X-ray harmonics made with long-wavelength lasers reach higher fluxes and photon energies when plasma effects dominate.

Measurements of the static permittivity of pure samples of eight polar liquids—acetone, butan-1-ol, dimethyl sulphoxide, ethanediol, ethanol, methanol, propan-1-ol and water—and two non-polar liquids—cyclohexane and silicone oil—were measured in a shielded micrometer-driven, parallel-plate admittance cell at temperatures from 5 to 50 °C. The experimental method, which requires measurements of capacitance using a four-terminal-pair impedance analyser, is described in detail. The full measurement results have been tabulated separately in a National Physical Laboratory report. The present paper provides a comparative study of these latest results with previously published data and also describes the details of the uncertainty analyses employed. Admittance cell measurement techniques are also briefly reviewed to put this work into context.

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We have performed a series of experiments which demonstrate the effect of open-ended coaxial diameter on the depth of penetration. We used a two layer configuration of a liquid and movable cylindrical piece of either Teflon or acrylic. The technique accurately demonstrates the depth in a sample for which a given probe diameter provides a reasonable measure of the bulk dielectric properties for a heterogeneous volume. In addition we have developed a technique for determining the effective depth for a given probe diameter size. Using a set of simulations mimicking four 50 Ω coaxial cable diameters, we demonstrate that the penetration depth in both water and saline has a clear dependence on probe diameter but is remarkably uniform over frequency and with respect to the intervening liquid permittivity. Two different 50 Ω commercial probes were similarly tested and confirm these observations. This result has significant implications to a range of dielectric measurements, most notably in the area of tissue property studies.