Thomas R. Jarboe

Exploration of Spherical Torus Physics in the NSTX Device

The National Spherical Torus Experiment (NSTX) is being built at the Princeton Plasma Physics Laboratory to test the fusion physics principles for the Spherical Torus (ST) concept at the MA level. The NSTX nominal plasma parameters are R {sub 0} = 85 cm, a = 67 cm, R/a greater than or equal to 1.26, B {sub T} = 3 kG, I {sub p} = 1 MA, q {sub 95} = 14, elongation {kappa} less than or equal to 2.2, triangularity {delta} less than or equal to 0.5, and plasma pulse length of up to 5 sec. The plasma heating/current drive (CD) tools are High Harmonic Fast Wave (HHFW) (6 MW, 5 sec), Neutral Beam Injection (NBI) (5 MW, 80 keV, 5 sec), and Coaxial Helicity Injection (CHI). Theoretical calculations predict that NSTX should provide exciting possibilities for exploring a number of important new physics regimes including very high plasma beta, naturally high plasma elongation, high bootstrap current fraction, absolute magnetic well, and high pressure driven sheared flow. In addition, the NSTX program plans to explore fully noninductive plasma start-up, as well as a dispersive scrape-off layer for heat and particle flux handling.

Review of spheromak research

Spheromak research from 1979 to the present is reviewed including over I60 references. Emphasis is on understanding and interpretation of results. In addition to summarizing results some new interpretations are presented. An introduction and brief history is followed by a discussion of generalized helicity and its time derivative. Formation and sustainment are discussed including five different methods, flux core, theta -pinch z-pinch, coaxial source, conical theta -pinch, and kinked z-pinch. All methods use helicity injections. Steady-state methods and rules for designing spheromak experiments are covered, followed by equilibrium and stability. Methods of stabilizing the tilt and shift modes are discussed as well as their impact on the reactor designs. Current-driven and pressure-driven instabilities as well as relaxation in general are covered. Energy confinement is discussed in terms of helicity decay time and and beta s limits. The confinement in high and low open-flux geometries are compared and the reactor implications discussed.

Formation and steady-state sustainment of a tokamak by coaxial helicity injection

A possible method for steady-state sustainment of a tokamak through coaxial helicity injection is presented. To keep the toroidal flux constant, two insulators are used, one that injects and one that absorbs toroidal flux. The scheme is made attractive by incorporating the toroidal field of the tokamak in the source to raise its impedance.

Physics design of the national spherical torus experiment

The mission of the National Spherical Torus Experiment (NSTX) is to prove the principles of spherical torus physics by producing high-beta toroidal plasmas that are non-inductively sustained, and whose current profiles are in steady-state. NSTX will be one of the first ultra low a[P(input) up to 11 MW] in order to produce high-beta toroidal (25 to 40%), low collisionality, high bootstrap fraction (less than or equal to 70%) discharges. Both radio-frequency (RF) and neutral-beam (NB) heating and current drive will be employed. Built into NSTX is sufficient configurational flexibility to study a range of operating space and the resulting dependences of the confinement, micro- and MHD stability, and particle and power handling properties. NSTX research will be carried out by a nationally based science team.

Initial results from coaxial helicity injection experiments in NSTX

Coaxial helicity injection has been investigated on the National Spherical Torus Experiment (NSTX). Initial experiments produced 130 kA of toroidal current without the use of the central solenoid. The corresponding injector current was 20 kA. Discharges with pulse lengths up to 130 ms have been produced.

Non-inductive current generation in NSTX using coaxial helicity injection

Coaxial helicity injection (CHI) on the National Spherical Torus Experiment (NSTX) has produced 240 kA of toroidal current without the use of the central solenoid. Values of the current multiplication ratio (CHI produced toroidal current/injector current) up to 10 were obtained, in agreement with predictions. The discharges, which lasted for up to 200 ms, limited only by the programmed waveform, are more than an order of magnitude longer in duration than any CHI discharges previously produced in a spheromak or a spherical torus.

Formation and sustainment of a low‐aspect ratio tokamak by coaxial helicity injection

Low‐aspect‐ratio tokamaks with toroidal currents, Ip, up to 250 kA are formed and sustained in the Helicity Injected Tokamak experiment [Nelson et al., Phys. Rev. Lett. 72, 3666 (1994)] using coaxial helicity injection. These plasmas are produced without use of a current drive transformer. Average toroidal currents are sustained at high values, 〈Ip〉=225 kA for 2 ms, where electron thermal energies are measured up to 80 eV with spectroscopy data suggesting burnthrough to the higher ionization states of oxygen. Currents can also be sustained for longer periods at lower values, 〈Ip〉=138 kA for 7 ms. These tokamaks are characterized by a rotating, n=1 distortion, with poloidal distortions approximately following the field line pitch, which only occur on the outer bad‐curvature region. Equilibrium reconstructions show these plasmas have a tokamak q profile (q0=5 – 8, q95=10 – 12, qcyl≂3.6), with a hollow toroidal current profile and up to 170 kA of closed field toroidal current in a low‐aspect‐ratio, A=1.68 co...

Initial physics results from the National Spherical Torus Experiment

The mission of the National Spherical Torus Experiment (NSTX) is to extend the understanding of toroidal physics to low aspect ratio (R/a approximately equal to 1.25) in low collisionality regimes. NSTX is designed to operate with up to 6 MW of High Harmonic Fast Wave (HHFW) heating and current drive, 5 MW of Neutral Beam Injection (NBI) and Co-Axial Helicity Injection (CHI) for non-inductive startup. Initial experiments focused on establishing conditions that will allow NSTX to achieve its aims of simultaneous high-bt and high-bootstrap current fraction, and to develop methods for non-inductive operation, which will be necessary for Spherical Torus power plants. Ohmic discharges with plasma currents up to 1 MA and with a range of shapes and configurations were produced. Density limits in deuterium and helium reached 80% and 120% of the Greenwald limit respectively. Significant electron heating was observed with up to 2.3 MW of HHFW. Up to 270 kA of toroidal current for up to 200 msec was produced noninductively using CHI. Initial NBI experiments were carried out with up to two beam sources (3.2 MW). Plasmas with stored energies of up to 140 kJ and bt =21% were produced.

Results from current drive experiments on the Helicity Injected Torus

The Helicity Injected Torus [T. R. Jarboe, Fusion Technol. 15, 7 (1989)] is a low aspect ratio tokamak that is formed and sustained by coaxial helicity injection with no transformer. Toroidal plasma currents of over 200 kA have been achieved with electron temperatures in the 100 eV range and electron density between 1019 and 1020 m−3. The major radius is 0.3 m and the minor radius is 0.2 m. New results from equilibrium and stability analysis of the external magnetic diagnostics and new results from the Transient Internal Probe (TIP), an internal magnetic field diagnostic, are presented. A mechanism for the transfer of current drive on the open to the closed flux regions is presented.

Progress with energy confinement time in the CTX spheromak

Large improvements in spheromak parameters and new understanding have been obtained from the CTX experiment at Los Alamos [Phys. Rev. Lett. 51, 39 (1983); 61, 2457 (1988)]. In one experiment the global energy confinement time has been increased an order of magnitude over previous experiments to 0.2 msec and the magnetic‐energy decay time increased to 2 msec. These results were achieved in a decaying spheromak by reducing the helicity dissipation in the edge. In another smaller spheromak, record electron temperatures (∼400 eV) and record magnetic field strengths (∼30 kG) have been obtained.