K. Knapp

A new high performance field reversed configuration operating regime in the C-2 devicea)

Large field reversed configurations (FRCs) are produced in the C-2 device by combining dynamic formation and merging processes. The good confinement of these FRCs must be further improved to achieve sustainment with neutral beam (NB) injection and pellet fuelling. A plasma gun is installed at one end of the C-2 device to attempt electric field control of the FRC edge layer. The gun inward radial electric field counters the usual FRC spin-up and mitigates the n = 2 rotational instability without applying quadrupole magnetic fields. Better plasma centering is also obtained, presumably from line-tying to the gun electrodes. The combined effects of the plasma gun and of neutral beam injection lead to the high performance FRC operating regime, with FRC lifetimes up to 3 ms and with FRC confinement times improved by factors 2 to 4.

Internal magnetic field measurement on C-2 field-reversed configuration plasmasa)

A long-lived field-reversed configuration (FRC) plasma has been produced in the C-2 device by dynamically colliding and merging two oppositely directed, highly supersonic compact toroids (CTs). The reversed-field structure of the translated CTs and final merged-FRC state have been directly verified by probing the internal magnetic field structure using a multi-channel magnetic probe array near the midplane of the C-2 confinement chamber. Each of the two translated CTs exhibits significant toroidal fields (B(t)) with opposite helicity, and a relatively large B(t) remains inside the separatrix after merging.

Far infrared laser polarimetry and far forward scattering diagnostics for the C-2 field reversed configuration plasmasa)

A two-chord far infrared (FIR) laser polarimeter for high speed sub-degree Faraday rotation measurements in the C-2 field reversed configuration experiment is described. It is based on high power proprietary FIR lasers with line width of about 330 Hz. The exceptionally low intrinsic instrument phase error is characterized with figures of merit. Significant toroidal magnetic field with rich dynamics is observed. Simultaneously obtained density fluctuation spectra by far forward scattering are presented.

Magnetic diagnostic suite of the C-2 field-reversed configuration experiment confinement vessela)

Magnetic measurements are a fundamental part of determining the size and shape of field-reversed configuration (FRC) plasmas in the C-2 device. The magnetic probe suite consists of 44 in-vessel and ex-vessel probes constructed using various technologies: ultra-high vacuum compatible mineral-insulated cable, nested triple axis coils hand-wound on ceramic bobbins, and commercial chip inductors mounted on printed circuit boards. Together, these probes measure the three-dimensional excluded flux profile of the FRC, which approximates the shape of the separatrix between the confined plasma volume and the scrape-off layer. High accuracy is achieved by using the extensive probe measurements to compensate for non-ideal effects such as flux leakage through the vacuum vessel and bulk motion of the FRC towards the wall. A subset of the probes is also used as a set of Mirnov arrays that provide sensitive detection of perturbations and oscillations of the FRC.

Magnetic diagnostic suite of the C-2W field-reversed configuration experiment

A fundamental component of any magnetically confined fusion experiment is a firm understanding of the magnetic field. The increased complexity of the C-2W machine warrants an equally enhanced diagnostic capability. C-2W is outfitted with over 700 magnetic field probes of various types. They are both internal and external to the vacuum vessel. Inside, a linear array of innovative in-vacuum annular flux loop/B-dot combination probes provide information about plasma shape, size, pressure, energy, temperature, and trapped flux when coupled with established theoretical interpretations. A linear array of B-dot probes complement the azimuthally averaged measurements. A Mirnov array of 64 3D probes, with both low and high frequency resolution, detail plasma motion and MHD modal content via singular value decomposition analysis. Internal Rogowski probes measure axial currents flowing in the plasma jet. Outside, every feed-through for an internal probe has an external axial field probe. There are many external loops that measure the plasma formation dynamics and the total external magnetic flux. The external measurements are primarily used to characterize eddy currents in the vessel during a plasma shot. Details of these probes and the data derived from their signals are described.

Langmuir probe diagnostic suite in the C-2 field-reversed configurationa)

Several in situ probes have been designed and implemented into the diagnostic array of the C-2 field-reversed configuration (FRC) at Tri Alpha Energy [M. Tuszewski et al. (the TAE Team), Phys. Rev. Lett. 108, 255008 (2012)]. The probes are all variations on the traditional Langmuir probe. They include linear arrays of triple probes, linear arrays of single-tipped swept probes, a multi-faced Gundestrup probe, and an ion-sensitive probe. The probes vary from 5 to 7 mm diameter in size to minimize plasma perturbations. They also have boron nitride outer casings that prevent unwanted electrical breakdown and reduce the introduction of impurities. The probes are mounted on motorized linear-actuators allowing for programmatic scans of the various plasma parameters over the course of several shots. Each probe has a custom set of electronics that allows for measurement of the desired signals. High frequency ( > 5MHz) analog optical-isolators ensure that plasma parameters can be measured at sub-microsecond time scales while providing electrical isolation between machine and data acquisition systems. With these probes time-resolved plasma parameters (temperature, density, spatial potential, flow, and electric field) can be directly/locally measured in the FRC jet and edge/scrape-off layer.

Fusion proton diagnostic for the C-2 field reversed configurationa)

Measurements of the flux of fusion products from high temperature plasmas provide valuable insights into the ion energy distribution, as the fusion reaction rate is a very sensitive function of ion energy. In C-2, where field reversed configuration plasmas are formed by the collision of two compact toroids and partially sustained by high power neutral beam injection [M. Binderbauer et al., Phys. Rev. Lett. 105, 045003 (2010); M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012)], measurements of DD fusion neutron flux are used to diagnose ion temperature and study fast ion confinement and dynamics. In this paper, we will describe the development of a new 3 MeV proton detector that will complement existing neutron detectors. The detector is a large area (50 cm(2)), partially depleted, ion implanted silicon diode operated in a pulse counting regime. While the scintillator-based neutron detectors allow for high time resolution measurements (∼100 kHz), they have no spatial or energy resolution. The proton detector will provide 10 cm spatial resolution, allowing us to determine if the axial distribution of fast ions is consistent with classical fast ion theory or whether anomalous scattering mechanisms are active. We will describe in detail the diagnostic design and present initial data from a neutral beam test chamber.

COMBINED FRC AND MIRROR PLASMA STUDIES IN THE C-2 DEVICE

Abstract The Field Reversed Configuration (FRC) is a high-beta Compact Toroid that includes closed and open field line regions of poloidal magnetic field. Improving the transport properties of both regions is important for the overall FRC confinement and may be attempted in the C-2 device. The goal of this experiment is to explore FRC sustainment by combining heating and current drive from neutral beam injection and particle fueling from a pellet injector. Additions to the C-2 device may include magnetic mirror plugs, plasma guns, and electrically-biased limiters. These additions would permit us to explore combined FRC and mirror physics, with emphasis on improving the FRC confinement.

Particle and heat flux diagnostics on the C-2W divertor electrodes

A suite of diagnostics was developed to measure particle and heat fluxes arriving at the divertor electrodes of the C-2W experiment at TAE Technologies. The divertor electrodes consist of 4 concentric rings, each equipped with a bolometer, electrostatic energy analyzer, and thermocouple mounted at two opposing azimuthal locations. These probes provide measurements of the power flux to the divertor electrodes as well as measurements of the ion current density, ion energy distribution, and total energy deposition. The thermocouples also provide calibration points for inferring the heat deposition profile via thermographic imaging of the electrodes with a fast infrared camera. The combined measurements enable the calculation of the energy lost per escaping electron/ion pair, which is an important metric for understanding electron heat transport in the open field lines that surround the field-reversed configuration plasma in C-2W.

Design of a custom insertable probe platform for measurements of C-2W inner divertor plasma parameters

A custom motor controlled probe system has been designed to make spatially resolved measurements of temperature, density, flow, and plasma potential in the C-2W inner divertors. Measurements in the inner divertors, which have a 1.7 m radius and are located on either end of the confinement vessel, are critical in order to gauge exactly how local settings affect the plasma conditions, confinement, and stability in the field-reversed configuration core. The inner Divertor Insertable Probe Platform (iDIPP) system consists of a custom motor controlled linear rack and pinion transporter that has a 1.9 m travel length in order to reach the center of the divertor. Mounted to the end of the transporter is a 1 m long segmented probe shaft made of individually floating stainless steel rings to prevent shorting out the electrode plates, which are biased up to 5 kV/m. A variety of interchangeable probe tips, including a triple Langmuir probe, a baffled probe, and a Gundestrup probe, can plug into the end of the probe shaft. Custom UHV coiled cabling comprised of 9 shielded conductors expands/retracts with the motion of the transporter in/out of the divertor. The physics motivating plasma parameter measurements in the inner divertors and the details of the design of the iDIPP system will be discussed.