A brief description of the Reconnection Scaling eXperiment

Introduction

RSX Experimental apparatus


 

Introduction

Magnetic reconnection refers to the local breaking of magnetic field lines and subsequent change in the global topology of the magnetic field in the presence of plasmas or conducting fluids. During this process, magnetic field lines of opposite polarity are convected toward each other by fluid flow in the so-called diffusion region. This region is centered around a neutral magnetic line, across which the reconnecting component of the magnetic field changes sign. In the diffusion region, the "frozen-in" condition of ideal MagnetoHydroDynamics (MHD) equations is broken. The magnetic field can diffuse through the plasma allowing the annihilation of opposite directed magnetic field lines and conversion of magnetic field energy into Ohmic heating and particle acceleration.
During the last four decades, a large effort has been devoted to study magnetic reconnection in plasmas since it is considered to play a crucial role in a variety of different astrophysical and laboratory phenomena. Examples are in the evolution of solar flares, in the dynamics of the earth magnetosphere and in the redistribution of the energy in the universe. In magnetically confined laboratory plasmas for fusion research, a major role is played by magnetic reconnection in determining the dynamics of relaxation processes, such as sawtooth oscillations and major disruptions in tokamaks or dynamo effects in Reversed Field Pinches.
Early 3D laboratory experiments on magnetic reconnection provided detailed measurements of the relaxation to a force-free state produced by the coalescence of two current channels inthe LPD linear device. Interacting current channels were produced by coating a large cathode source nonuniformly and then biasing it with respect to an external anode. Although 3D features of magnetic reconnection were studied, the Lundquist number of the interacting current channels was limited by the plasma production scheme and ions were not magnetized (EMHD regime). Over the past few years, developments in plasma gun technology have made plasma guns a reliable laboratory source capable of producing plasmas with large Lundquist number. The Reconnection Scaling Experiment uses this innovative plasma gun technology to study the coalescence of parallel current channels in a 3D linear geometry. The emphasis will be on using many repetitive plasma discharges to acquire detailed spatial and temporal measurements as the two current channels merge along the axis of the linear device. One major advantage of the plasma gun technology is that it allows a high degree of flexibility in scaling independently the different parameters important in the reconnection process.


RSX Experimental apparatus

A view of the RSX device is shown in Fig. 1 (a),(b) together with a schematic view of the linear geometry of two interacting current channels, Fig.1 (c). For a picture of the actual status of RSX, click here. In this section, the main elements of the RSX device are described together with the plasma production technique. An overview is presented of the control and data acquisition system and main diagnostics.

Figure 1. Cross section of RSX device: (a) lateral view, (b) head on view; and (c) schematics of two interacting current channels along the axis of the device. RSX main elements are shown: (1) plasma guns, (2) head flange accomodating a large window, (3) magnet coils, (4) external anode, (5) stainless steel vacuum vessel, (6) schematics of the vacuum system, and (7) electro mechanical valve for gas puffing.

Vacuum vessel and vacuum system

The RSX vessel consists of a cylindrical vacuum chamber with approximately 4m length and 20 cm radius, Fig. 1 (a), which is formed by 3 sections 48 inch in length recycled from the Field Reversed Configuration experiment FRX-C. Each section is fabricated from stainless steel (SS304) tubing with a 40.6 cm nominal outside diameter, a 0.48 cm wall thickness and has 16 x 2.75 inch and 12 x 6 inch Conflat flanges facing radially, allowing for easy placement of diagnostics and flexibility in the plasma gun geometry arrangement. The use of standard Conflat hardware ensures ultrahigh vacuum capability at reasonable cost. The conductivity of the vessel and the numerous Conflat flanges adds a delay on the order of $600 \mu sec$ for magnetic field penetration from the external solenoid coils. End flanges accomodate large windows, probe access, and eventually mounts for plasma gun arrays. The vacuum system consists of a 5500 liter/sec roughing and backing mechanical pumps with a 110 liter/sec turbo pump which provides a 10^{-7} Torr base vacuum before plasma formation. The vacuum control system encompasses an automatic gauge controller and safety interlock which closes appropriate valves and shuts down the turbo pump in the case of an accidental pressure or power failure. During operation of the plasma guns, this system is disabled to prevent its intervention following the plasma formation and the subsequent pressure increase. 

Plasma guns, pulser supply and discharge sequence

Presently, RSX is equipped with four plasma guns radially inserted through 2.75 inch flanges into the vacuum chamber and equally spaced in the poloidal direction as shown in Fig. 1(b). The plasma guns, originally designed for current profile modification in the Madison Symmetric Torus, allow the injection of parallel current channels along the axial direction of the RSX device as it is schematically shown in Fig. 1(c). The radial distance between plasma guns can be modified to provide flexibility in the geometry of the interacting current channels.
Each plasma gun, with a 2.5 cm external radius, contains a miniaturized plasma source with a circular 0.79 cm^2 nozzle aperture in which a cylindrical plasma is produced by an arc discharge between a molybdenum anode and a cathode. A stack of molybdenum and boron nitride washers 1.5 cm high defines the arc channel between the anode and the cathode. The gas, usually H or He, is supplied through the cathode by an electromagnetic valve, Fig. 1, which is pulsed 12 ms before the arc discharge. This time delay corresponds to the gas travelling time from the puff valve to the miniaturized discharge chamber. When the arc anode is negatively biased with respect to an external electrode, which in the following we will refer to as external anode, a fraction of the arc current is diverted towards the external anode as plasma current. In Fig. 1, the external anode is shown consisting of a 50 cm^2 SS304 plate bolted to copper rod which is electrically insulated from the vacuum vessel by a ceramic feedthrough. The external anode can be moved along the RSX vessel to modify the length of a single current channel. In Fig. 2, the schematic of the discharge pulser circuit is shown.

Figure 2. Schematic view of the exciter circuit for the plasma guns. When the arc anode is negatively biased with respect to the external anode (see Fig. 1), a fraction of the arc current I_arc is diverted towards the external anode as plasma current. Shown are the pulse forming network (PFN), the silicon controlled rectifier (SCR), the arc and bias power supply (PS) with switch relays (S1,S3).

The arc plasma is maintained by a Pulse Forming Network (PFN) which sets the internal gun arc voltage to V_arc~ 80-100 V and determines the discharge pulse length which is presently approximately 10 ms. The impedance of the PFN matches the arc impedance and is set to ~100 mOhms. The PFN capacitors are charged up to a maximum voltage V_cap~1 kV and the arc discharge is initiated by applying a gate pulse to a silicon controlled rectifier (SCR), SCR1 in Fig. 2. The arc current, I_arc can be varied from 0.3 to 1 kA. The bias pulser, shown on the left of Fig. 3 is designed to lower the potential of the arc anode below that of the grounded external anode so that by varying the external bias voltage, V_bias=0-300 V$, a desired fraction of the arc current can be extracted as external anode (i.e. plasma) current. The bias discharge is initiated by applying a gate pulse to SCR2 approximately 2 ms after the beginning of the arc discharge corresponding to the maximum of I_arc. The duration of the bias discharge can be manually varied between 0.5 to 8 ms by shorting the bias capacitor bank into a dump resistor through SCR3. 

Magnet coils and switching

Figure 3 presents a schematic of the pulse exciter and basic control circuitry for the magnet coils.

Figure3. Schematic view of the exciter circuit for the magnet coils together with the MOSFET driver for the SCR switch starting the discharge. Also shown are the capacitor bank, power supply (PS) and safety circuitry encompassing two high power switches (S1, S2) to dump the capacitor bank before and after each discharge.

A set of 12 identical copper coils with 125 cm external diameter and 53 cm internal diameter per coil surrounds the RSX vessel (see Fig. 1) and provides the axial guide field B_z. Each coil has self-inductance of 820 microH and resistance of 80 mOhms. In the present arrangement, a limited set of 5 pairs of series coils are connected in parallel and energized by an 0.16 F, electrolytic capacitor bank with a quarter cycle time of 16 ms. The capacitor bank is charged by a 800 V, 10 Amps power supply and for the present bank and magnet configuration the maximum central magnetic field is B_z0=0.1 T at 1 kA current per coil.
At this current level, magnet operation only requires straight forward SCR switched network for a 32 ms half cycle pulse length, with diode crowbar at the zero voltage crossing time. The SCR is controlled by a MOSFET switch which is gated by a +24 V, 35 ms pulse provided by the control system described in the last section}. The power supply control system encompasses an automatic safety circuitry (see high voltage switches S2, S3 in Fig. 3) that shorts the positive and negative side of the capacitor bank before and after each shot to prevent from accidental charging of the capacitor bank. Initial operation at pulse repetition of 60-120 s can be achieved with air cooling of the coils. Water cooling at 10 gpm using a closed loop deionized water system can be easily added to extend the operation of the device to much higher repetition rate. 

Diagnostics

 RSX is equipped with an initial set of diagnostics consisting of internal probes and noninvasive optical systems. Presently, internal probes are inserted into the vacuum vessel through flexible bellows and sliding seals allowing a two dimensional scanning of plasma parameters in the plane perpendicular to the axis of the device. Each probe can be moved along the axis of the device into a different porthole. The three component of the magnetic field are measured using a single probe consisting of electrostatically shielded, small (3.175 mm diameter), orthogonal magnetic loops with 30 turns each. A miniaturized Rogowski coil with 1 cm internal diameter allows measurements of the radial profile of current channels. A double Langmuir probe consisting of two electrically insulated tungsten wires provides measurements of electron density and temperature. A CCD single frame camera, fast framing camera \cite{phantom4 camera} and multiple photomultiplier tubes allow monitoring of visible light emission from the plasma through portholes. A feature of paramount importance in 3D magnetic reconnection experiments is the ability of measuring plasma parameters and magnetic field structures and fluctuations in three dimensions A new three axis moveable probe drive, which will be the subject of a future article, has been designed and is presently under construction. This system will allow a complete three dimensional scan of the plasma with an interchangeable probe head which accomodates different types of probes. 

Control and data acquisition system

The RSX control, data acquisition and data storage system is based on a distributed architecture including a National Instruments FP-2000 Ethernet module and commercial personal computers (PCs). All the machines communicate through Ethernet links.
The FP-2000 module runs embedded LabView Real Time and controls a National Instruments FP-DO-401 discrete output module. This module provides $+24$ V pulses for the timing sequence with a time accuracy within few ms. Events which require higher accuracy (i.e. gun arc and bias start and stop) are provided by digital hardware delays.

The RSX data acquisition system consists of digital Tektronix oscilloscopes and multiple Le Croy transient recorders which are housed in CAMAC crates controlled via GPIB interfaces by a commercial PC running Windows 2000. The sequence of configuring and arming the digitizers followed by data retrieval is performed by Labview6 routines which are grouped in a single GUI. A high level programming language such as Labview6 allows use and integration of many different digitizers with modest software development.

The PC which controls the acquisition system is linked via Ethernet to the RSX data storage system. This consists of a commercial PC running Linux RedHat7.2 operating system in which a MDS+ server database is installed. The data are archived on a 60 GB disk drive in MDS+ compressed format and analyzed via Matlab and IDL routines, which allow compatibility with Apple, Windows, LINUX and UNIX platforms.


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