NSTX: National Spherical Torus Experiment

NSTX Vacuum Vessel


The National Spherical Torus Experiment (NSTX) is an innovative magnetic fusion device being used to study the physics principles of spherically shaped plasmas -- hot ionized gases in which nuclear fusion will occur under the appropriate conditions of temperature, density, and confinement in a magnetic field.

Located at the Plasma Physics Laboratory (PPPL), research on NSTX is conducted by a collaborative international research team of physics and engineers. Building on the world-class expertise acquired from work on DIII-D, PBX-M, TEXTOR, TCV, CCT, and CER’s own PISCES group, UCSD’s activities at NSTX are based on a physics program that includes elements of fusion science addressed by the PT plasma that include:

Confinement and Transport

ST plasma provides a radically different environment from the existing moderate aspect ratio machines. The unique characteristics of ST configuration provide opportunities for unraveling some of the complex phenomena underlying transport. Among the potentially important ST characteristics, we find that possible low instability growth rates and large ExB shearing rates contribute to the scaling databases. IT is of the utmost importance that the basic elements characterizing confinement and transport and compiled early in the program. Among the physics issues that UCSD proposes to explore are:

  • Characterization and scale of turbulence levels at the edge and SOL, and their relation to global confinement
  • Search for H-mode threshold and characterization of pedestal property profiles
  • SOL profile scalings: this is an important issue for higher power levels and the performance limitations of ST’s by plasma-surface interactions
  • Understanding the influence of ST-characteristic properties on turbulence

The ST plasmas are thought to be highly stable against microinstability growth and the highly sheared geometry will provide intrinsically high EXB shearing rates that are stabilizing for microturbulence. UCSD will contribute to the following areas:

  • Study possible links between core shear stabilization and edge microturbulence
  • Characterize edge microturbulence and its effect on particle transport
  • Perform basic scalings on stabilizing parameters
  • Effect of ion orbits on edge radial electric field / radial conductivity. ST poloidal flows are expected to be very damped, but orbit squeezing is expected to modify the viscosity as well.
  • Address the issue of poloidal asymmetries in a ST configuration. Since microturbulence is dominant in the unstable outboard of existing tokamaks (ballooning behavior) it is very relevant to address the issue of stabilizing effect for ST on the edge.
  • Evaluate local particle and energy fluxes and compare to gyrokinetic simulations

Scrape-Off-Layer and Divertor Physics

High power operation is required to reach high beta (Beta_T) and high bootstrap current fractions; consequently the heat and particle fluxes present in the scrape-off layer (SOL) and incident on the plasma facing components (PCFs) will be very high. The SOL of an ST plasma is expected to be dominated by short connection length and high mirror ratio, and therefore very different from known tokamak devices. The UCSD program will address the following:

  • Characterization and modeling of the SOL and edge plasma, and integration with the core
  • It has been demonstrated that achieving high confinement and performance core plasmas is dependent on controlling boundary conditions; namely impurity and ion fluxes and edge instabilities. In order to achieve this understanding in the largely uncharted ST plasma we need to:
    • Characterize the edge and SOL. Scale the SOL parameters.
    • Utilize data in to model the edge and SOL, to further understand ST edge/SOL plasma.
    • Extrapolate understanding to full NSTX heating power, in order to develop a program for control of expected heat and particle fluxes.
    • Develop analytical understanding of the SOL and incorporate the particular ST conditions.
  • Power balance and control of the Heat Flux
  • The ST boundary possesses a natural divertor, with flux expansion of about 10. High divertor heat fluxes are expected at full power operation due to ST plasma characteristics such as short connection lengths, steep angle of incidence on the PFCs, and low flux expansion. A way to mitigate and control heat flux to PFCs will be paramount for increasing performance and proving fitness as a reactor concept. A radiative divertor or boundary will therefore be integral to the NSTX program. UCSD has accumulated experience in radiative boundary layers and divertors at TEXTOR and DIII-D and will contribute to this important aspect program by:
    • Providing characterization of the boundary and divertor plasmas.
    • Studying the effect of recombination and convection in a radiative divertor by using Mach probes.
    • Providing input and modeling, in order to understand boundary stability and flow reversal.

Recycling and Impurity Control

Impurity and recycling control are key elements for obtaining high core performance. UCSD will support efforts to characterize the recycling and evaluate the particle inventory by furbishing:

  • High resolution edge and SOL profiles in order to accurately quantify the sources and sinks.
  • Quantification of the effects of wall conditioning on edge and SOL conditions and concomitant influence on the recycling and impurity content.

Non-Inductive Operation

The ability to operate non-inductively is an important element of the NSTX program. Therefore characterization of the plasma boundary in front of launching elements, and the ability to control it, are necessary elements for effective use of RF tools.

  • UCSD can provide high resolution edge and SOL profiles of Ne and Te in order to support the RF efforts.
  • Evaluation of the RF ponderomotive effects on the edge profiles is a potentially important element in coupling RF power to the plasma.

MHD Stability

The ST plasma will present new challenges in stability and stability limits. The ability to characterize modes in plasma edge and possible will require, among other tools, profile control at the plasma edge and edge ergodization. Another topic of interest is the causality between stationary magnetic perturbations (SMPs), transport, and edge phenomena. Well resolved edge measurements will be essential in order to characterize the edge profiles, resolve sharp edge gradients in density and temperature and evaluate the effects of edge ergodization.


Principal Investigators: Jose Boedo


Program Accomplishments

  • Discovered intermittency is dominant in the edge of NSTX
  • Measured density decay length
  • Found density decay length is power dependent (inversely)

Image Source: PPPL