Flute Instabilities during Fast Magnetic Compression of Collisionlessβ=1Plasmas

End-on framing photographs taken at the Garching 500-kV theta pinch during fast magnetic plasma compression [${n}_{0}=(1.5\ensuremath{-}4)\ifmmode\times\else\texttimes\fi{}{10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$] show distinct radial filaments that are due to flute instabilities of high azimuthal-mode number. The observed wavelengths are of the order $\frac{c}{{\ensuremath{\omega}}_{{p}_{i}}}$ they decrease with increasing density, but no mass dependence is found. The flute instabilities are responsible for the existence of a plasma halo which surrounds the thermonuclear $\ensuremath{\beta}=1$ plasma column and has a major influence on its behavior.