Measurements of Correlation-Enhanced Collision Rates

We measure the perp-to-parallel collision rate $\nu_{\perp \|} $ in laser-cooled Magnesium ion plasmas in the strongly-magnetized and correlated regime; and obtain close agreement with the ``Salpeter correlation enhancement'' first studied for fusion in dense plasmas such as stars.\footnote{E.E. Salpeter and H.M. Van Horn, Astrophys. J. {\bf 155}, 183 (1969).} The cyclotron energy, like nuclear energy, is released only through rare close-range collisions. These close collisions are suppressed by strong magnetization, because collisional impact distances are rarely as small as a cyclotron radius $r_c$. However, theory\footnote{D.H.E. Dubin, Phys. Rev. Lett. {\bf 94}, 025002 (2005).} predicts that particle correlations reduce this suppression of collisionality, enhancing the rare close collisions by $e^\Gamma$, where $\Gamma \equiv e^2 / aT$ is the correlation parameter. We control the plasma temperature over the range $4 \! \times \! 10^{-6} < T < 1$eV, giving correlation parameters up to $\Gamma \! \sim \! 20$, with measured collision rates $2 < \nu_{\perp \|} \! < 2 \! \times 10^4$ sec$^{-1}$. At low temperatures, the measured $\nu_{\perp \|}$ are enhanced by up to $10^9$ compared to uncorrelated theory, consistent with the predicted correlation enhancement. When the plasma density is reduced from 2 to 0.12 $ \times 10^7$cm$^{-3}$, the correlations are eliminated and the measured $\nu_{\perp \|}$ agree with uncorrelated theory.