Initial Vacuum Magnetic Field Mapping in the Compact Toroidal Hybrid

The Compact Toroidal Hybrid (CTH) is a low aspect ratio (R/a_plasma ≥ 3.5, R₀ = 0.75 m, a_vessel = 0.29 m, B ≤ 0.6 T) torsatron with a highly flexible vacuum magnetic field configuration designed for current-driven instability studies. Vacuum magnetic field mapping of the completed configuration is performed using a movable electron gun and phosphor-coated screen. These experiments compare the actual magnetic configuration with the design, verify the range of accessible magnetic configurations, and identify vacuum field errors. The main helical field is produced by a continuously-wound helical coil, and the vacuum rotational transform is varied with a set of toroidal field coils. Four independent poloidal field coil sets provide equilibrium control and shaping, and are also used for ohmic current drive.     

Proto-CIRCUS tilted-coil tokamak–torsatron hybrid: Design and construction

We present the field-line modeling, design, and construction of a prototype circular-coil tokamak–torsatron hybrid called Proto-CIRCUS. The device has a major radius R = 16 cm and minor radius a < 5 cm. The six “toroidal field” coils are planar as in a tokamak, but they are tilted. This, combined with induced or driven plasma current, is expected to generate rotational transform, as seen in field-line tracing and equilibrium calculations. The device is expected to operate at lower plasma current than a tokamak of comparable size and magnetic field, which might have interesting implications for disruptions and steady-state operation. Additionally, the toroidal magnetic ripple is less pronounced than in an equivalent tokamak in which the coils are not tilted. The tilted coils are interlocked, resulting in a relatively low aspect ratio, and can be moved, both radially and in tilt angle, between discharges. This capability will be exploited for detailed comparisons between calculations and field-line mapping measurements. Such comparisons will reveal whether this relatively simple concept can generate the expected rotational transform.     

Contributions of the “Saturn” to Modern Stellarator-Torsatron Research

It has been 40 years since the startup and first plasma from the Saturn stellarator-torsatron. The l = 3 stellarator-torsatron “Saturn” was the first of a series of torsatron-like devices built in Kharkov Institute of Physics and Technology (KIPT) in the 1970s: later appeared “Vint-20”, “U-3”, “U-3M”. The Saturn device was used for comparative investigations on the same device two magnetic configurations in stellarator and torsatron regimes and for experimental examination of their effects on plasma confinement. The thorough measurements of magnetic structure in both regimes demonstrated their high equivalence. Investigations of torsatron without toroidal coils supported the principal possibility to have a spatial divertor configuration, which was later realized in U-3, the first torsatron with a divertor. The results on Saturn, obtained for the first time with a pure torsatron configuration, have opened the prospect for torsatrons to be an alternative to tokamaks in the development of a fusion reactor. After various magnetic configurations were studied, the Saturn device was used for providing investigations of confinement of injected hydrogen plasma and of ECR plasma in different gases. In this paper we summarize the main Saturn results and try to find the bridge between them and the present experiments on existing stellarator-type fusion devices with an aim to see what particular Saturn results were supported by those obtained later.     

Anisotropic rearrangement of the substrate atoms during Ar bombardment on Pd(0 0 1) surface

Using a three-dimensional molecular dynamics (MD) simulation, we investigated the atomic scale rearrangement that occurs on a Pd(0 0 1) surface after energetic bombardment by Ar at room temperature. High energy Ar bombardment provoked the significant rearrangement of Pd atoms in a ballistic manner with a fourfold symmetric lateral distribution aligned along the 〈1 1 0〉 direction. The MD simulation of uniform Ar bombardment at normal incidence on a Pd surface reproduced the experimentally observed fourfold symmetric nano-scale surface structure. The present result supports that the ballistic rearrangement of the substrate atoms plays an important role in the ion induced surface structure evolution.     

Oxygen deficiency and excess of rutile titania (1 1 0) surfaces analyzed by ion scattering coupled with elastic recoil detection

Oxygen deficiency and excess of rutile titania (TiO₂) surfaces are important factors for catalytic activities of metal nano-particles on the TiO₂ supports. Medium energy ion scattering (MEIS; 80 keV He⁺) coupled with elastic recoil detection analysis (ERD; 150 keV Ne⁺) can determine the numbers of bridging O (O_br) vacancies (V_O) and excess O atoms adsorbed on the 5-fold Ti rows of TiO₂(1 1 0) surfaces. The amounts of V_O and adsorbed O were derived by H₂O and ¹⁸O₂ exposure followed by ERD and MEIS analyses, respectively. The present analysis revealed that only about a half of V_O are filled and a comparable amount of O atoms are adsorbed on the reduced TiO₂(1 1 0) surface after exposure to O₂ (1000 L; 1 L = 1 × 10⁻⁶ Torr s) at room temperature (RT). We also detected the adsorbed O for the hydroxylated TiO₂(1 1 0) after ¹⁸O₂ exposure at RT. Finally, it is shown that the O adsorbed on the Ti rows reacts with CO probably to form CO₂ at RT. Based on the results obtained here, we clarify the reason why only a half of V_O are filled by exposing reduced surface to O₂ at RT and what is the primary source of subsurface excess electronic charge, which acts as a leading part of the surface electrochemistry and gives the defect state in the band gap seen in the valence band spectra for reduced and hydroxylated TiO₂(1 1 0) surfaces.     

“Ultimate” Configurations of the L = 1, m = 13 Torsatron and the “VINT-20” Torsatron as a Possibility of Creating a Tandem Stellarator–Mirror Trap Hybrid System

The paper is concerned with numerical investigation of various magnetic configurations of a single-pole toroidal L = 1, m = 13 “ultimate” torsatron having a low aspect ratio A _h = 4.345 and the modulation factors (?0.23) ≤ α ≤ 0.71 (K = 1, β = 0) of the helix law ?mφ = θ?αKSinθ?βKSin2θ. The extremums of the basic properties of closed magnetic surfaces as functions of the coefficient α in the neighborhood of α ≈ 0.4, as well as the structures of the edge magnetic field (substructures of virtual current) and the separatrix have been first determined for the L = 1, m = 13 “ultimate” torsatrons and the “Vint-20” torsatron. Unusual positions of the separatrix X-points in the cross sections for 1/4- and 3/4- magnetic field periods have been identified. The formula that takes into account the modulation factor α of the helix law was first derived to determine the average vertical magnetic field on the geometrical axis of the torus.     

Effects of ferromagnetic components on energetic ion confinement in ITER

By using a fully three dimensional magnetic field orbit-following Monte-Carlo code, the energetic ion confinement was investigated for the current conceptual design of the ferromagnetic components in ITER which will be employed for reducing the toroidal magnetic field (TF) ripple. The ferromagnetic insert is effective in the reference standard scenario with Q = 10 (Scenario No. 2) and steady state scenario with Q = 5 (Scenario No. 4) to improve the energetic ion confinement. Over-compensation appears at half of the full toroidal magnetic field and its effect becomes stronger when the quantity of the ferromagnetic insert is increased in order to more reduce the TF ripple at the full toroidal magnetic field. Though the current design is acceptable, whether to increase the ferromagnetic insert to achieve lower TF ripple amplitude at the full field operation depends on how prospected are possibilities of lower field operations. Planned test blanket modules do not induce large loss (<1%) at the full field in Scenario No. 4. At the half field, however, the loss reaches ∼10% for the alpha particles due to localized large TF ripple.     

Engineering design for the HL-2M tokamak components

The HL-2A tokamak will be modified into HL-2M. The Bt at the plasma center (major radius R = 1.78 m) is 2.2 T, the minor radius is 0.65 m. The plasma current I_P of HL-2M will reach up to 2.5 MA, the elongation and triangularity is more than 1.8 and more than 0.5, respectively. The vacuum vessel torus consists of 20 sectors with “D” shaped cross-section and double wall structure. 20 toroidal field coil bundles comprise 140 turns which are designed with demountable joints, the poloidal field coils system consists of 25 coils. The engineering design and calculation for field coil system, vacuum vessel, support structure, etc. are finished, many key issues for manufacture process have been discussed with industry and the fabrication of main components of HL-2M tokamak will be carried out in factories.     

A modular commercial tokamak reactor with day long pulses

We discuss the design features of a commercial tokamak reactor in which day long pulses are provided by the volt-second capability of the ohmic heating transformer. Illustrative parameters are a major radius of 9.7 m, a minor radius of 1.95 m, an average toroidal beta of 0.036, a magnetic field on axis of 6.1 T, a neutron wall loading of 2.3 MW/m² and a thermal power level of 4000 MW. The tokamak is modularized into units which consist of two toroidal field coils, blanket and shield and first wall. The removal of the two toroidal field coils associated with a module would be carried out without warming up the rest of the toroidal field coil set.     

Revisiting the poloidal magnetic confinement

This note proposes a closed poloidal magnetic configuration with an in-vessel coil system held by shielded supports. A dipole field is bounded by external coils and constrained into a hollow torus aiming at uniform intensity. In the horizontal mid-plane region the external coils and the dipole outer coils are broken in four arcs and bridged by couple of straight branches. Arcs and straight branches build a set of four side coils. In the clearance between their straight branches four tunnels in the poloidal magnetic field are achieved, to pass the supports and the feeders of the in-vessel coil system.A poloidal machine with a plasma thick as those of present large experiments is outlined. The dipole radius is 5.4 m, the plasma about it has a constant poloidal cross-section about 40 m², a volume about 1300 m³ and a minimum thickness 1 m in the outboard. The magnetic field ranges from 1.4 to 1.8 T.     

Tokamak field error measurements with an electron beam in KSTAR

It is possible to detect the presence of small field errors in a tokamak with an electron beam. This was demonstrated earlier on T-15 and TEXTOR. This paper discusses the concept, past experience on these tokamaks, calculations for the Korea Superconducting Tokamak Advanced Research (KSTAR) device, an electron beam source, measurement devices for these measurements, and some results. It is shown that small toroidally averaged field errors can be detected by this method. A low voltage electron beam (e-beam) gun and fluorescent screen were mounted in a vertical port and inserted into the vacuum vessel at the end of the KSTAR 2nd campaign plasma experiments. A camera with a narrow field of view was mounted in midplane port in a tube tangent to the field lines at R ∼ 1.3 m and photographed the beam striking the screen. The poloidal field (PF) currents were held constant during the camera exposure period. Many shots with various PF coils energized were made and the deflections of the e-beam were measured. The measurements were made with a camera integration time of 300 ms because of the low light intensity. The results show that there are large field errors that diminish as the PF currents are raised. There appears to be no significant up-down asymmetry for static fields. Measurements with a 7 PF coil scenario with a calculated field null located at e-beam radial position show much larger fields than calculated. KSTAR was constructed with Incoloy 908 conduit using cable-in-conduit conductors (CICC) in 10 of the 14 PF coils and all 16 of the toroidal field (TF) coils. Incoloy 908 has a relative magnetic permeability, μ, of about 10. The field errors appear to be largely due to Incoloy 908.     

The design and the manufacturing process of the superconducting toroidal field magnet system for EAST device

The toroidal field (TF) magnet system of EAST (HT-7U), which consists of 16 superconducting coils enclosed in steel cases, has been manufactured to generate the magnetic field of 3.5 T at the plasma center to maintain plasma in a tokamak configuration with a current up to 1 MA. The TF coils have an approximately D-shape geometry of 2.6 m wide and 4.0 m high, and operate at a maximum field of 5.8 T. The conductor used in the TF coil is NbTi/Cu cable-in conduit (CIC) conductor, and its operating current is 14.3 kA.In March 2006, the first cooling down of the EAST device has been carried out successfully. The total of TF magnet system has been cooled down from room temperature to 4.5 K, and the TF system has been energized up to 8.2 kA with 5 A/s ramp rate. In September 2006, full performances of the TF magnet system have been reached, and the device of EAST has delivered its first plasma. In addition, the TF magnet system has been routinely operated with a current maintained constant on a whole day basis, for a preliminary program of more than 500 shots.In this paper, the main parts of the design, developmental tests, and the fabrication and assembly of TF coils are described in detail.     

Research plan for divertor simulation making use of a large tandem mirror device

We are planning to start a study of divertor simulation under the closely resemble to actual fusion plasma environment making use of the advantage of open magnetic field configuration and to contribute the solution for realizing the divertor in ITER as a future research plan of Plasma Research Center of the University of Tsukuba. In the research plan, the concepts of two divertor devices are introduced. One has an axi-symmetric divertor configuration with the separatrix which is similar to toroidal divertor of torus systems and the other is a high heat flux divertor simulator by using an end-mirror exit of the existing tandem mirror device. Development of magnetic field configuration for ensuring the MHD stability is under way and a designed example is investigated under the optimal condition for plasma production. Consideration of plasma heating scheme using Fokker-Planck simulation code was successfully performed at both axi-symmetric divertor and end-mirror regions. Preparative experiments using calorimeter, Mach probe and high-speed camera have been started at the end-mirror region and the heat flux density of the level in 1-10 MW m⁻² was achieved in standard hot-ion mode plasma-confining experiments, which gives a clear prospect of generating the required heat flux density for divertor studies.     

A Systematic study of modular coil characteristics for 2-field periods quasi-axisymmetric stellarator QAS-LA

Modular coil characteristics of a 2-field periods quasi-axisymmetric stellarator QAS-LA configuration with an aspect ratio A_p = 3, magnetic pressure ∼4% and rotational transform ι ∼ 0.15 per field period supplied by its own shaping have been detailed studied. In addition, the characteristics of modular coils for QAS-LA were compared with those of an intermediate QA configuration QAS-LAx and a tokamak based on the same center magnet field B₀, aspect ratio and number of coils. As expected, the B_max/B₀, force F and overturning moment M, increase with the increased complexity of the coil shape. The relationships between the modular coils’ parameters (such as radius curvature ρ, distance from coil to coil Δ_c–c and the cross-section of coils) and the electromagnetic characteristics have been systematically summarized. The approximate formula for the maximum magnetic field in the coil body as functions of modular coil parameters (Δ_c–c, ρ) was derived for a simple two wire system which will be useful when optimizations of coil properties are called for.     

Alkali ion scattering from Ag(0 0 1) and Ag thin films at low and hyperthermal energies

We have investigated the scattering of K⁺ and Cs⁺ ions from a single crystal Ag(0 0 1) surface and from a Ag-Si(1 0 0) Schottky diode structure. For the K⁺ ions, incident energies of 25 eV to 1 keV were used to obtain energy-resolved spectra of scattered ions at θ_i = θ_f = 45°. These results are compared to the classical trajectory simulation safari and show features indicative of light atom-surface scattering where sequential binary collisions can describe the observed energy loss spectra. Energy-resolved spectra obtained for Cs⁺ ions at incident energies of 75 eV and 200 eV also show features consistent with binary collisions. However, for this heavy atom-surface scattering system, the dominant trajectory type involves at least two surface atoms, as large angular deflections are not classically allowed for any single scattering event. In addition, a significant deviation from the classical double-collision prediction is observed for incident energies around 100 eV, and molecular dynamics studies are proposed to investigate the role of collective lattice effects. Data are also presented for the scattering of K⁺ ions from a Schottky diode structure, which is a prototype device for the development of active targets to probe energy loss at a surface.     

Primary research of heat flux deposition pattern on the surface of HT-7 movable limiter

This paper presents a method of characterizing the heat flux deposition pattern on HT-7 movable limiter, a new component in the spring experimental campaign of year 2006. A new modeling establishment combines with heat transfer simulation with ANSYS code, and a shot of long pulse discharge is chosen with small radius 265 mm of movable limiter, which is 5 mm smaller than that of the main toroidal limiters. Both parallel and perpendicular heat flux are taken into account and different ratios of them are also tested in simulations. The simulation temperature values match well with the results of the IR-camera. Temperature distribution shows that ?_||/?_⊥ = 5 is quite suitable in HT-7 device, and the maximum heat flux is about 10 MW/m² and average heat flux is about 5 MW/m² on the movable limiter. This method can be also used in other tokamak devices with limiter configuration and circular cross-section shape.     

Grazing scattering of 1-2 MeV HeH ions from KCl(0 0 1): Effect of surface track potential

Three dimensional (3D) distributions (energy E, scattering angle θ and azimuth angle φ) of the fragment protons dissociated from HeH⁺ during grazing angle scattering from a KCl(0 0 1) are measured using a magnetic spectrometer in order to study the effect of the surface track potential. The distributions of the fragment protons scattered from a SnTe(0 0 1) are also measured as a reference. Although the observed distributions for KCl(0 0 1) and SnTe(0 0 1) are basically the same, there is small differences, especially in the scattering angle distribution. While the fragment protons are scattered at the specular angle from SnTe(0 0 1), the protons are scattered at slightly larger angles from KCl(0 0 1). The observed angular shift is more pronounced for the trailing protons than the leading protons. It is also found that the angular shift increases with decreasing ion energy. The observed angular shift can be qualitatively explained by the surface track potential induced by the partner He ions using a simple model of the surface track potential.     

Heliotron E Results and Large Helical Project

Heliotron E(H-E) experiment was started in 1980. Until 1987 high power heating experiments for improving plasma parameters have almost finished. H-E firstly demonstrated that ECR heated plasmas are usable for target plasmas of NBI or ICRF heating to obtain high density and high temperature currentless plasmas. The highest electron temperature is 1.5keV and ion temperature is 1.6keV and both are realized in the low density regime of <n> (average density) ≤10¹³cm^?3.

H-E also showed that the currentless plasmas have no major disruption and quasi-steady plasmas are confined with controlling impurity ions by titanium gettering and carbon coating.

H-E also obtained <β> (average β) –2%, which is the highest value realized in helical systems, with <n–8×l0¹³cm^?3 and Te(0)–T_i(0)–350 eV at B₀ (magnetic field at the magnetic axis) =0.94 T. In the high β experiments pressure-driven instabilities were observed for peaked pressure profiles and sometimes relaxation oscillations similar to the tokamak internal disruptions were observed.

In the ECRH plasmas neoclassical transport is dominant in the region inside the half radius. However, global confinement time τ_E follows the scaling law τ_E ∝<n>^0.66P_heat ^?0.53 which is different from the neoclassical scaling law. Here P_heat denotes the net heating power.

Based on the H-E results, a new large helical system design study has started in 1986. The plasma parameters entering the regime of <n>τ_E<T> (2–3)× 10¹⁹m^?3?S?keV is investigated, which is about one tenth of fusion plasma condition. From the transport code studies and empirical scaling law based on the H-E results, R=(4×5)m, ā=(50–60)cm and B_o=4T are required to satisfy the above condition with P_heat=20MW. The design study to fix the magnetic field configuration is progressing. Expected one is l=2 and m=10 with additional poloidal coils, where m is a toroidal period number. The magnetic field is produced by superconducting coil and long pulse operation will be tested, if continuous heating is available.     

Effect of adsorbates on the formation of doubly excited He atoms during impact of He ions on a Ni(1 1 0) surface

The formation of doubly excited states of He atoms during impact of He²⁺ ions with projectile energies of 60-1000 eV under near-grazing angles of incidence of 5°-20° on clean and adsorbate-covered Ni(1 1 0) surfaces is studied by means of Auger electron spectroscopy. Pronounced dependencies of electron spectra from autoionization of atoms in doubly excited 2s², 2s2p and 2p² configurations on the coverage of the target surface with adsorbates are observed. These are directly related to work function changes, which are studied for the controlled adsorption of oxygen. Changes of the electron spectra on the target temperature are found for adsorbate-covered surfaces only, which puts into question recent interpretations of similar electron spectra in terms of a high local electron spin polarization of Ni(1 1 0) by an alternative interpretation based on thermal desorption or dissolution into bulk of surface contaminations. The formation of doubly excited states is studied for the oxygen p(2 × l) and p(3 × l) superstructures on Ni(1 1 0) in order to provide well-defined experimental data for theoretical investigations.     

Possibility of pure thermal sensitization in the pre-dose mechanism of the 110 °C TL peak of quartz

The role of thermal activation in the pre-dose sensitization of the 110 °C TL glow peak of quartz has been hypothesized to transfer holes from reservoir centre, R, to luminescence centre, L. But, the thermal treatment has been reported to sensitize this peak, also, in an independent mode. So, a comparative analysis of the impact of thermal and pre-dose treatment on the TL properties of the 110 °C signal, observed by various workers, has been undertaken in this paper. Also experiments were carried out to observe thermal sensitization component at the temperatures which are used in the pre-dose sensitization measurements. The study seems to support the existence of an inherent pure thermal sensitization component in the pre-dose sensitization mechanism.