A. von Stechow

VINETA II: A linear magnetic reconnection experiment

A linear experiment dedicated to the study of driven magnetic reconnection is presented. The new device (VINETA II) is suitable for investigating both collisional and near collisionless reconnection. Reconnection is achieved by externally driving magnetic field lines towards an X-point, inducing a current in the background plasma which consequently modifies the magnetic field topology. Owing to the open field line configuration of the experiment, the current is limited by the axial sheath boundary conditions. A plasma gun is used as an additional electron source in order to counterbalance the charge separation effects and supply the required current. Two drive methods are used in the device. First, an oscillating current through two parallel conductors drive the reconnection. Second, a stationary X-point topology is formed by the parallel conductors, and the drive is achieved by an oscillating current through a third conductor. In the first setup, the magnetic field of the axial plasma current dominates the field topology near the X-point throughout most of the drive. The second setup allows for the amplitude of the plasma current as well as the motion of the flux to be set independently of the X-point topology of the parallel conductors.

Overview of the Wendelstein 7-X phase contrast imaging diagnostic

A phase contrast imaging (PCI) diagnostic has been developed for the Wendelstein 7-X (W7-X) stellarator. This diagnostic, funded by the U.S. Department of Energy through the Office of Fusion Energy Sciences, is a collaboration between the Max Planck Institute for Plasmaphysics, MIT, and SUNY Cortland. The primary motivation for the development of the PCI diagnostic is measurement of turbulent fluctuations, such as the ion temperature gradient, electron temperature gradient, and the trapped electron mode instabilities. Understanding how the magnetic geometry and other externally controllable parameters, such as the fueling method and heating scheme, modify the amplitude and spectrum of turbulence is important for finding operational scenarios that can lead to improved performance at fusion-relevant temperatures and densities. The PCI system is also sensitive to coherent fluctuations, as may arise from Alfvén eigenmodes or other MHD activity, for example. The PCI method creates an image of line-integrated variations in the index of refraction. For a plasma, the image created is proportional to the line-integral of electron density fluctuations. This paper provides an overview of some key features of the hardware and the optical system and presents two examples of recent measurements from the W7-X OP1.2a experimental campaign.

Measurements of plasma profiles using a fast swept Langmuir probe in the VINETA-II magnetic reconnection experiment

A fast-swept Langmuir probe capable to be biased at a high voltages has been constructed and successfully operated at the VINETA-II magnetic reconnection experiment. The presented circuit has two main features beneficial for fast transient parameter changes in laboratory experiments as, e.g., plasma guns or magnetic reconnection: the implementation simplicity and the high voltage sweep range. This work presents its design and performance for time-dependent measurements of VINETA-II plasmas. The probe is biased with a sinusoidal voltage at a fixed frequency. Current − voltage characteristics are measured along the falling and rising slopes of the probe bias. The sweep frequency is fsweep= 150 kHz. The spatiotemporal evolution of radial plasma profiles is obtained by evaluation of the probe characteristics. The plasma density measurements agree with those derived from a microwave interferometer, demonstrating the reliability of the measurements. As a model plasma system, a plasma gun discharge with typical pulse times of 60 μ s is chosen.