Nagoya Bumpy Torus Research Articles

An Energy Limit of Hot Electrons in a Bumpy Torus

Numerical estimation shows that an upper energy limit of electrons confined in the Nagoya Bumpy Torus, NBT-1M, exists under electron-cyclotron heating (ECH). This energy limit is determined from the power balance between the microwave input and the losses due to synchrotron radiation and Coulomb drag by bulk electrons, when the maximum harmonic number of ECH corresponding to the energy limit plays an important role. The calculation result of the orbit of the particle reveals us that mirror-trapped electrons can have energies of 10 MeV, while electrons passing through linked mirrors cannot be confined if their energies are above 3 MeV for the magnetic current of 4.2 kA. Successive ECH to a higher harmonic resonance with an injected microwave accelerates electrons, especially mirror-trapped ones, to the energy limit. However, the resonant heating efficiency estimated from the wave damping decreases as the harmonic number n H increases, and also the total power loss increases as heated electrons become more ...

A heavy ion beam probe for the Madison Symmetric Torus

A heavy ion beam probe (HIBP) is being developed for application on the Madison Symmetric Torus (MST). It will be used to make measurements of the plasma space potential Φ(r), fluctuations in potential Φ̃(r), the electron density ne(r), fluctuating electron density ñe(r), and radial electric field Er(r) from the core to the edge region of the plasma in MST. While information on these quantities can and has been obtained with probes inserted in the surface region, none of the above measurements have been made in the core of a hot reversed field pinch. Measurements of Φ(r), Er(r), and ne(r) have been well established in previous HIBP systems on tokamaks such as Impurities Studies Experiment, Texas Experimental Tokamak (TEXT), and TEXT Upgrade, stellarators such as Advanced Toroidal Facility and Compact Helical System and Bumpy Tori such as Elmo Bumpy Torus and Nagoya Bumpy Torus. Less well developed in terms of HIBP measurements are equilibrium and fluctuating magnetic fields. Because the confining field on MST is determined by plasma conditions, some effort has been made in the design of the MST beam probe to make it possible to characterize B before, during, and after the plasma discharge.

Density fluctuations and confinement in the Nagoya Bumpy Torus (NBT-1M)

The stabilization of low-frequency density fluctuations is evidenced in the Nagoya Bumpy Torus (NBT-1M) [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1985), Vol. 2, p. 551] in the presence of microwave-heated hot-electron rings. Flute-type fluctuations, which are considered to be stabilized by the charge-uncovering effect of the rings, are found to cause large plasma losses, and to affect radial density profiles in the way that the lower fluctuation level yields the steeper density gradient. The particle confinement is, therefore, improved by the hot-electron rings to some extent, but is mainly determined by the plasma convection, which is expected from the discrepancy between density and potential profiles. It is also found that fluctuations in a toroidal plasma inside the ring grow when a weak negative ambipolar potential and a steep density gradient are formed, and are reduced to a low level when a deep potential well is achieved.

Bumpy torus plasma confinement (NBT)

A survey is made of experiments in different versions of the Nagoya Bumpy Torus (NBT). Hot-electron-plasmas with β-values up to 5% were routinely produced by electron-cyclotron heating. Plasma confinement studies of the toroidal plasma have revealed that the confinement times fall short of neoclassical predictions, but global stability can be achieved. A density of 1013cm−3 and an ion temperature of 100 eV have been obtained simultaneously on NBT-1M by the use of hot-electron rings in combination with ICRF heating. The electrons are still in the collisional regime.

Stability of electrostatic drift waves in bumpy tori

Nonlocal properties of electrostatic drift waves in a straight magnetic field in a collisionless and low-β plasma with a hot-electron component are discussed analytically in detail. Interesting drift wave features, such as eigenfrequency, growth rate, radial wavenumber, and position of localization, are clarified. Effects of the hot-electron component and the ambipolar potential on the stability of drift waves are also studied and related to low-frequency fluctuations measured in ELMO Bumpy Torus and Nagoya Bumpy Torus experiments. It is found that the hot-electron component has a destabilizing effect, but a strong ambipolar field has a stabilizing influence on drift waves inside the hot-electron ring.

Heavy Ion Beam Study of Negative and Positive Potential Formation in Bumpy Torus Plasma

Potential formation is studied in Nagoya Bumpy Torus (NBT-1M) by the use of a heavy ion beam probe. The concentric negative potential well and its positive rim are observed in the standard operation. The position of the rim moves with the second harmonic ECRH zone at the midplane of each mirror section. It is confirmed that the core electron heating is essential for the potential well formation. A stable positive potential is observed near T-M transition with a smooth change from the negative profile.

Heavy Ion Beam Probe for the Study of Plasma Confinement in Nagoya Bumpy Torus

The heavy ion beam probing system has been constructed for the study of plasma confinement in Nagoya Bumpy Torus (NBT-1/1M). Not only the plasma space potential but also the electron density multiplied by a function of the electron temperature nf(Te) can be measured with good spatial resolution. The feedback controlled detection technique and the time resolved fast detection technique are used, depending on the operation mode. The effects of the hot electron beta and the potential on the beam orbit are analyzed. It was confirmed that these effects on the observation points are usually small, and that the hot electron beta can be estimated from the shifts of the primary beam orbits of many cords.

I.1.4. The bumpy-torus

The Institute of Plasma Physics at Nagoya University and the College of Science and Technology at Nihon University discuss plasma confinement in the Nagoya Bumpy Torus. Empirical scaling laws for ring parameters are obtained from experiments of various mirror devices, EBT-1/S and NBT-1/M, as shown in figures. Confinement of the main toroidal core plasma is considered to be governed by neoclassical transport greatly improved by a radial electric field. Alternative approaches to reach reactor-grade Bumpy Torus plasmas are presented. Figures show the increase in ion temperature by high-power ion-cyclotron heating. Bumpy Torus concept development is discussed, along with device parameters and results of the modified Bumpy Torus.

Development of Concentric Equipotential Surfaces in Bumpy Torus Plasma

Radial profiles of the plasma space potential are measured in Nagoya Bumpy Torus (NBT-1) by the use of a heavy ion beam probe. Asymmetric potential profiles owing to toroidal drift are observed in high pressure operation (C-mode). As the pressure is decreased, toroidal plasma is effectively heated (T-mode), poloidal precessional frequency overcomes the electron collision frequency and the equipotential surfaces becomes concentric inside the hot electron ring.

Overview of EBT reactor projections

The ELMO Bumpy Torus (EBT) concept provides a unique basis for a truly steady-state fusion reactor in a favorable geometry with a significant Q engineering (> 10) value. The similarity of the dimensionless parameters of the present experiments, EBT-I/S and Nagoya Bumpy Torus, to a reactor-grade plasma and observed confinement characteristics indicates plausible extrapolations and projections for a reactor. Past and present reactor studies have indicated that EBTs have many attractive reactor features. There are, of course, a number of physics and technology issues yet to be resolved. Recent developments are reviewed, and their implications to reactor physics/technology/engineering issues are discussed.