At magnetic confinement and laser inertial confinement nuclear fusion research facilities, plasma imaging and spectroscopy are critical tools for understanding the dynamics of the confined fuel and getting closer to the goal of clean, sustainable power. These plasma diagnostics are enabled by precision bent crystal x-ray optics designed and fabricated to offer high spatial and energy resolutions for particular ranges of x-ray energies. These optics operate by using the interplanar spacing at a given crystal orientation as a diffraction grating.

The soft x-ray range is of particular interest to nuclear fusion energy researchers, thus driving a demand for x-ray crystals with sufficiently wide interplanar spacing. The list of suitable crystals is short, but several of candidates can be found within the family of acid phthalate crystals (KAP, RbAP, CsAP, and TlAP) which offer plane spacings from 25.69 Å to 26.66 Å. This corresponds to x-ray energies down to 470 eV, making it possible to resolve characteristic x-rays from low Z elements like O, F, Si, Na, Mg, Al, and Si. Although KAP offers the widest plane spacing, RbAP, CsAP, and TlAP have greater integral reflectivities.

Potassium Acid Pthalate (KAP) single crystal boules grown at Inrad Optics Inc.

As part of an effort to establish a domestic source of high quality acid phthalate crystals in support of fusion energy research, the Inrad Optics crystal growth team undertook a phase I SBIR project. The scope of this project comprised development of techniques to grow high quality acid phthalate crystals, cleave or polish thin crystals, and bend the thinned crystals to curvatures suitable for x-ray spectroscopy. To date we have been successful in all three of these development areas and are continuing to improve processes and expand our capabilities. During the course of the phase I effort, we grew three large single-crystal boules of KAP which displayed highly uniform microstructure and few inclusions, cleaved thin plates of KAP (001) using a prototype apparatus which currently achieves a plate thickness accuracy of 20µm, and bent and epoxied these cleaved crystals onto cylindrically curved backings.

Future work will include growth of RbAP, CsAP, and TlAP, as well as refinement of growth techniques and efforts to optimize the process of cleaving and bending crystal plates. For more information about x-ray crystals, visit or email us at This email address is being protected from spambots. You need JavaScript enabled to view it.