Achieving a symmetric, low adiabat CH implosion in a low-gas fill hohlraum*

To reach ignition in inertial confinement fusion (ICF), the burning central fuel must be surrounded by a symmetric shell of high areal mass, to maximally capture the energy of the produced fusion products.  In indirect drive, this needs a symmetric hohlraum x-ray drive during the entire laser pulse, a requirement that is challenging in low-gas fill hohlraums (proven to reduce Raman and Brillouin backscatter losses) driven by long laser pulses, typically used in high-compression low-adiabat CH targets.  Specifically, the control of the low-order mode 2 is difficult due to hohlraum wall expansion limiting the inner beam transport, and outer beams may be stopped by the endcap blow-off.  Additionally, larger excursions of the outer beam spots lead to significant swings in mode 4 which may cause severe shell areal density variations at stagnation. In this talk, we contend, using simulations, that these problems can be efficiently overcome by larger laser-entrance-holes and outer beam time staggering.  Furthermore, we show that the dynamics of laser-induced wall ablation is different for pulses with long troughs, permitting an effective control of inner beam propagation.  As a result, we show that a symmetric high-compression CH design driven by a 1 MJ-class laser is possible.