In toroidal plasmas immersed in a toroidal field of B ϕ , the electrons drift downwards, while the ions drift upwards due to the field gradient and curvature. The electrons that drift down to the bottom would return through the conducting vessel to the top, on which they recombine with the ions, completing the current circulation. Helical field lines by the superposition of a vertical field B Z would provide another return path of internal current circulation, along which the electrons flow toroidally, generating a toroidal current. These conjectures have been examined on ECR plasmas in the Low Aspect ratio Torus Experiment device with current-collecting electrodes at the top and bottom of the vacuum chamber. Upon the blocking of external return path the discharges terminate when B Z = 0, while they survive with a toroidal current when B Z is applied, showing that current circulation is vital to maintain the discharge. The detailed studies reveal the characteristics of current circulation and equilibrium. When B Z =0, the electron pressure profile is a uniform vertical ridge along the ECR layer in accordance with the electron vertical drift current circulating via the external circuit, while an upwardly-shifted potential hill arises, providing the ions E × B drift paths around the potential peak to the vicinity of the top electrode. When a B Z is applied, the central electron pressure rises with an upwardly shifted peak, being relaxed from the vertical uniformity by addition of the internal return circuit. The space potential V S decreases and the electron density obeys the Boltzmann law under V S. The vertical and toroidal currents are found to be equilibrium currents driven by the base and excess portions of electron pressure, respectively, to ensure the radial force balance of the plasma torus.
Generation of initial closed flux surfaces by electron cyclotron heating (ECH) at a conventional aspect ratio of R/a ∼ 3 is investigated on the Low Aspect ratio Torus Experiment (LATE) device and further tested on the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) tokamak. In both experiments, a plasma current is initiated and increased by ECH under steady external fields composed of a toroidal field and a weak vertical field. In the LATE experiments, a movable inboard limiter is used to change the aspect ratio of plasmas up to R/a ∼ 3. The results show that the formation of closed flux surfaces is still possible up to R/a ∼ 3 while higher decay indices of vertical field are required as the aspect ratio increases. Similar current start-up discharges are performed on JT-60U, and a plasma current is initiated and increased up to 20 kA under a vertical field having a high decay index.
Suppose an axisymmetric toroidal plasma is immersed in a toroidal magnetic field B φ in the cylindrical coordinates of (R, φ, Z) with Z coordinate on the symmetric axis. The electrons drift downward while the ions drift upward due to the field gradient and curvature by the velocity, v Z = m(v 2 + v 2 ⊥ /2)/(qRB φ ). It was conjectured that the electrons that reached the bottom wall flowed through the conducting vessel to the top where they recombined with the ions, thus a closed circuit being completed for the vertical charge-separation current. This current has for the first time been directly measured by current-collecting electrodes newly fabricated in the Low Aspect ratio Torus Experiment (LATE) device for a plasma maintained by a microwave power at the electron cyclotron range of frequency. The amount of current is found to be consistent with the above vertical drift model for electrons. The discharge terminates quickly during microwave injection when the current circuit is blocked by the artificial insertion of a large resistance.
Abstract. In order to investigate vitamin D metabolism in insulin-deficient diabetic rats, plasma vitamin D metabolites were measured at various periods after induction of diabetes by iv administration of 60 mg/kg streptozotocin (STZ). After STZ injection, plasma insulin was significantly decreased and plasma urea nitrogen increased with the duration of diabetes, while plasma creatinine remained unchanged. Plasma calcium, 25-hydroxyvitamin D (25(OH)D), and 24,25-dihydroxyvitamin D (24,25(OH)2D) progressively decreased. On the other hand, plasma 1,25-dihydroxyvitamin D (1,25(OH)2D) did not change at any period, but the ratio of 1,25(OH)2D to 25(OH)D became high in proportion to the severity of hypocalcaemia. Since significantly lower 25(OH)D and 24,25(OH)2D levels were observed at the later stage of diabetes, it is suggested that the altered vitamin D metabolism in diabetes is secondary to the disturbances in metabolic homeostasis derived form the insulin deficiency.
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