Laser-driven ion and electron acceleration from near-critical density gas targets: Towards high-repetition rate operation in the 1 PW, sub-100 fs laser interaction regime

Abstract

Ion acceleration from gaseous targets driven by relativistic-intensity lasers was demonstrated as early as the late 1990s, yet most of the experiments conducted to date have involved picosecond-duration, Nd:glass lasers operating at low repetition rate. Here, we present measurements on the interaction of ultraintense <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>(</a:mo><a:mo>≈</a:mo><a:msup><a:mn>10</a:mn><a:mn>20</a:mn></a:msup><a:mspace width="0.28em"/><a:mi mathvariant="normal">W</a:mi><a:mspace width="0.16em"/><a:msup><a:mi>cm</a:mi><a:mrow><a:mo>−</a:mo><a:mn>2</a:mn></a:mrow></a:msup><a:mo>,</a:mo><a:mo> </a:mo><a:mn>1</a:mn><a:mo> </a:mo><a:mi>PW</a:mi><a:mo>)</a:mo></a:mrow></a:math>, ultrashort <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mo>(</e:mo><e:mo>≈</e:mo><e:mn>70</e:mn><e:mspace width="0.28em"/><e:mi>fs</e:mi><e:mo>)</e:mo></e:mrow></e:math> Ti:Sa laser pulses with near-critical <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mo>(</g:mo><g:mo>≈</g:mo><g:msup><g:mn>10</g:mn><g:mn>20</g:mn></g:msup><g:mspace width="0.28em"/><g:msup><g:mi>cm</g:mi><g:mrow><g:mo>−</g:mo><g:mn>3</g:mn></g:mrow></g:msup><g:mo>)</g:mo></g:mrow></g:math> helium gas jets, a debris-free targetry with the potential for future compatibility with high <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mo>(</i:mo><i:mo>≈</i:mo><i:mn>1</i:mn><i:mo> </i:mo><i:mi>Hz</i:mi><i:mo>)</i:mo></i:mrow></i:math> repetition rate operation. We provide evidence of <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mi>α</j:mi></j:math> particles being forward accelerated up to <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mrow><k:mo>≈</k:mo><k:mn>2.7</k:mn><k:mtext>−</k:mtext><k:mi>MeV</k:mi></k:mrow></k:math> energy with a total flux of <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mrow><l:mo>≈</l:mo><l:msup><l:mn>10</l:mn><l:mn>11</l:mn></l:msup><l:mspace width="0.28em"/><l:msup><l:mi>sr</l:mi><l:mrow><l:mo>−</l:mo><l:mn>1</l:mn></l:mrow></l:msup></l:mrow></l:math> as integrated over <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"><n:mrow><n:mo>&gt;</n:mo><n:mn>0.1</n:mn><n:mtext>−</n:mtext><n:mi>MeV</n:mi></n:mrow></n:math> energies and detected within a <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:mn>0.5</o:mn><o:mtext>−</o:mtext><o:mi>mrad</o:mi></o:mrow></o:math> solid angle. We also report on on-axis emission of relativistic electrons with an exponentially decaying spectrum characterized by a <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:mrow><p:mo>≈</p:mo><p:mn>10</p:mn><p:mtext>−</p:mtext><p:mi>MeV</p:mi></p:mrow></p:math> slope, i.e., five times larger than the standard ponderomotive scaling. The total charge of these electrons with energy above 2 MeV is estimated to be of <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:mrow><q:mo>≈</q:mo><q:mn>1</q:mn><q:mspace width="0.28em"/><q:mi>nC</q:mi></q:mrow></q:math>, corresponding to <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"><s:mrow><s:mo>≈</s:mo><s:mn>0.1</s:mn><s:mo>%</s:mo></s:mrow></s:math> of the laser drive energy. In addition, we observe the formation of a plasma channel, extending longitudinally across the gas density maximum and expanding radially with time. These results are well captured by large-scale particle-in-cell simulations, which reveal that the detected fast ions most likely originate from reflection off the rapidly expanding channel walls. The latter process is predicted to yield ion energies in the MeV range, which compare well with the measurements. Finally, direct laser acceleration is shown to be the dominant mechanism behind the observed electron energization. Published by the American Physical Society 2024