![]() Alternatively, higher atomic number materials can also provide strong L-shell emissions 9 with ∼10 keV photon energy however, the multiple discrete emission lines characteristic of these sources are not preferred for powder x-ray diffraction. ![]() Lower atomic number elements have been determined to have higher CE in the same irradiance conditions 7,8 but have a smaller observable range of d-spacings in the TARDIS geometry 1–3 due to having longer probe wavelength. It is the highest atomic number, readily available element that can efficiently generate He α emission with the few beams available to heat the XRS foil in the TARDIS configuration. We have determined that Ge is optimal for most TARDIS experiments. 7,8 The probe wavelength determines the observable range of crystalline plane spacings through the Bragg equation m λ = 2 d sin θ, where m is the reflection order ( m = 1), λ is the probe wavelength, d is the crystal plane spacing, and θ is the Bragg angle. X-ray source material selection is particularly important as it defines the probe wavelength and strongly influences CE. ![]() 6 This motivates careful design and characterization of the laser-plasma conditions. This suggests an optimal laser irradiance regime, high enough for the IB absorption to heat the XRS plasma enough to generate He-like states, but not so high as to cause significant instabilities. Both SRS and TPD generate plasma waves that can become large enough to break, which produces non-thermal “hot” electrons that have enough kinetic energy to escape the local region and reduce the conversion efficiency (CE) from laser energy to He α emission. For L μ m = 200 μm, T e = 3.5 keV, the SRS onset irradiance is 5.7 × 10 15 W/cm 2, and the TPD onset irradiance is 2.5 × 10 14 W/cm 2. Montgomery 5 determined relations for the onset irradiance of SRS and TPD, where I is the laser irradiance, λ μ m is the laser wavelength (0.351 μm at the NIF), L μ m is the plasma density gradient scale length, and T e is the plasma electron temperature. Instabilities like stimulated Raman scattering (SRS) and two-plasmon decay (TPD), 5 which can occur at high laser irradiances, 6 may also be active in the XRS plasma of TARDIS experiments. These free electrons undergo collisions that dephase them from the laser's electric field, leading to permanent energy gain and heating the plasma. 4 In IB, electrons are freed and then oscillate in the electric field of the laser. ![]() For laser irradiance in the vicinity of 1 × 10 15 W/cm 2 as in TARDIS experiments, laser energy is predominantly absorbed in the XRS foil via inverse bremsstrahlung (IB). ![]()
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