Performance Scaling in Magnetized Liner Inertial Fusion Experiments

We present experimental results from the first systematic study of performance scaling with drive parameters for a magnetoinertial fusion concept. In magnetized liner inertial fusion experiments, the burn-averaged ion temperature doubles to 3.1 keV and the primary deuterium-deuterium neutron yield increases by more than an order of magnitude to $1.1\times {10}^{13}$ (2 kJ deuterium-tritium equivalent) through a simultaneous increase in the applied magnetic field (from 10.4 to 15.9 T), laser preheat energy (from 0.46 to 1.2 kJ), and current coupling (from 16 to 20 MA). Individual parametric scans of the initial magnetic field and laser preheat energy show the expected trends, demonstrating the importance of magnetic insulation and the impact of the Nernst effect for this concept. A drive-current scan shows that present experiments operate close to the point where implosion stability is a limiting factor in performance, demonstrating the need to raise fuel pressure as drive current is increased. Simulations that capture these experimental trends indicate that another order of magnitude increase in yield on the $Z$ facility is possible with additional increases of input parameters.

Figure 1

(a) Cross section of the target configuration used in these experiments. The laser enters the target through the laser-entrance-hole (LEH) window at $z=12.6\mathrm{mm}$. The imploding portion of the target spans from $z=0$ to 10 mm. The target aspect ratio is defined as the outer radius divided by the wall thickness ($\mathrm{AR}=r_O/\mathrm{\Delta }r$). (b),(c) Cross sections of the load configurations used in the experiments conducted at 16 and 20 MA, respectively. The target is shown in gray, fuel in yellow, anode in blue, cathode in red, coil windings in orange, internal coil support in cyan, external coil support in dark gray, and laser in green. With a single coil above the target, the amplitude of the axial field in (c) decreased from the top to the bottom of the target by 30%. The average field is reported for experiments using this configuration. (d) Plot of the load current (determined with velocimetry [34]) for the two transmission line geometries. The transition to dashed lines after peak current indicates reduced fidelity of the velocimetry measurement due to relaxation of the drive pressure. The higher inductance and smaller anode-cathode gap geometry (b) has greater current loss, which impacts the shape and peak of the current pulse.

Figure 2

Plot of the primary neutron yield as a function of ion temperature for all 10 mm tall, 5.58 mm outer diameter, $\mathrm{AR}=6$ MagLIF experiments (squares) with the reactivity parameter for $\mathrm{DD}\text{−}{}^3\mathrm{He}$ fusion [37] scaled by $3.9\times {10}^{38}$ overlaid (dashed line) for comparison. Experiments that utilized protocols to reduce radiative loss (magenta) tended to have higher ion temperatures and consequently neutron yields than those that did not (teal). Increasing from 0.68 to $1.0\mathrm{mg}/{\mathrm{cm}}^3$, 10.4 to 15.9 T, 0.46 to 1.2 kJ, and 16 to 20 MA doubled the ion temperature and increased the yield by an order of magnitude.

Figure 3

(a),(b) Plots of primary neutron yield and burn-averaged ion temperature as a function of laser preheat energy absorbed in the fuel, respectively, showing increasing performance followed by a plateau. (c) Plot of the magnetic field-radius product (BR) as a function of preheat energy showing a decay in magnetization at stagnation as preheat energy increases, which is consistent with increasing magnetic flux loss due to the Nernst effect. Experimental values (solid squares) and simulation results (dashed lines) are shown. An experiment with zero preheat energy produced ${10}^{10}$ primary neutrons (about 3 times the detection threshold) but provided an insufficient signal to determine the ion temperature or BR. The ion temperature for this experiment (shown as an open square) was estimated using the reactivity curve in Fig. 2 and the measured yield.

Figure 4

Plot of primary neutron yield as a function of applied magnetic field. Experiments with dielectric-coated targets that have been mass matched to $\mathrm{AR}=9$ targets (cAR9) [40] are shown as solid triangles, and 2D lasnex simulations with AR9 targets (scaled by $1/8$) are plotted as inverted open triangles. The CR in these simulations was between 46 and 49, resulting in a simulation-to-experimental-yield ratio of approximately 8.

Figure 5

(a),(b) X-ray self-emission images obtained with a spherical crystal imager in experiments with 16 and 20 MA peak load currents, respectively, while other parameters remained fixed. The transverse spatial resolution was approximately 40 and $20\mu \mathrm{m}$ for (a) and (b), respectively, which accounts for some of the increased structure in (b). (c) Plot of the inferred radius [30] as a function of axial position (solid lines) and average radius (dashed lines) for the two experiments.