Illumination uniformity of a capsule directly driven by a laser facility with 32 or 48 directions of irradiation

Abstract

The uniformity of the illumination of a spherical capsule directly driven by laser beams has been assessed numerically. Two schemes characterized by 32 and 48 directions of irradiation with associated a single laser beam or a bundle of laser beams characterized by a super-Gaussian intensity profile are considered. The calculations take into account beam imperfections as power imbalance and pointing errors. It is found that the optimum laser intensity profile, which minimizes the illumination uniformity, depends on the beams’ imperfections. A comparison between both schemes shows that the 48 beams configuration allows better illumination uniformity than the 32 beams’ configuration.

Stochastic homogenization of the laser intensity to improve the irradiation uniformity of capsules directly driven by thousands laser beams

“…However, these non-uniformities (ai) have been calculated for the optimum láser beam parameters that minimize ero which in general do not correspond to the minimum of ai. The optimum láser intensity proñle which minimizes ai depends on the beam imperfections [23]. Indeed, for the beam imperfections ap¡ = 1% and app = 5% it is found that the minima of (TII^VD] move from large focal spots (A/ro > 1.1 at ap¡ = 1% and a PE = 5%) towards the position of the minimum intrinsic non-uniformity as the errors tend to zero.…”

“…of the láser intensity over a sphere of radius T-Q (J is the average intensity and the integral covers the whole sphere surface) [17][18][19][20][21][22][23], The intrinsic non-uniformity ao, which is a characteristic of each facility, has been calculated neglecting any láser imperfection as e.g. beam-to-beam power imbalance (api) and pointing error (aPE).…”

Irradiation uniformity at the Laser MegaJoule facility in the context of the shock ignition scheme

“…In the LMJ facility, the laser beams are grouped in quads; thus the power imbalance benefits from a statistical factor which reduces it to σ PI = 5% (quad-to-quad). The illumination non-uniformity, evaluated taking into account these beam uncertainties, is usually measured as an average value (σ ) estimated over a large number of calculations [72][73][74][75][76][77] . In these calculations, each of the three parameters (laser power, laser pointing, and target position) varies randomly and follows a Gaussian distribution centred to their nominal values and characterized by the corresponding standard deviation σ PI , σ PE , or σ TP .…”

Illumination uniformity of capsules directly driven by a facility with thousands of laser beams

“…In this phase the expansion of the plasma corona does not affect the laser reflection and the uniformity of the energy deposition could be analyzed assuming a direct illumination of the capsule [11]. The laser beams assembled in a facility are affected by unavoidable imperfections as power imbalance and pointing error [11][12][13][14][15][16][17][18]. In this paper we analyze the illumination uniformity provided by five irradiation configurations characterized by N D = 12, 20, 32, 48 and 60 irradiation directions [12].…”

“…In this paper we analyze the illumination uniformity provided by five irradiation configurations characterized by N D = 12, 20, 32, 48 and 60 irradiation directions [12]. It is shown that the optimal focal spot shape depends on the beam imperfections [13] and that taking beam imperfection into account the illumination uniformity is degraded by about one order of magnitude [11]. Finally, it is shown that the use of bundles [14,15] of N B beamlets (laser beam unit) associated to each irradiation direction introduces a stochastic homogenization of the laser intensity which benefits the irradiation uniformity [16,17].…”