiency. The particle confinement time of SMBI lies between the values of GP and PI and its value is about 3–5 times of the energy confinement time.

Electron density and temperature determination using the concept of particle confinement time uniqueness

The use of atomic hydrogen line emission to determine the particle confinement time τp of a tokamak plasma is a well-known diagnostic technique. Using such a method, for any one of the emission lines, be it from Lyman, Balmer, or Paschen series, the same (i.e., unique) value of τp must be obtained. Furthermore, this measurement is directly related to the local values of the electron temperature and density. We have developed a method based on the Hα, Hβ, and Hγ hydrogen line emissions and on the concept of τp uniqueness for a tokamak plasma, to determine the local electron density and temperature. The technique has been applied to plasma discharges generated in the NOVA-UNICAMP tokamak. The results show good agreement with measurements from multichannel Thomson scattering and Langmuir probe. A procedure to simulate the Hα emissivity radial profile using the obtained results is also discussed.

Optimization of Stellarator Reactor Parameters

Four quasi-axisymmetric compact stellarator plasma and coil configurations are analyzed for their potential as reactors. A 0-D (volume-average) approach for optimizing the main reactor parameters allows study of the relationship between global parameters and the compatibility of different constraints for a given power output including plasma-coil spacing, coil-coil spacing, maximum field and coil current density, neutron wall loading, plasma beta value, etc. The result is reactor candidates with average major radii <R> in the 6-7 m range, a factor of two smaller than those of previous studies. A 1-D power balance code is used to study the ignited operating point and the effect of different plasma and confinement assumptions including density and temperature profiles, alpha particle losses, and helium particle confinement time for the different plasma and coil configurations.

DIVIMP modeling of the toroidally symmetrical injection of 13CH4 into the upper SOL of DIII-D

As part of a study of carbon–tritium co-deposition, we carried out an experiment on DIII-D involving a toroidally symmetric injection of 13CH4 at the top of a LSN discharge. A Monte Carlo code, DIVIMP-HC, which includes molecular breakup of hydrocarbons, was used to model the region near the puff. The interpretive analysis indicates a parallel flow in the SOL of M∥∼0.4 directed toward the inner divertor. The CH4 is ionized in the periphery of the SOL and so the particle confinement time, τc, is not high, only ∼5ms, and about 4X lower than if the CH4 were ionized at the separatrix. For such a wall injection location, however, approximately 60–75% of the CH4 gets ionized to C+, C2+, etc., and is efficiently transported along the SOL to the inner divertor, trapping hydrogen by co-deposition there.

Research of edge plasma structure on the HL-1M tokamak

In this paper, experimental results of plasma characteristics in several discharges have been measured in the plasma boundary region of the HL-1M tokamak using a multiarray of Mach/Langmuir 6 probes, electrostatic 4 probes and 20 group of 3 probes. In the experiments of Ohmic discharge, lower hybrid current drive, supersonic molecular beam injection, multi-shot pellet injection and neutral beam injection, etc., the correlation between the Reynolds stress and poloidal flow in the edge plasma is obtaimed. The experimental results suggest that the confinement improvement is related to the increasing rotation velocity and to the decreasing edge fluctuation. The results indicate that the sheared poloidal flow can be generated in tokamak plasma due to the radially varying Reynolds stress. The particle confinement with low-density discharge can increase 1—2 times in LHCD. The particle confinement time and poloidal rotation velocity can at least be doubled after MPI, and that the particle confinement time can increase one order of magnitude and can obtain high performance plasma during the experiment of SMBI.

Radiated Power and Impurity Concentrations in the EXTRAP-T2R Reversed-Field Pinch

A numerical and experimental study of the impurity concentration and radiation in the EXTRAP-T2R device is reported. The experimental setup consists of an 8-chord bolometer system providing the plasma radiated power and a vacuum-ultraviolet spectrometer providing information on the plasma impurity content. The plasma emissivity profile as measured by the bolometric system is peaked in the plasma centre. A one dimensional Onion Skin Collisional-Radiative model (OSCR) has been developed to compute the density and radiation distributions of the main impurities. The observed centrally peaked emissivity profile can be reproduced by OSCR simulations only if finite particle confinement time and charge-exchange processes between plasma impurities and neutral hydrogen are taken into account. The neutral hydrogen density profile is computed with a recycling code. Simulations show that recycling on metal first wall such as in EXTRAP-T2R (stainless steel vacuum vessel and molybdenum limiters) is compatible with a rather high neutral hydrogen density in the plasma centre. Assuming an impurity concentration of 10% for oxygen and 3% for carbon compared with the electron density, the OSCR calculation including lines and continuum emission reproduces about 60% of the total radiated power with a similarly centrally peaked emissivity profile. The centrally peaked emissivity profile is due to low ionisation stages and strongly radiating species in the plasma core, mainly O4+ (Be-like) and C3+ Li-like.

Boundary turbulence and transport in density scanning experiment on TH-7 tokamak

The boundary turbulence and transport in density scanning experiment were investigated using a fast reciprocating Langmuir probe in the HT-7 tokamak. With the increase of the central line averaged plasma density, an enhanced shear of the radial electric field was observed at plasma edge, which could account for the reduction of edge transport and the improvement of global particle confinement. The reduced particle transport was demonstrated by the increase of the particle confinement time and the decrease of the perpendicular diffusion coefficient. The features of the ionization-and-radiation-driven turbulence, e/Te~ñe/ne and θ−eñe~π, were found in the plasma edge region, which suggests the importance of the ionization and radiation in the turbulence driving.

Effects of a Vertical Magnetic Field on Particle Confinement in a Magnetized Plasma Torus

The particle confinement in a magnetized plasma torus with superimposed vertical magnetic field is modeled and measured experimentally. The formation of an equilibrium characterized by a parallel plasma current canceling out the grad B and curvature drifts is described using a two-fluid model. Characteristic response frequencies and relaxation rates are calculated. The predictions for the particle confinement time as a function of the vertical magnetic field are verified in a systematic experimental study on the TORPEX device, including the existence of an optimal vertical field and the anticorrelation between confinement time and density.

Evaluation of energetic particle confinement using CXNPA with NB-blip experiments on Large Helical Device

A new experimental technique to investigate the particle confinement of energetic ions during their slowing-down process is developed for charge exchange neutral particle diagnostics. The method is based on the maximum entropy and the maximum likelihood method and is used for the neutral beam short pulse experiment. The method assumes that the energy loss of the fast ions is given by the classical deceleration rate at their birth orbit. It will not require the bulk neutral distributions and the energetic ion distribution in plasmas. The method provides the spatial distribution of confinement time of energetic particles on their orbit during the slowing-down processes. The method of the analysis is described and the application to the experimental data is shown in this article.

Study of particle behaviour at the edge in HT-7 tokamak

The cross-field diffusion coefficient (D⊥) at the edge in the HT-7 tokamak is close to the Bohm value when the line average electron density ranges from 1.5×1019 to 3.0×1019m−3. The energy profile of the particles is derived directly from the Hα(Dα) line shape; the dissociative excitation of molecules is dominating when the local electron temperature is above 10eV. By means of the Monte Carlo method the Dα line shape is also simulated. We find that the molecular dissociation contributes to 57% of neutral atoms and 53% of emission intensity in front of the limiter and 85% of neutral atoms and 82% of emission intensity in front of the wall. The influence of atomic and molecular processes on the energy balance is discussed for the scrape-off layer (SOL) and the power loss from molecular dissociation is found to be 6×104kW at the SOL. The ion Bernstein wave (IBW) can effectively suppress the magnetohydrodynamic behaviour, the fluctuation levels and the turbulence; the D⊥ in front of the limiter declines from 0.84 to 0.2m2.s−1 and the particle confinement time rises from 9 to 12ms.

Enhanced Dα confinement mode: a theoretical model

It has been shown in a previous paper that the neoclassical theory of plasma rotation explains well the large toroidal velocity that is measured in the core of edge localized mode (ELM)-free ALCATOR C-Mod H-mode discharges. It has also been noted that the gradient of the toroidal/parallel velocity estimated at the pedestal inflexion point approaches the value required for the onset of the parallel velocity shear Kelvin–Helmholtz (PVS K–H) instability when the discharge is about to undergo an ELM-free to enhanced Dα (EDA) transition. The wavenumbers and frequencies of weakly unstable PVS K–H oscillations are consistent with those of the quasi-coherent mode that is observed in the EDA, but not in the ELM-free, regime—hence the suggestion, which is explored here further, that the transition is the consequence of PVS K–H instability onset. It is at first noted that the neoclassical expression for the toroidal velocity gradient warrants that large values of the safety factor, q, and of the triangularity parameter, δ, favour the transition, as observed. It is also shown that outward convection of toroidal momentum reduces the toroidal velocity gradient and, therefore, the instability growth rate. The anomalous particle flow and the particle confinement time in EDA discharges are then estimated on the assumption that anomalous transport maintains the velocity gradient at the threshold value. The power balance consideration shows that heat transport across the H-mode pedestal is neoclassical, at most. A simple expression of the ratio of particle and energy fluxes is then obtained; the ratio τP/τE of the respective confinement times is in the range of reported values (although it must be noted that it is difficult to quantify τP as it varies rapidly across the edge layer). The scenario assumes that the H-mode pedestal of EDA discharges (the unstable layer where anomalous transport occurs) is impermeable for neutrals since charged particles issuing from ionization would otherwise accumulate in the core; experimental parameters justify this assumption. The possibility of matching simultaneously the requirements for PVS K–H instability onset and non-permeability to neutrals in large devices is discussed.