B.L. Stansfield

Gas-phase synthesis of SWNT by an atmospheric pressure plasma jet

We present here a new method for producing single wall carbon nanotubes (SWNT) based on the atomization of a gaseous mixture (composed of argon, ferrocene and ethylene) in an atmospheric plasma jet. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy were performed to study the samples obtained. They contain SWNT with diameters and structure comparable to those produced by laser ablation and arc discharge techniques. Since this method is continuous and easily scalable, we feel it has potential for large-scale commercial production of SWNT.

Growth of carbon nanotubes on Ohmically heated carbon paper

Abstract Multi-walled carbon nanotubes (MWCNTs) have been grown from ethylene on the fibers of carbon paper by Ohmically heating Co–Ni or Fe catalytic sites supported on the carbon fibers. MWCNTs are mainly tip-grown and have diameters ranging from 10 to 30 nm, depending on the catalyst used. The MWCNTs adhere firmly to the carbon paper with which they are in electrical contact. Cyclic voltammetry of the electrode thus formed indicates that MWCNTs behave like a graphitic material.

Growth of carbon nanotubes on carbon paper by Ohmically heating silane-dispersed catalytic sites

Abstract A large density of multiwall carbon nanotubes (MWCNTs) has been grown on the fibers of a carbon paper using a sulfonated silane intermediary to uniformly disperse (1:1) Ni–Co catalytic sites used in the decomposition of ethylene. The nanotubes were obtained at 800 °C by Ohmically heating the carbon paper support. The MWCNTs are typically up to 20 μm in length and between 30 and 50 nm in diameter. These tubes are easily opened at room temperature, while remaining firmly attached to the carbon paper.

Experimental demonstration of tokamak fuelling by compact toroid injection

The most promising concept for deep fuelling a reactor is by the injection of compact toroid (CT) plasmoids. The first results showing CT fuelling of a tokamak plasma, without any adverse perturbation to the tokamak discharge, are reported. The Compact Toroid Fueller (CTF) device was used to inject a CT-spheromak plasmoid into the TdeV tokamak. Following the CT penetration, the tokamak particle inventory increased by 16%, the loop voltage and the plasma current did not change, and there was no increase in magnetohydrodynamic (MHD) activity. The number of injected impurities was low and dominated by non-metallic elements. The plasma diamagnetic energy and the energy confinement time increased by more than 35%

Biasing of closed double null poloidal divertor plates on TdeV

Abstract Bias voltages applied to the divertor plates in TdeV (Tokamak de Varennes) permit fine control of several main plasma parameters, including the poloidal rotation velocity near the separatrix, microturbulence, the sawtooth period and the heat pulse propagation speed. Biasing also improves the divertor efficiency in either the top or bottom closed divertor chamber depending on polarity, in agreement with E × B drifts. Negative biasing reduces carbon and CO fluxes from the wall, the loop voltage and the X-ray emissivity, all indicative of decreasing impurity contamination.

Various divertor biasing configurations and improved divertor performance with biasing on Tokamak de Varennes (TdeV)

Electrically insulated divertor plates are used on TdeV (Tokamak de Varennes) [18th EPS Conference on Controlled Fusion and Plasma Physics Berlin (European Physical Society, Petit‐Lancy, 1991), Vol. 15C, Part I, pp. 1–141] to produce various biasing configurations, which can be decomposed into two basic modes. Plasma biasing, with a radial electric field Er in the scrape‐off layer (SOL), is most promising for divertor applications. The Er field is produced with a particular divertor plate geometry, causing a nonambipolar radial current and a particle flow in the Er×BT direction, toward one of the divertors (the active divertor). The pressure and impurity retention in the active divertor are shown, in the Ohmic regime, to be strongly increased by biasing. He exhaust through this divertor is increased by a factor of almost 3 with modest biasing voltages and currents scalable to larger devices. Biasing also modifies the power repartition between the divertors, with the active divertor also receiving a larger...

Effects of divertor plate biasing on radial and poloidal edge fluxes in the TdeV

The divertor plates of TdeV, a tokamak with a double-null divertor and closed divertor chambers, have been electrically biased with respect to the walls. The authors discuss the resulting effects on the edge electron density profile, on the neutral pressures and impurity fluxes in the main vacuum chamber and the divertor chambers, and on the plasma flow to the divertors. As a function of the bias voltage, which was varied between-180 V and +160 V, the electron density scrape-off width and the wall impurity influxes increase monotonically; the flows to the top and bottom divertors vary strongly, in qualitative agreement with an E*B/B2 rotation, but not symmetrically. With negative biasing, the electrostatic barrier and the rotation combine to give a strong improvement of the divertor efficiency

Microturbulence measurements during the divertor biasing

The application of a bias voltage to a neutralization plate of the upper divertor with respect to the vacuum chamber in the Tokamak de Varennes (TdeV) influences the plasma well inside the separatrix. In particular, the unbiased Ohmic poloidal rotation edge velocity measured by visible spectroscopy is found to be in the electron diamagnetic drift direction (2-3 km/s) and increases by a factor of two for Vbias = 100 V. This coincides with a major reduction of the microturbulence signal at low frequencies (50 kHz < f < 1 MHz) and for all wave numbers probed (20 cm-1 < k < 60 cm-1), as determined from coherent laser scattering measurements. One possible explanation is that the turbulence signal is simply Doppler shifted to frequencies outside the accessible range. This scenario is, however, difficult to reconcile with some observations. Another explanation invokes a reduction of the turbulence level. The variation of the turbulence signal as a function of the applied bias voltage can indeed be reproduced with a theoretical model based on radial and poloidal decorrelation mechanisms, the latter corresponding to poloidal velocity shear stabilization. This model also explains the steepening of the k-spectrum decay during biasing. Biasing also modifies the electron density profile inside the separatrix. These changes of ∇ne cannot explain the behaviour of microturbulence in terms of their effect on the free energy term driving the instability. On the contrary, the observed turbulence behavior, when explained in terms of stabilization, would agree with the plasma maintaining a steeper electron density gradient

Measurement of flow in the scrape-off layer of TdeV

Two techniques are used to monitor the flow in the scrape-off layer of Tokamak de Varennes (TdeV); one is based on a new multipin Langmuir/Mach probe called Gundestrup while the other depends on the measurement of the upstream/downstream asymmetry of the power absorbed by a test limiter inserted into the plasma edge. Gundestrup has been used to measure the components of velocity parallel and perpendicular to the magnetic field as a function of the radial electric field. Both components vary linearly with the radial electric field and inversely as the magnetic field ( U ‖ ∝ E r / B θ and U ⊥ ∝ E r /B ). The pattern of power deposition on the test limiter implies that the flow is in the same direction as that measured by Gundestrup and the e-folding length for the power deposition is in agreement with Gundestrup measurements of temperature and density. The test limiter observations indicate that the flow reverses just inside the separatrix.

Measurement of pre-sheath flow velocities by laser-induced fluorescence

Abstract For the first time, the pre-sheath ion flow velocity has been measured using the Doppler shift of laser-induced fluorescence in singly-ionized argon ions. The velocity shows a monotonic increase, from a value of about 0.15 of the sound speed VS far from the target to 0.5 of Vs at a distance of 5 mm from the surface. The temperature, the floating potential and the density are calculated from cylindrical probe measurements taken in the same region under identical conditions. These experimental results are compared with those from a 1D isothermal single-ion fluid model of the pre-sheath and a kinetic electron/fluid ion model. Both models agree well with the density profile, but underestimate the potential change and overestimate the velocity. In addition, the bulk flow velocity has been independently determined from “Mach probe” measurements, using various candidate theories to relate the Mach number to the ratio of the upstream to downstream saturation currents. Comparison with the optical measurements indicate that the probe models which include viscosity provide reasonable agreement with our Mach probe data.