Heating of the on-axis plasma in long-implosion plasma radiation sources

We discuss new opportunities of using long-implosion Z-pinch plasma radiation sources for generating Ar K-shell radiation and harder keV quanta. Until recently it was believed that multi-MA current pulses with 300 to 1000 ns rise time cannot be used for K-shell radiation production because the initial load diameter needed for matching the implosion to the generator must be very large, and the pinch plasma would be distorted by Rayleigh-Taylor instability too much to become an efficient radiator at stagnation. Recent experiments demonstrated that this does not have to be the case: implosions of structured gas-puff loads (shell-on-shell, or shell-on-shell-on gas fill) can produce high quality, tight pinches at stagnation, generating reasonable amounts of keV x-rays. In view of these new developments, the conventional limitations on the PRS performance should be reconsidered. We discuss an approach to keV x-ray generation based upon an analogy with laser fusion, where the imploding shell compressionally heats the low-density inner mass. Our design of a Z-pinch load suitable for producing K-shell emission in long implosions consists of one or two heavy outer shell(s) with low-density on-axis mass (i. e., central gas jet) producing most of the radiation. The heavy outer shell does not need to have a very large initial diameter for its implosion to be matched to the long-pulse current driver. This is because the outer shell is not supposed to radiate, and therefore does not need to have high specific energy characterized by the large eta parameter. Rather, we want to couple a large amount of energy from the driver to this heavy shell to make it slowly implode and eventually convert much of this energy to the thermal energy of the low-density on-axis plasma. ID RMHD simulations for double-shell-Ar-on-Kr-jet demonstrate that the kinetic energy coupled to the outer shell could be efficiently converted into the thermal energy of the on-axis plasma via compressional heating and thermal conduction, achieving sufficiently high values of the effective eta parameter in this plasma rapidly enough to produce K-shell emission and to mitigate the soft x-ray energy losses.