King´s College laser plasma x-ray source design

Abstract

The aim of this work is to design and build a source for a range of applications, with optimized multilayer structures in order to use the source output as efficiently as possible. The source is built around a Nd:YAG laser with fundamental wavelength 1064 nm, frequency doubled 532 nm (green) and tripled 355 nm, with a pulse length of about 800 ps and a repetition rate up to 50 Hz. The target material is Mylar (C 10 H 8 O 4 ) tape, which is cheap, readily available and has many benefits as explained in this article. A versatile cubic target chamber and a set of computer controlled stage motors are used to allow positioning of the X-ray emission point. A range of measures is used to protect delicate components and optics, including a glass slide between the focusing lens and the target to prevent the lens being coated with debris. A low pressure gas (typically 3-6 mbar) is used inside the chamber as collision of atomic size debris particles with gas molecules reduces their kinetic energy and consequently their adhesion to the surrounding surfaces. The gas used is typically helium or nitrogen, the latter also acting as a spectral filter. Finally, the chamber is continually pumped to ensure that more than 70% of the debris particles are pumped out of the chamber.

Development of a low-debris laser driven tape drive soft x-ray source

“…The Shock wave is only partially reflected from the back surface as most of it will penetrate through the target material and it is energetic enough to punch a hole through the tape as the material will be vaporized. Because the ablation front travels much slower into the material, at around the sonic-subsonic speed [14], with respect to the shockwave, by the time it reaches the molten materials of the target either a hole is being punched through the tape by the shock effect or the tape material has been weakened enough by the shock, thus offering lower resistance to the ablation front, resulting in most of the debris particles being emitted in the forward direction of the laser beam as shown in figure 1(c) [8]. In thin tapes, the Shock wave is partially reflected from the back surface and is energetic enough to punch a hole through the tape as the material will be vaporized (NOT spallated).…”

“…Here the target material was VHS tape, Fuji Super high grade/SHG180/Hi-Fi professional/double coating, 15 µm thick, 13 mm width, consist of Mylar (C10H8O4) as a carrier film and fine magnetic powder of both Fe 2 O 3 and chrome dioxide and a backing layer of carbon black for static control and binding agent (polyester-polyurethane) added. The 15 µm thickness of the tape should act to limit debris as the shock wave escapes through the rear of the target instead of being reflected [8]. However, VHS video tapes can contain carbon black added to reduce the buildup of static charge.…”

“…The gas used may also act as a spectral filter, for example nitrogen can be used in this way. The volume around the target may also be continually pumped to ensure that debris particles are removed from the chamber, here to efficiencies of order 70% [8]. Finally, the use of the combined effect of both magnetic field and buffer gas appeared to be an effective method also in reducing the larger macroscopic debris particles.…”

“…(c)In thin tapes, the Shock wave is partially reflected from the back surface and is energetic enough to punch a hole through the tape as the material will be vaporized (NOT spallated). Most of the debris particles are then emitted in the forward direction of the laser beam[8]. The illustration of this model is based on the work described by[13].…”