Rotamak Plasmas Research Articles

High sensitivity, low noise Mirnov coil array on Prairie View rotamak

An array of 32 Mirnov coils with novel features of high sensitivity and low noise has been installed on the outside chamber surface of Prairie View rotamak. This B(R)-oriented coil array has proven to be very reliable in the plasma driven by rotating magnetic field; it can resolve magnetic perturbation signals of 0.1 G. With this new diagnostic, the n = 1 tilt, radial shift, and kink modes are observed for the first time in rotamak plasmas.

Effect of toroidal magnetic field on n = 1 mode stability in rotamak plasmas

To study the effect of toroidal magnetic field on n = 1 mode stability, a series of experiments with linearly ramping the axial current Iz, which makes field-reversed configuration (FRC) to spherical tokamak (ST) transition, have been conducted in rotamak. Results clearly demonstrate that the tilt mode can be completely suppressed by small Iz around 0.4 kA (in comparison with 2.0 kA plasma current). An unknown new mode with larger magnetic perturbations is triggered when Iz reaches 0.5 kA. This instability mode keeps saturation while plasma current is boosted when Iz is in the range of 0.6–1.4 kA. When Iz exceeds 1.6 kA, the new mode suddenly disappears and discharge is free from instability modes.

Experiments on rotamak plasma equilibrium and shape control

A set of magnetic shaping coils and copper rings is installed in cylindrical chamber rotamak to allow for an active equilibrium control in 40 ms plasma discharges. The coils, which are powered by programmable current source, are used to control both the plasma shape and the boundary poloidal magnetic flux. Without the active equilibrium control, the boundary flux drops from its vacuum value of 0.3 mWb to zero after the plasma current is generated. If the coils are activated, the boundary magnetic flux can be sustained within the 0.2–0.3 mWb range, thus keeping the separatrix away from chamber wall during whole period of the shot. The passive copper rings help in eliminating the fast variations of the boundary magnetic flux. The response of rotamak plasma to the active equilibrium control is drastically different in regimes with or without external toroidal field. A model is presented that describes the change in plasma shape, plasma current, and pressure under the effect of active equilibrium coils.

An Extreme Ultraviolet Source for Photolithographic Applications Based on Rotamak Discharge

Xenon plasma produced in the Kansai Rotamak has been used to generate about 66 W of extreme ultraviolet (EUV) radiation. The plasma in the Kansai Rotamak is produced using a rotating magnetic field (RMF) generated using a pair of 200 kHz/180 kW oscillators. The plasma can be sustained for a period of 5 ms limited only by the duration of applied RMF. The rotamak plasma discharge is electrodeless and its geometry is sufficiently simple to allow easy access to the emitted EUV radiation.

Comparison of rotamak plasma discharges in cylindrical and spherical devices

A new cylindrical chamber rotamak with one magnetic coil added in the midplane was constructed, and a series of experiments were performed at this device for comparison with the experiments previously conducted at a spherical chamber device. The experiments have been conducted in two regimes: the field-reversed configuration (FRC) and the spherical tokamak (ST) configuration when a steady toroidal magnetic field is added. The results from the new device further confirm that within a certain range of the equilibrium magnetic field Bv, the plasma current driven by a rotating magnetic field (RMF) grows linearly with Bv in both FRC and ST rotamak discharges. The ST configuration is more stable and allows one to achieve higher plasma current Ip with a proper choice of the toroidal field (TF); plasma current is found to reach its maximum at an optimum value of the applied TF. The measured plasma current, electron density and temperature in the cylindrical device are lower than those measured in the spherical device. The quadrupole structure of the self-generated toroidal magnetic field in the FRC regime is observed in both devices but with different magnitudes. In the cylindrical device, when a current is applied in the middle coil, plasma current can be enhanced up to 250% depending on the initial conditions.

The effect of toroidal field on the rotating magnetic field current drive in rotamak plasmas

A rotamak is one kind of compact spherically shaped magnetic-confinement device. In a rotamak the plasma current is driven by means of rotating magnetic field (RMF). The driven current can reverse the original equilibrium field and generate a field-reversed-configuration. In a conventional rotamak, a toroidal field (TF) is not necessary for the RMF to drive plasma current, but it was found that the present of an additional TF can influence the RMF current drive. In this paper the effect of TF on the RMF current drive in a rotamak are investigated in some detail. The experimental results show that addition of TF increases the RMF driven current greatly and enhances the RMF penetration dramatically. Without TF, the RMF can only penetrate into plasma in the edge region. When a TF is added, the RMF can reach almost the whole plasma region. This is an optimal strength of toroidal magnetic field for getting maximum plasma current when Bv and radio frequency generator power are fixed. Besides driving current, the RMF generates high harmonic fields in rotamak plasma. The effect of TF on the harmonic field spectra are also reported.

Comparison of rotamak plasmas in FRC and ST configurations

The results of experiments in a newly rebuilt rotamak are presented. A comparison is made for two types of operations: the common field-reversed configuration (FRC) produced by driving plasma current with a rotating magnetic field (RMF), and the spherical tokamak (ST) configuration when a steady toroidal magnetic field is added. In both cases the driven plasma current develops two current rings, but in the ST configuration the inner ring current density is about three times larger than that in the FRC case. The addition of the toroidal field enables the ST to overcome the current limit for a given radio-frequency power. The total plasma current is found to have a peak at an optimum value of the applied toroidal field; the optimum toroidal field depends on the value of the equilibrium magnetic field. With proper choice of the toroidal field magnitude, the plasma current can be enhanced two- or three-fold in comparison with the limits in the cases of too a small or too large toroidal field. In the rotamak–ST, the temperature of electrons is increased by at least 50%. The density profile is triangular-shaped in the ST case and almost uniform in the FRC. The measurements of the oscillating fields indicate that the penetration of RMF into plasma is greatly enhanced in the ST case. The analysis of RMF profiles in the plasma supports the hypothesis that the improved penetration is due to the excitation of a whistler wave mode.

Current drive for rotamak plasmas

Experiments which have been undertaken over a number of years have shown that a rotating magnetic field can drive a significant non-linear Hall current in a plasma. Successful experiments of this concept have been made with a device called rotamak. In its original configuration this device was a field reversed configuration without a toroidal magnetic field but with a vertical field to establish the MHD equilibrium. However, modifications have shown that current can also be driven if a central current-carrying rod is used to provide an applied toroidal field. The new rotamak has then a spherical tokamak magnetic field structure.

High harmonic fields in a rotamak plasma

High harmonic oscillations of the magnetic fields in a rotamak plasma are measured for the first time in experiments. Experimental results indicate that besides the fundamental rotating magnetic field, harmonic fields are stimulated simultaneously. The features of the harmonics in the discharges with and without toroidal field are compared. In the case without toroidal field the harmonics are dominated by odd orders, as expected by theory. The presence of toroidal field enhances the even harmonics greatly. The ratio of amplitudes of second harmonic and the fundamental one can be as high as 45% in this case. Numerical simulation confirms that the Hall term J×B is the cause of the harmonic generation.

Investigation of the localized co-interchange instabilities in a rotamak plasma

If the rotamak is regarded as a spherical field-reversed mirror then, according to conventional ideal MHD analysis, it should be unstable to co-interchange modes localized near the vortex point of the magnetic field. It is shown that to study these instabilities in a typical rotamak plasma, the Hall term in Ohm's law cannot be ignored. The effect of the Hall term on the ideal MHD analysis of co-interchange modes is investigated and a stability criterion is derived.

Effect of a Rotating Magnetic Field on the Tilting Instability of a Prolate Rotamak

The effect of a rotating magnetic field on the stability of a rotamak is investigated. Using a simple equilibrium model and the linearised equations of ideal magnetohydrodynamics, it is shown that a rotating field can be a contributing factor in the stabilisation of the tilting instability of a slightly prolate rotamak plasma.

Power Balance Models for Rotamak Plasmas

Energy transfer processes are calculated for a range of rotamak plasmas which are modelled by equilibria with plasma pressure proportional to either the poloidal flux function or the square of this function. The latter case is more realistic, so it is used to study the conditions for which global power balance can be obtained for both the electrons and the ions. The absence of a toroidal magnetic field means that there is no neo-classical transport. Provided that the particle and energy diffusion remains classical when the plasma temperature increases, the energy confinement time is predicted to scale as n3/2 R8, where n is the density and R is the radius of the separatrix. As the plasma temperature increases, the dominant energy loss process changes from plasma recycling to electron thermal conductivity and then to ion thermal conductivity.

Calculation of the axisymmetric eigenmodes of a field reversed mirror

The axisymmetric eigenmodes of a field reversed mirror plasma are investigated using numerical solution of the ideal MHD wave equation. Ignoring the azimuthal component, the wave equation is reduced to a one-dimensional eigenvalue problem involving an infinite system of coupled second order differential equations. A numerical method for solving this system is given. A variety of eigenmodes are obtained and comparison is made with recently observed oscillations of a rotamak plasma.

Magnetohydrodynamic equilibrium conditions for a plasma in the presence of a rapidly rotating magnetic field

The ideal magnetohydrodynamic equations are used to investigate the equilibrium configurations of a plasma when it is subjected to an external rotating magnetic field. It is shown that the rigid rotor model for the steady toroidal current is an exact solution of the equations provided that there are no oscillating secondary currents within the plasma and that there is no toroidal component of the steady magnetic field. Experimental observations of rotamak plasmas indicate that these conditions are often not satisfied. The rigid rotor model may therefore not be applicable to these plasmas. A perturbation method is used to generalise the theory to include the effects of small oscillating secondary currents and small toroidal magnetic fields.