Search

A new experiment, called the PHAse Space MApping (PHASMA) experiment, features laser induced fluorescence diagnostics for ion measurements, Thomson scattering diagnostics for electron velocity distribution function measurements, and a microwave scattering system for turbulence measurements. PHASMA is designed to enable the direct measurement of ion and electron vdfs in space-relevant plasma phenomena including reconnection, shocks, and turbulence. To create the conditions necessary for different experimental regimes, PHASMA employs a 2 kW, steady-state helicon source capable of generating variable-density background hydrogen, helium, argon, krypton, and xenon plasmas with controllable plasma pressure (relative to the magnetic pressure), collisionality, and azimuthal flow shear. Reconnecting flux ropes arise through the merging of discharges from two pulsed plasma guns.

A new incoherent Thomson scattering system measures the evolution of electron velocity distribution functions perpendicular and parallel to the ambient magnetic field during kinking of a single flux rope and merging of two flux ropes through magnetic reconnection. The Thomson scattering system provides sub-millimeter spatial resolution, sufficient to diagnose the several millimeters sized magnetic reconnection electron diffusion region in the PHAse Space MAppgin experiment. Due to the relatively modest plasma density ∼1019 m−3 and electron temperature ∼1 eV, stray light suppression is critical for these measurements. Two volume Bragg gratings are used in series as a notch filter with a spectral bandwidth 47% is used as the detector in a 1.3 m spectrometer. Preliminary results of gun plasma electron temperature will be reported and compared with measurements obtained from a triple Langmuir probe.

Magnetic reconnection is a fundamental process in plasmas that can release magnetic energy via changing field line topology, resulting in a variety of violent phenomena. The reconnection has been extensively studied in low temperature experiments. Field reversed configuration (FRC) is a compact toroid with closed field geometry and is being pursued as an alternative fusion concept. We propose to use collision-merging FRC to study the reconnection in high parameter regime. Preliminary results from KMAX-FRC shows the evolution of field reversed configure measured by internal probe, and burst of light emission during merging taken by fast camera.

Magnetic flux ropes have been successfully created with plasma guns in the newly commissioned PHAse Space MApping (PHASMA) experiment. The flux ropes exhibit the expected m = 1 kink instability. The observed threshold current for the onset of this kink instability is half of the Kruskal–Shafranov current limit, consistent with predictions for the non-line tied boundary condition of PHASMA. The helicity, paramagnetism, and growth rate of the observed magnetic fluctuations are also consistent with kink instability predictions. The observed fluctuation frequency appears to be a superposition of a real frequency due to a Doppler shift of the kink mode arising from plasma flow ( ∼ 2 kHz) and a contribution from a wave mode ( ∼ 5 kHz). The dispersion of the wave mode is consistent with an Alfven wave. Distinct from most previous laboratory studies of flux ropes, the working gas in PHASMA is argon. Thus, the ion cyclotron frequency in PHASMA is quite low and the frequency of the Alfvenic mode plateaus at ∼ 0.5 of the ion gyro frequency with increasing background magnetic field strength.

A novel magnetic mirror concept with field-reversed configurations (FRCs) formed via rotating magnetic fields (RMFs) serving as end plugs is proposed to improve the mirror's axial confinement. Single-particle orbit calculations suggest that the FRCs in the end plugs can reflect ions back into the central cell if their parallel speeds are not so fast that they can overcome the magnetic field gradient force from the X-point of the FRC to the midplane outside of the FRC. However, this effect is limited and is no different from that of adding a weak mirror cell to the central cell. When the inward Hall electric field generated by the RMFs is considered, an additional Lorentz force emerges that pushes the incoming ions back to the central mirror, thereby dramatically improving the confinement. The Lorentz force is related to the azimuthal drift speed times the radial component of the magnetic field. By surveying the particle phase space of the speeds, we find that this Lorentz force can reflect back or trap >90% of ions escaping from the central mirror given a sufficient Hall electric field in the RMF region. Finally, preliminary experimental results from the Keda Mirror with AXisymmetricity RMF/FRC are reported and show that with the RMFs on, the axial mirror confinement can increase by a factor of ∼1.4 on average.

Rayleigh–Taylor instability (RTI) is a primary hurdle for many different fusion approaches, most of which rely on external pressure to stabilize the plasma by impeding plasma displacement. In this paper, we report a novel method that utilizes a rotating magnetic field (RMF) to drive an azimuthal electron current to reduce the charge separation caused by RTI. The fluctuation measured in the central cell of the mirror device, approximately half a device length away from the RMF, is identified as the m = 1 mode and is suppressed by the RMF in the plug cell. The azimuthal electric fields of the fluctuation are found to decrease to almost zero, and the radial confinement is improved by more than a factor of ten. The separation of the RMF region from the central cell makes this stabilization method unique because the RMF, which can complicate the local magnetic field lines, has little influence on the magnetic field configuration in the central cell. This study may shed light on the use of resonant magnetic perturbations in tokamaks as well as on stabilization methods for many other fusion experiments.Rayleigh–Taylor instability (RTI) is a primary hurdle for many different fusion approaches, most of which rely on external pressure to stabilize the plasma by impeding plasma displacement. In this paper, we report a novel method that utilizes a rotating magnetic field (RMF) to drive an azimuthal electron current to reduce the charge separation caused by RTI. The fluctuation measured in the central cell of the mirror device, approximately half a device length away from the RMF, is identified as the m = 1 mode and is suppressed by the RMF in the plug cell. The azimuthal electric fields of the fluctuation are found to decrease to almost zero, and the radial confinement is improved by more than a factor of ten. The separation of the RMF region from the central cell makes this stabilization method unique because the RMF, which can complicate the local magnetic field lines, has little influence on the magnetic field configuration in the central cell. This study may shed light on the use of resonant magnetic p...

The azimuthal electric field E θ ω, which is induced by the axial oscillating magnetic field B z ω of the odd-parity Rotating Magnetic Field (RMF) scheme, is experimentally found to play an important role in driving the toroidal plasma current. In the odd-parity RMF scheme, E θ ω at the middle plane of the field reversed configuration can generate the so-called punctuated betatron-orbital electrons to drive the toroidal current in addition to the current driven by the transverse component of RMF B r ω, whereas B r ω is the only driving mechanism in the even-parity scheme. In this work, E θ ω (or B z ω) and B r ω are each studied to account for different current driving mechanisms. We report that E θ ω accounts for the more efficient current driving in our experiments.

A Field-Reversed Configuration (FRC) is formed by an in-vessel odd-parity rotating magnetic field (RMF) antenna in a tandem mirror device, Keda mirror with axisymmetricity. The 40-cm diameter antenna is fed independently by four IGBT-based power units with an output current of 1500 A each at 84 kHz, and their phases are adjustable to launch odd or even parity RMFs. A medium-sized washer gun is utilized to generate a highly ionized seed hydrogen plasma. Driven by RMF, the resultant FRC is formed with a separatrix radius of ∼17 cm, external field of ∼50 G, and trapped poloidal flux of ∼0.15 mWb. The formation process of FRCs is evidenced by the measurement of an array of internal two-dimensional probes; a comparison with the rigid rotor model is presented in this paper. In addition, substantial toroidal electron current is found to be driven, despite the partial RMF penetration. Moreover, the dependence of the driven current on the antenna current is reported and is found to be consistent with the RMF current driving model.

In this paper, we describe the engineering work involved in constructing two ion cyclotron resonance heating (ICRH) systems for use in the Keda Mirror with AXisymmetry tandem mirror experiment. Because they offer an effective and robust heating method, ICRH systems have been widely used in a variety of plasma experiments. The goal of our system is to heat the hydrogen plasma contained in the central cell using the fundamental ion cyclotron frequency. Both systems can deliver a radiofrequency power of ∼120 kW with adjustable operating frequencies that are tuned to be slightly lower than their local ion cyclotron frequencies. Two types of antennas are installed in the central cell in an attempt to launch both slow and fast waves. The heating mechanism is reliant on the magnetic beach effect for slow waves.