The ZaP Flow Z-Pinch project is experimentally studying the effect of sheared flows on Z-pinch stability It has been shown theoretically that when dVz∕dr exceeds 01kVA the kink (m=1) mode is stabilized [U Shumlak and C W Hartman, Phys Rev Lett 75, 3285 (1995)] Z pinches with an embedded axial flow are formed in ZaP with a coaxial accelerator coupled with a 1m assembly region Long-lived, quiescent Z pinches are generated throughout the first half cycle of the current During the initial plasma acceleration phase, the axial motion of the current sheet is consistent with snowplow models Magnetic probes in the assembly region measure the azimuthal modes of the magnetic field The amplitude of the m=1 mode is proportional to the radial displacement of the Z-pinch plasma current The magnetic mode levels show a quiescent period which is over 2000 times the growth time of a static Z pinch The axial velocity is measured along 20 chords through the plasma and deconvolved to provide a radial profile U

Formation of a sheared flow Z pinch

An optical interferometer matrix which includes a reference waveguide (10) for optically transmitting a reference signal. The optical interferometer matrix further includes a signal source waveguide (12) for optically transmitting a source signal which has been modulated with respect to the reference signal in response to an applied physical effect. The physical effect may be, for example, thermal, acoustic, hydrodynamic, electrical, electro-mechanical or magnetic. The reference and signal waveguides (10, 12) are then optically coupled to one another at two different points (38, 40) so that interference between the light in the waveguides occurs under two different phase conditions, with a phase difference between these conditions which approaches pi/2 radians. In the preferred embodiment, the two points of optical coupling are confined in a small space so that thermal- or pressure-induced differential phase changes of light between the first and second coupling points (38, 40) are negligable.

Quadrature fiber-optic interferometer matrix

Turbulence has been studied in laboratory plasmas for decades. Magnetic and electrostatic turbulence fluctuations have been implicated in degraded confinement in fusion devices so understanding turbulent transport is critical for those devices. The externally applied magnetic field in most laboratory plasmas has a strong effect on the character of the turbulence (particularly parallel and perpendicular to the applied field). A new turbulent plasma source is described with several unique features. First, the magnetohydrodynamic (MHD) wind tunnel configuration has no applied magnetic field and has no net axial magnetic flux. Second, the plasma flow speed is on the order of the local sound speed (M = 1), so flow energy is comparable to thermal energy. Third, the plasma β (ratio of thermal to magnetic pressure) is of order unity so thermal energy is comparable to magnetic energy. We will review sources of magnetic turbulence in laboratory plasmas and discuss the main analytical tools used in the study of plasma turbulence. Some initial results from the MHD plasma wind tunnel will be presented.

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Laboratory sources of turbulent plasma: a unique MHD plasma wind tunnel

An interferometer has been developed with which it is possible to make simultaneous measurements of plasma density along several chords. A single Bragg cell and laser are used, and the phase detection is in quadrature in order to obtain unambiguous and uniformly sensitive readings.

Heterodyne quadrature interferometer for simultaneous measurements of plasma density along several chords

Through compact toroid (CT) injection experiments on the TEXT-U tokamak (with BT simeq 10 kG and IP simeq 100 kA), it has been shown that the acceleration electrode configuration, particularly in the vicinity of the toroidal field (TF) coils of the tokamak, has a strong effect on penetration performance. In initial experiments, premature stopping of CTs within the injector was seen at anomalously low TF strengths. Two modifications were found to greatly improve performance: (a) removal of a section of the inner electrode and (b) increased diameter of the 'drift tube' (which guides the CT into the tokamak after acceleration). It is proposed that the primary drag mechanism slowing CTs is toroidal flux trapping, which occurs when a CT displaces transverse TF trapped within the flux conserving walls of the acceleration electrodes (or drift tube). For a simple two dimensional (2-D) geometry, a magnetostatic analysis produces a CT kinetic energy requirement of 1/2ρv2 ≥ α(B02/2μ0), with α = 2/(1-a2/R2) a dimensionless number that is dependent on the CT radius a normalized by the drift tube radius R. For a typical CT, this can greatly increase the required energies. A numerical analysis in 3-D confirms the analytical result for long CTs (with length L such that L/a gtrsim 10). In addition to flux trapping, the CT shape is also shown to affect the energy criterion. These findings indicate that a realistic assessment of the kinetic energy required for a CT to penetrate a particular tokamak TF must take into account the interaction of the magnetic field with the electrode walls of the injector.

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Effects of CT injector acceleration electrode configuration on tokamak penetration

An interferometer has been developed with which it is possible to make accurate plasma density measurements in the presence of density gradients. A modified Mach–Zehnder interferometer employing a linearly polarized scene beam and a circularly polarized reference beam generates signals proportional to the sine and cosine of the phase shift introduced by the plasma. The differential sensitivity of the phase shift measurements is independent of the instantaneous phase angle, and the phase angle may be unambiguously followed through multifringe excursions of the interferometer. The phase shift measurement is also independent of fringe contrast fluctuations caused by the refractive bending of the scene beam by the plasma.

A. R. Jacobson

Papers

Quadrature interferometer for plasma density measurements