By use of a streak camera aligned either along the axis of the pinched plasma or at 90 degrees to it, the distance along the axis between first and second compression, h, and the minimum radius of the pinch, r0, have been measured as a function of the plasma current, I, and filling pressure, p0. The plasma current was measured by magnetic-pickup loops calibrated to 35 MHz with Helmholtz coils. The results are that h varies as Ixp0y where x=-0.02+or-0.12 and y=0.25+or-0.13 and that r0 varies as Iap0b where a=-0.18+or-0.11 and b=0.2+or-0.03. Thus within the experimental uncertainties h= alpha r0 where alpha =8.6+or-2.4. These results may be correlated to the stability period tau by use of the zippering effect; the latter predicts the radial collapse velocity nu r should be directly related to the speed nu c of the compression wave traveling along the column. These latter two quantities are found to exhibit the same dependences with I and p0 within less than 10% experimental uncertainties in their power-law slopes. The empirical scaling of tau with p0 and I found is consistent with results of other plasma focus devices and is consistent with ideal MHD stability theory. Experimentally, tau is found to be enhanced over that of a straight cylinder by a factor alpha '=2.7+or-0.6. Two simple models consistent with ideal MHD theory could explain this enhancement.
Alpha-particle heating in an open-field-line plasma