Megagauss (and higher) DC magnetic fields which have been observed
in experiments with laser-irradiated solid targets may play an
important role in both energy absorption and transport in laser-irradiated-pellet
fausion. The short temporal and spatial scales in laser experiments,
however, make detailed measurements of these magnetic fields difficult.
In microwave-plasma experiments, precise measurements of the growth
and saturation of self-generated magnetic fields were made over
a five orders-of-magnitude variation in incident field intensity.
We report on experiments employing microwaves in larger tenuous
plasmas where precise measurements can be more easily.. We have
investigated the growth and saturation of self-generated magnetic
fields over five-orders-of-magnitude variation in incident field
intensity, and we have investigated the effects of finite-bandwidth
pumps on magnetic field generation.
Microwave radiation(fo ~ 3 Ghz) is launched from a gridded horn along the z axis of a cylindrical unmagnetized plasma (60 cm diam, 80 cm length) produced by a multifilament discharge.
Figure 1 shows the growth of By near the axis as a function of
axial position for a 3 kW rf pulse showing the formation of a
well-defined current sheet in the vicinity of the resonance layer
at early times up to rf turn-off (t=1 usec)
and subsequent magnetic field reconnection resulting in bubble
formation. Radial measurements indicate that the field is apparently
formed by currents parallel to Ex flowing within a
few centimeters of the critical layer.
A variety of interactions occur in the vicinity of the critical layer shortly after the onset of microwave pulses of typical rise time ~ 10nsec. In this experimental, we observe density steepening near the critical layer, electron main-body heating, the appearance of ion waves, exponentially growing high frequency fields characteristic of the tail of the electron distribution. In addition, we observe the growth and diffusion of magnetic fields in plasma.
Figure 2 shows the growth of By near the axis as a function of axial position at a moderate power level (~10-3). The rf was turned on at t=0 and remained on for the times indicated. Radial measurements indicate that the field is apparently formed by currents parallel to EX flowing within a few centimeters of the critical layer. The magnetic field diffuses at a rate consistent with the calculated plasma resistivity . Upon cessation of rf input, the magnetic fields persist for several microseconds, decaying a rate consistent with their spatial extent and the plasma resistivity (see Fig. 1 inset).
Our experimental indicate that near the critical layer, the fields are produced by currents parallel to Ex as one would expect for field generation due to resonance absorption.
The growth of magnetic field as a function of power was investigated. At moderate power levels (<= 10-2) the magnetic field in the overdense and underdense plasma well removed (by a few centimeters) from the critical-density layer monotonically increases in amplitude to a saturated value which remains constant for the longest microwave pulsed (~ 15u) employed. The saturated magnitude of By in the overdense plasma is displayed in Fig.3 as a function of power.
At very low powers (P<50W) the magnetic field increases nearly linearly with incident power. At powers in excess of approximately 50W there is a sharp break in the curve of saturated field versus power together with a sudden increase in saturation time. At higher power levels the saturation time decreases to ~2 microseconds. This behavior is consistent with the onset of parametric instability.
In previous experiments, the effect of infinite pump bandwidth on parametric instability, density-profile modification and hot-electron production was examined.
The saturated level of By as a function of pump bandwidth is shown in Fig.4 at two power levels.
The higher power(340 W) is located just above the region where the magnetic field is a very rapidly increasing function of power, while the lower power is located in the linear regime (B~P).At the higher power, bandwidths as small as 1% yielded significant reduction in the magnetic field. At a lower power level (P~17W), finite bandwidth has a much smaller effect on the magnetic field generation.