Using nuclear data and Monte Carlo techniques to study areal density and mix in D2 implosions

Abstract

Measurements from three classes of direct-drive implosions at the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] were combined with Monte-Carlo simulations to investigate models for determining hot-fuel areal density (ρR hot ) in compressed, D 2 -filled capsules, and to assess the impact of mix and other factors on the determination of ρR hot . The results of the Monte-Carlo calculations were compared to predictions of simple commonly used models that use ratios of either secondary D 3 He proton yields or secondary DT neutron yields to primary DD neutron yields to provide estimates ρR hot,p or ρR hot,n , respectively, for ρR hot . For the first class of implosions, where ρR hot is low (≤ 3 mg/cm 2 ), ρR hot,p and ρR hot,n often agree with each other and are often good estimates of the actual ρR hot . For the second class of implosions, where ρR hot is of order 10 mg/cm 2 , ρR hot,p often underestimates the actual value due to secondary proton yield saturation. In addition, fuel-shell mix causes ρR hot,p to further underestimate, and ρR hot,n to overestimate, ρR hot . As a result, values of ρR hot,p and ρR hot,n can be interpreted as lower and upper limits, respectively. For the third class of implosions, involving cryogenic capsules, secondary protons and neutrons are produced mainly in the hot and cold fuel regions, respectively, and the effects of the mixing of hot and cold fuel must be taken into account when interpreting the values of ρR hot,p and ρR hot,n . From these data sets, we conclude that accurate inference of ρR hot requires comprehensive measurements in combination with detailed modeling. a) Also visiting Senior Scientist,

Effects of magnetization on fusion product trapping and secondary neutron spectra

“…In unmagnetized, spherical capsule implosions of pure deuterium fuel, the ratio of primary-to-secondary neutron yields was shown to be proportional to fuel qR in the limit that the tritons do not significantly slow down before they leave the fuel. [26][27][28] In this limit, the probability of a DT reaction occurring is P DT / hq D 'ir DT , where ' is the path length of an average triton within the fuel, r DT is the DT fusion crosssection at a triton energy of 1 MeV, q D is the background deuterium density and the angle brackets indicate an ensemble average over the tritons. In the hot-spot approximation, 27 all tritons are born at the center of a uniform, spherically symmetric mass of deuterium, and hq D 'i q D R.…”

“…In addition, the LFP collisional model provides a complete account of the influence of the background plasma on the fast tritons. We avoid the use of simplified stopping power models, for example, which are commonly used in secondary reaction calculations, [26][27][28]32 and instead employ a description that retains energy drag, diffusion, and pitch-angle scattering due to all the plasma constituents. So, while the slowing-down physics, particularly between fast ions and plasma electrons, captured in a stopping power model indeed dominates the collisional processes for a significant portion of the triton's life-cycle, the role played by other parts of the full collision operator increases as the triton loses energy.…”

Understanding Fuel Magnetization and Mix Using Secondary Nuclear Reactions in Magneto-Inertial Fusion

“…Analysis ofȲ and the anisotropies in the secondary neutron energy spectra provide two relatively independent methods to obtain estimates of the volumeaveraged magnetization, leading unambiguously to the conclusion that the first MagLIF experiments achieved significant fuel magnetization during burn. In addition, secondary yields are known to correlate with mix in unmagnetized ICF [25,26]. We show thatȲ can constrain the amount of volumetric mix during burn in MIF, providing evidence that MagLIF experiments also achieved a relatively low-mix hot spot.…”

“…Near the collisionally thin limit, a modest increase inȲ occurs with mix, since σ DT increases as the triton slows [25,26]. One might suspect, then, based on Fig.…”

Investigation of ion kinetic effects in direct-drive exploding-pusher implosions at the NIF

“…[51][52][53] For shot N110131, a secondary-proton yield of 2.0±0.5×10 8 (see Figure 2b) and a primary DD-neutron yield of 3.0±0.3×10 11 gives a fuel ρR of 4.6±1.1 mg/cm 2 , using a model of uniform fusion production throughout the fuel. 52 Similar measurements give a fuel ρR of 3.6±1.1 mg/cm 2 for shot N120328 and 4.6±1.1 mg/cm 2 for shot N130129.…”