Time dependence of fast electron beam divergence in ultraintense laser-plasma interactions
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We report on the measurement and computer simulation of the divergence of fast electrons generated in an ultraintense laser-plasma interaction (LPI) and the subsequent propagation in a nonrefluxing target. We show that, at I lambda(2) of 10(20) Wcm(-2) mu m(2), the time-integrated electron beam full divergence angle is (60 +/- 5)degrees. However, our time-resolved 2D particle-in-cell simulations show the initial beam divergence to be much smaller (<= 30 degrees). Our simulations show the divergence to monotonically increase with time, reaching a final value of (68 +/- 7)degrees after the passage of the laser pulse, consistent with the experimental time-integrated measurements. By revealing the time-dependent nature of the LPI, we find that a substantial fraction of the laser energy (similar to 7%) is transported up to 100 mu m with a divergence of 32 degrees.