Ultrahigh flux of direct laser-accelerated electrons, MeV photons, and neutrons from overdense polymer foams

Ultrabright, well-collimated MeV bremsstrahlung radiation was generated through the interaction of high-current electron beams produced via direct laser acceleration (DLA) with a high- <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>Z</a:mi> </a:math> converter. The DLA mechanism was initiated by a 200 TW, subpicosecond PHELIX laser pulse at a moderately relativistic intensity of (1–2) <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mo>×</c:mo> <c:mspace width="0.1em"/> <c:msup> <c:mn>10</c:mn> <c:mn>19</c:mn> </c:msup> <c:mspace width="0.1em"/> <c:mrow> <c:mrow> <c:mi mathvariant="normal">W</c:mi> </c:mrow> </c:mrow> <c:mspace width="0.1em"/> <c:msup> <c:mi>cm</c:mi> <c:mrow> <c:mo>−</c:mo> <c:mn>2</c:mn> </c:mrow> </c:msup> </c:math> , delivering approximately 60 J of energy into preionized, overcritical-density foam targets. The electron spectrum measured along the laser axis exhibited an effective temperature of approximately <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mn>30</i:mn> <i:mspace width="0.1em"/> <i:mi>MeV</i:mi> </i:math> and energies exceeding 100 MeV, with a total charge of about 300 nC for electrons with energies <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mo>&gt;</l:mo> <l:mrow> <l:mn>1.5</l:mn> </l:mrow> <l:mspace width="0.1em"/> <l:mi>MeV</l:mi> </l:math> (ponderomotive potential), while 100 nC of them is directed along the laser axis within a half-angle cone of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:msup> <o:mn>12</o:mn> <o:mi>∘</o:mi> </o:msup> </o:math> . The directed fraction of DLA electrons with energies exceeding 7.5 MeV carries a charge of 20 to 30 nC, corresponding to a flux up to <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:mn>2</q:mn> <q:mo>×</q:mo> <q:msup> <q:mn>10</q:mn> <q:mn>24</q:mn> </q:msup> <q:mspace width="0.1em"/> <q:msup> <q:mi>sr</q:mi> <q:mrow> <q:mo>−</q:mo> <q:mn>1</q:mn> </q:mrow> </q:msup> <q:mspace width="0.1em"/> <q:msup> <q:mrow> <q:mrow> <q:mi mathvariant="normal">s</q:mi> </q:mrow> </q:mrow> <q:mrow> <q:mo>−</q:mo> <q:mn>1</q:mn> </q:mrow> </q:msup> </q:math> . These high-current relativistic electron beams efficiently generate MeV x-rays, enabling the subsequent production of isotopes, positrons, and neutrons with exceptional yield and application potential. In laser shots employing overcritical-density foam targets placed in front of a high- <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"> <v:mi>Z</v:mi> </v:math> converter, bremsstrahlung photons with energies up to 70 MeV were generated and analyzed via nuclear activation of tantalum and gold. The formation of the isotopes <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"> <x:msup> <x:mi/> <x:mn>174</x:mn> </x:msup> <x:mi>Ta</x:mi> </x:math> and <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"> <z:msup> <z:mi/> <z:mn>190</z:mn> </z:msup> <z:mi>Au</z:mi> </z:math> , whose photonuclear cross-section peaks at approximately 65 MeV, confirmed the presence of high-energy photons. In contrast, no activation was observed in control shots where the laser was directed onto the converter without foam, indicating that high-energy photon generation is intrinsically linked to the DLA process in the preionized foam targets. Autoradiographic measurements revealed a divergence of the bremsstrahlung beam of approximately <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"> <bb:msup> <bb:mn>22</bb:mn> <bb:mi>∘</bb:mi> </bb:msup> </bb:math> (half-angle) in the 14–21-MeV range. These diagnostics indicate an unprecedented photon flux of approximately <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"> <db:mn>2</db:mn> <db:mo>×</db:mo> <db:msup> <db:mn>10</db:mn> <db:mn>22</db:mn> </db:msup> <db:mspace width="0.1em"/> <db:msup> <db:mi>sr</db:mi> <db:mrow> <db:mo>−</db:mo> <db:mn>1</db:mn> </db:mrow> </db:msup> <db:mspace width="0.1em"/> <db:msup> <db:mrow> <db:mrow> <db:mi mathvariant="normal">s</db:mi> </db:mrow> </db:mrow> <db:mrow> <db:mo>−</db:mo> <db:mn>1</db:mn> </db:mrow> </db:msup> </db:math> , corresponding to about <ib:math xmlns:ib="http://www.w3.org/1998/Math/MathML" display="inline"> <ib:msup> <ib:mn>10</ib:mn> <ib:mn>11</ib:mn> </ib:msup> </ib:math> photons per shot with energies exceeding 7.5 MeV. The conversion efficiency of focused laser energy to bremsstrahlung photons is greater than 1% (within the FWHM of x-ray beam and laser beam). More than <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline"> <kb:mn>2</kb:mn> <kb:mo>×</kb:mo> <kb:msup> <kb:mn>10</kb:mn> <kb:mn>9</kb:mn> </kb:msup> </kb:math> photoneutrons per shot were emitted isotropically, corresponding to a peak flux of <mb:math xmlns:mb="http://www.w3.org/1998/Math/MathML" display="inline"> <mb:mn>2</mb:mn> <mb:mo>×</mb:mo> <mb:msup> <mb:mn>10</mb:mn> <mb:mn>20</mb:mn> </mb:msup> <mb:mspace width="0.1em"/> <mb:msup> <mb:mi>cm</mb:mi> <mb:mrow> <mb:mo>−</mb:mo> <mb:mn>2</mb:mn> </mb:mrow> </mb:msup> <mb:mspace width="0.1em"/> <mb:msup> <mb:mrow> <mb:mrow> <mb:mi mathvariant="normal">s</mb:mi> </mb:mrow> </mb:mrow> <mb:mrow> <mb:mo>−</mb:mo> <mb:mn>1</mb:mn> </mb:mrow> </mb:msup> </mb:math> ( <rb:math xmlns:rb="http://www.w3.org/1998/Math/MathML" display="inline"> <rb:mn>4</rb:mn> <rb:mo>×</rb:mo> <rb:msup> <rb:mn>10</rb:mn> <rb:mn>18</rb:mn> </rb:msup> <rb:mspace width="0.1em"/> <rb:msup> <rb:mi>cm</rb:mi> <rb:mrow> <rb:mo>−</rb:mo> <rb:mn>2</rb:mn>