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

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10J115 (2018)
Design of a custom insertable probe platform for measurements
of C-2W inner divertor plasma parameters
A. M. DuBois,a) V. Sokolov, K. Knapp, M. C. Thompson, and TAE Teamb) TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, California 92610, USA
(Presented 16 April 2018; received 20 April 2018; accepted 4 June 2018; published online 13 August 2018)
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. Published by AIP Publishing. https://doi.org/10.1063/1.5037118
I. INTRODUCTION
The C-2W1 experiment [Fig. 1(a)] is a field-reversed con- figuration (FRC) which uses neutral beam injection and an edge biasing scheme to suppress turbulence and stabilize the plasma. The FRC core is located within the separatrix in the confinement vessel (CV), and the edge is characterized by a region of open field lines called the scrape-off-layer (SOL). The open field lines of the SOL terminate on electrode plates located in the inner and outer divertors. Figure 1(b) shows an illustration of the magnetic field lines in a flared field con- figuration (black) and the plasma density (color contours) as modeled by MHD codes. This provides favorable boundary conditions for the core plasma and handles energy and parti- cle flows away from the FRC. In situ measurements of inner divertor plasma parameters using Langmuir probes in the inner divertors are therefore critical in order to gauge how local set- tings affect the plasma conditions, confinement, and stability in the FRC core.
Langmuir probes are one of the earliest diagnostics used to measure basic plasma parameters such as the floating potential (V f ), plasma potential (V p ), electron density (ne ), and elec- tron temperature (T e ).2 In fusion plasmas, where the plasma core is too hot and dense for Langmuir probes, in situ mea- surements are limited to the plasma edge and the SOL. A fast reciprocating Langmuir probe technique has been used in tokamaks to make spatially resolved profiles of local mea- surements in the SOL during a single plasma discharge.3–10
Note: Paper published as part of the Proceedings of the 22nd Topical Confer- ence on High-Temperature Plasma Diagnostics, San Diego, California, April 2018.
a)Electronic mail: adubois@tae.com.
These reciprocating probes generally inject the probe no more than 50 cm at rates on the order of 2 m/s. This technique is not a viable solution for radial measurements on C-2W due to the vast size of the divertors (r = 1.7 m).
Recently, a custom motor controlled probe system has been designed for spatially resolved measurements of plasma parameters in the C-2W inner divertors called the inner diver- tor insertable probe platform (iDIPP). This paper discusses the physics of the inner divertor (Sec. II), which motivate the need for the iDIPP. Section III A discusses the custom motion system, Sec. III B reviews the custom cabling, and Secs. III C and III D describe the probe shaft and probe tip assemblies.
II. PHYSICS OF THE C-2W INNER DIVERTOR
The edge of C-2W plasmas is characterized by a region of open field lines called the scrape-off-layer (SOL). These open field lines terminate on electrode plates in the inner diver- tor, which are located on either end of the CV. Mirror coil magnets, positioned between the CV and inner divertors, gen- erate a flared magnetic field in the inner divertor [Fig. 1(b)] to reduce the particle loss from the SOL11 and maintain the high temperature of the SOL electrons, which is the main pur- pose of the inner divertors. The thickness of the SOL can be controlled by varying the current in the mirror coils. A funnel limiter, located on the high field side of the inner diver- tor (closest to the CV), defines the radial size of the SOL and can also be used as an additional electrode during edge biasing.
In order to maintain a high Te in the SOL, potential sources of cold electrons, such as secondary electrons and neu- tral gas, must be eliminated. Large particle flux entering the
b)TAE Team members are listed in Nucl. Fusion 57, 116021 (2017). 0034-6748/2018/89(10)/10J115/5/$30.00
89, 10J115-1 Published by AIP Publishing.