Abstract-We describe an experiment to form and characterize a section of a spherically imploding plasma liner by merging six supersonic plasma jets that are launched by newly designed contoured-gap coaxial plasma guns. This experiment is a prelude to forming a fully spherical imploding plasma liner using many dozens of plasma guns, as a standoff driver for plasma-jet-driven magneto-inertial fusion. The objectives of the six-jet experiments are to assess the evolution and scalings of liner Mach number and uniformity, which are important metrics for spherically imploding plasma liners to compress magnetized target plasmas to fusion conditions. This paper describes the design of the coaxial plasma guns, experimental characterization of the plasma jets, six-jet experimental setup and diagnostics, initial diagnostic data from three-and six-jet experiments, and the high-level objectives of associated numerical modeling.

No abstract

When the driving magnetic pressure profile is unbalanced by the plasma density profile in a coaxial plasma accelerator, a portion of the plasma and associated current sheet may “blow by” the remaining bulk of the plasma. The fast moving plasma creates a significant back electromotive force, draining energy otherwise available for acceleration. The onset of blow-by for an unmagnetized plasma of finite length in a straight coaxial plasma accelerator was studied by exploring systematically the effects of the initial density profile, driving current, plasma temperature, inductance gradient, and electrode radii scale size on the blow-by time. In order to avoid the onset of blow-by, the principal acceleration phase needs to be less than the characteristic blow-by time, which is a function of electrode geometry, plasma conditions, and circuit parameters. For an initially uniform density profile, the total impulse at blow-by time is proportional to the current, plasma mass, jet length, and reciprocal of the electrode radius ratio.

In order to maximize the K-line radiation from imploding gas puff loads driven by high current, pulsed power generators such as the DECADE Module 2 (DM2), initial puff diameters of 5-10 cm are required to provide implosion times of 200-300 ns. To date, such implosions have been found to produce lower-than-predicted yields. Lack of uniformity in the initial current flow through the neutral gas is offered as one of the possible reasons. If so, sufficient pre-ionization of the gas should mitigate this problem.A uv source was designed to pre-ionize a 7-cm diameter argon gas puff to be employed on the DM2 generator at Primex Physics International Company. The source is a 19-cm diameter cylinder consisting of two flashboards bent into semi-circles. The flashboard plasma is directed toward the axis of the cylinder which is co-linear with the axis of the gas nozzle. The plane of the flashboard is located 18 cm from the nozzle exit so that the ionizing uv is emitted at 60-80" angles and promptly illuminates the gas while the flashboard plasma arrives at the PRS load region about 4 ps later. The pre-ionization is measured absolutely using a high-sensitivity interferometer with which we can discriminate between phase shifts due to neutral argon and those due to free electrons as a function of space and time. Pre-ionization levels in the I-10% range were measured near the outer edge of the DM2 nozzle rendering this outer layer sufficiently conductive to carry the DM2 current.In addition to the DM2 application, it is anticipated that a version of this uv pre-ionization scheme will be utilized on neon gas puffs driven by the NRL Hawk generator.An innovative fusion scheme, embodying the principles of magnetized target fusion (MTF), in which the initial magnetized target and a plasma liner containing a cold fuel layer are introduced into the reactor vessel in a stand-off manner, is discussed. Two compact toroids containing fusionable materials are introduced into a spherical reactor target chamber in a diametrically opposing maimer. Embedded in the compact toroids are force-free magnetic fields in Woltjer-Wells-Taylor's state of minimum energy, which are known experimentally to be extraordinarily stable. They collide in the center to form an initial magnetized target plasma. A spherical distribution of plasma jets are then launched fiom the periphery of the vessel, coalescing to form a converging spherical plasma liner. On impact with the central plasma, the plasma liner sends a shock wave through it, shock heating it to some elevated temperature (above 100 eV) which sets the initial adiabat for subsequent compression. The high temperature immediately raises the electrical conductivity of the plasma to the extent that it traps the magnetic flux inside the central plasma. The central plasma is further compressed by the plasma liner and heated nearly adiabatically to conditions for thermonuclear burn, the magnetic flux being compressed with it. The thermal loss rate, greatly reduced by the high magnetic fields, are sufficiently lo...