Length scaling of dynamic-hohlraum axial radiation

Radiation generated within a 10-mm-long foam-target DH (dynamic hohlraum) is used for high-temperature (<200 eV) radiation-flow and inertial-confinement-fusion studies [Sanford et al., Phys. Plasmas 9, 3573 (2002)]. The length of this DH is varied from 5 to 20 mm, keeping the mass/unit length constant in an effort to study the scaling of axial radiation power with length, and better understand its production. Measurements show a greater variation in this power with length than would be expected from simple arguments [Slutz et al., Phys. Plasmas 8, 1673 (2001)]. Maximum axial power of ∼10 TW is produced with a length of ∼7.5 mm, similar to the typical power for the baseline 10 mm DH. The decreasing axial power (at a rate of ∼0.65 TW per mm at longer lengths) is bounded by radiation-magnetohydrodynamic simulations [Peterson et al., Phys. Plasmas 6, 2178 (1999)] that include the development of the magnetic Rayleigh-Taylor instability in the r-z plane. The dramatic drop in axial power below 7.5 mm, by contrast, was unanticipated. This decrease suggests the presence of differing mechanisms for limiting power at short and long lengths