An Alternative Concept for the Structure of an X-Ray Emitting $Z$ -Pinch

We propose a more nuanced view of the structure of the X-ray emitting plasma in a
Z
-pinch. There are four relevant observations. First, in K-shell emitting
Z
-pinches, K-shell pulsewidth and K-shell pinch diameter seem linearly correlated. Second, on a given generator, peak K-power is not strongly correlated with the K-yield. Third, we sometimes see very narrow ?filaments? (as small as 100-?m in diameter, using a camera based on digital imaging plates) of intense K-shell emission that can extend for over a centimeter axially. Yet, on most shots, the observed pinch structure is quite amorphous; pinches with large average diameter show the least structural detail. Fourth, power flow losses upstream of the pinch within 10 ns of the start of K emission correlate with reduced K-yield. These observations can be reconciled if apparent pinch ?diameter? is actually due to the transverse (radial) motion of the filaments. That is, the filaments are kink unstable, with transverse velocity on the order of ~ 40 cm/?s (in argon). Hence, a large pulsewidth pinch shows no filaments because time-integrated pinhole images are blurred by motion of the filament(s). Thus, the basic K radiating ?elements? of a pinch are a few thin (~ 0.1 mm), long (few millimeters), and high-density filaments that can propagate for many millimeters axially. Each filament radiates at a relatively fixed peak power. Implied ion densities in a filament are > 10
²⁰
/cm
³
with actual temperatures that are well below values commonly reported (> 1.5 keV). Overall, yield will depend on the duration of the pinch which is due to the lifetime of the filament(s) rather than a transit time based on a ?sound speed.? The continued push from the magnetic field controls the filament lifetime. Hence, late time power flow losses can reduce the field and hence the filament lifetime and the K-yield. This interpretation of a pinch is in contrast to the notion of a many-millimeter-wide column of hot plasma that simply radiates away the energy that was accumulated during the implosion.