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 [$n_0=(1.5-4)\mathrm{}\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$] show distinct radial filaments that are due to flute instabilities of high azimuthal-mode number. The observed wavelengths are of the order $\frac{c}{{\omega }_{p_i}}$ 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 $\beta =1\mathrm{}$ plasma column and has a major influence on its behavior.