An Alternative Concept for the Structure of an Argon Z-pinch

Summary form only given. Several recent observations suggest an alternative view of the structure of a z-pinch. First, it is well known that for K-shell emitting z-pinches, pulse width and pinch size seem linearly correlated. Implosions driven by currents of 3.5 to >15 MA continue to show such a width/size correlation. The implied "velocity" of this correlation is ~40 cm/mus, and it is not a strong function of the generator (i.e., peak current). It is close to the 43 cm/mus that is the minimum ("eta" =1) needed to excite the argon K-shell. But the nominal "eta" for these implosions is >3.5 or 84 cm/mus. If we treat this implied '"velocity" as a sound speed, the corresponding temperature is 0.25 keV which is well below the >1.5 keV derived from emission spectra. Second, on a given generator, peak K-power seen in axially-resolved imaging data is not strongly correlated with the K yield. Third, we sometimes see very narrow "filaments" (as small as 100 microns diameter) of intense 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 within 10 ns of the start of K emission correlate with reduced K yield. These observations can be explained if pinch size is due to the transverse (radial) motion of the filaments. That is, the filaments are kink-unstable, with transverse velocity of order ~40 cm/mus. Hence a large pulse width pinch shows no filaments because the time-integrated pinhole image is blurred by motion of the filament(s). Thus the basic K radiating "elements" of a pinch are a few thin (~0.1 mm), long (>10 mm), high-density filaments. Each filament radiates at a relatively fixed peak power. Implied ion densities in a filament are >10
²⁰
/cc with actual temperatures that are well below the 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 "sound speed". What controls the filament lifetime? Is a continued push from the magnetic field needed'.' Observed late time power flow losses reduce the field, hence the filament lifetime, hence K yield. This interpretation of a pinch is in contrast to the notion of a many millimeter wide column of plasma that simply radiates away its energy accumulated during the implosion. Other consequences of this alternative view will be discussed.