Orbital-angular-momentum-enhanced laser absorption and neutron generation

We experimentally demonstrate enhanced absorption of near-relativistic optical vortex beams in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi mathvariant="normal">D</a:mi><a:mn>2</a:mn></a:msub><a:mi mathvariant="normal">O</a:mi></a:mrow></a:math> plasmas to generate a record fast-neutron yield of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mn>1.45</d:mn><d:mo>×</d:mo><d:msup><d:mn>10</d:mn><d:mn>6</d:mn></d:msup></d:mrow></d:math> n/s/sr. Beams with a topological charge of 5 were shown to deliver up to a 3.3 times enhancement of fast-neutron yield over a Gaussian focus of the same energy and an order of magnitude higher intensity. This result was achieved via mJ-class, high-repetition-rate (480 Hz) laser of 16 mJ and a pulse duration of 67 fs striking a free-flowing, 15-<e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>µ</e:mi><e:mi mathvariant="normal">m</e:mi></e:mrow></e:math> diameter heavy-water stream. Three-dimensional Particle-in-Cell simulations show unique vortex pulse filamentation and subsequent laser absorption. The orbital angular momentum beam evolution within the nonuniform plasma column leads to the excitation of sheared plasma waves, resulting in net transfer of angular momentum, enhanced volumetric heating, and subsequently increased D-D reactions.