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A new method of measuring the plasma potential using emissive probes is presented; the ‘‘droop’’ method. This method may be used in a steady‐state experiment in which Te≫Tw, where Te and Tw are the electron and filament temperatures, respectively. The method is based on the observation that the floating potential of an emissive probe depends on Tw. A filament temperature which changes in time causes the floating potential also to vary. The proportionality constant between the two is extremely sensitive to whether the probe is above or below the plasma potential. Thus, a floating emissive probe which is allowed to cool can be used to discriminate between voltages above and below the space potential.

This account provides information on the biology of round-leaved mallow, Malva pusilla Sm. (= M. rotundifolia L.). Primarily a weed of gardens and waste places, it has recently become troublesome in field crops causing yield losses of up to 90% in flax and lentil and up to 30% in wheat. Introduced from Eurasia, it is common mainly in the prairie provinces. Although often confused with two other weedy mallows, M. neglecta Wallr. and M. parviflora L., it has a different geographical distribution and can easily be distinguished by its short petals with bearded claws. Malva pusilla propagates by seed which are impermeable to water, exhibiting low germination unless scarified, and therefore capable of remaining dormant for long periods. Plant growth form varies greatly with competition. Round-leaved mallow is difficult to control chemically, but it can be effectively controlled with a fungus which is being developed as a commercial bioherbicide.Key words: Malva neglecta, Malva parviflora, Round-leaved mallow, weed biology

A drift‐cyclotron instability is observed at low magnetic field strengths in a linear multiple mirror. It is characterized by a frequency which decreases with increasing magnetic field strength, propagation in the direction of the electron diamagnetic current, and an azimuthal mode number of 2. The mode is observed when the ion‐cyclotron frequency is less than the ion diamagnetic frequency. The frequency is Doppler‐shifted by an E×B drift. Effects of particles trapped in the mirror cells alter the characteristics of the instability. The frequency decreases slightly and the fluctuation level increases with increasing mirror ratio for mirror ratios less than 1.1, but then levels off at higher mirror ratios. The drift wave is analyzed by a linear theory which treats finite ion gyroradius, an electron temperature gradient, and trapped electrons. The theory predicts three unstable modes, only one of which is consistent with all of the above measurements. This instability is a Doppler‐shifted drift‐cyclotron instability modified by trapped particle effects.

A slide rule has been developed to facilitate rapid calculations based on the Planck radiation formula. Quantities such as the radiant flux density in a given wave‐length region, the spectral radiant flux density at a given wave‐length, or the corresponding quantities expressed in photon units, can be obtained readily for a black body over a range λT=2×102 to λT=4×106 micron degrees with an accuracy of about 1 percent. Simple extension rules can be used for larger values of λT.

The Microwave Tokamak Experiment (MTX) at LLNL is investigating the heating of high density Tokamak plasmas using an intense pulse FEL. Our first experiments, now beginning, will study the absorption and plasma heating of single FEL pulses (20 ns pulse length and peak power up to 2 GW) at a frequency of 140 GHz. A later phase of experiments also at 140 GHz (FY 90) will study FEL heating at 5 kHz rate for a pulse train up to 50 pulses (35 ns pulse length and peak power up to 4 GW). Future operations are planned at 250 GHz with an average power of 2 MW for a pulse train of 0.5 s. The microwave output of the FEL is transported quasi‐optically to the tokamak through a window‐less, evacuated pipe of 20 in diameter, using a six mirror system. Computational modelling of the non‐linear absorption for the MTX geometry predicts single‐pass absorption of 40% at a density and temperature of 1.8×1020 m−3 and 1 keV, respectively. To measure plasma microwave absorption and backscatter, diagnostics are available to measure forward and reflected power (parallel wire grid beam‐splitter and mirror directional couplers) and power transmitted through the plasma (segmented calorimeter and waveguide detector). Other fast diagnostics include ECE, Thompson scattering, soft x‐rays, and fast magnetic probes.The Microwave Tokamak Experiment (MTX) at LLNL is investigating the heating of high density Tokamak plasmas using an intense pulse FEL. Our first experiments, now beginning, will study the absorption and plasma heating of single FEL pulses (20 ns pulse length and peak power up to 2 GW) at a frequency of 140 GHz. A later phase of experiments also at 140 GHz (FY 90) will study FEL heating at 5 kHz rate for a pulse train up to 50 pulses (35 ns pulse length and peak power up to 4 GW). Future operations are planned at 250 GHz with an average power of 2 MW for a pulse train of 0.5 s. The microwave output of the FEL is transported quasi‐optically to the tokamak through a window‐less, evacuated pipe of 20 in diameter, using a six mirror system. Computational modelling of the non‐linear absorption for the MTX geometry predicts single‐pass absorption of 40% at a density and temperature of 1.8×1020 m−3 and 1 keV, respectively. To measure plasma microwave absorption and backscatter, diagnostics are available to measu...

Edge localized mode (ELM) pacing using modulated n = 3 non-axisymmetric fields has been demonstrated on DIII-D over a wide range of conditions, including significant variations in temperature, βN, density and shape. At low collisionality, the pacing results in a clear reduction in the ELM size and peak heat flux to the divertor, up to a factor of 5–6 for short time windows, although only a factor of two for sustained periods with the present hardware capability. At higher collisionality, although similar increases in the ELM frequency have been demonstrated, no meaningful reduction in the heat flux is typically observed as a direct result of the pacing. However, it appears that the ELM size may be reduced indirectly via changes in the L–H power threshold as a result of density pumpout associated with the application of non-axisymmetric fields. At this stage, it remains unclear whether the failure to reduce the ELM size with modulated fields is a limitation associated with high collisionality, high density, or relative proximity to the L–H power threshold.

The concern of Cartier and Merlino regarding ac pickup in the emissive probe technique is not believed to be a problem since the sheath potential is low when the probe is in the emissive condition. This is consistent with both their and our data.

Two drift waves with similar frequencies are observed to be simultaneously unstable under the same plasma parameters. The modes are distinct below a critical magnetic field strength. By varying the mirror ratio and the magnetic field strength the frequencies of the two modes can be made to coalesce. As the critical magnetic field is exceeded the two modes again separate in frequency, but in addition a large‐amplitude, low‐frequency (