Byungmoo Song

Electron-beam-generated x rays from X pinches

X pinches are well known to produce very small, dense plasma pinches (“micropinches”) that emit short bursts of 1.5–8keV radiation [Shelkovenko et al., Phys. Plasmas 9, 2165 (2002)]. X-ray radiation in the 8–100keV range is also emitted, only a small portion of which is associated with the micropinches. Beginning immediately after the soft x-ray burst, higher energy x-ray emission is observed that is attributed to energetic electrons accelerated in the gaps that appear in the X-pinch plasma structure. The temporal, spectral, and spatial properties of this higher energy radiation (8–100keV) have been studied using two ∼0.1μs pulsed power generators, one operating at up to 450kA peak current and the other up to 270kA. This radiation was also used for imaging in a low magnification configuration, and spatial resolution of a few tens of micrometers was demonstrated.

Studies of energetic electrons with space and time resolution in Mo and W X-pinches from measurements of x rays >9 keV

Electron beams in 400 kA peak current Mo and W X-pinches have been studied using 1 ns time-resolution Si diodes to monitor x rays >9 keV. Softer x rays were monitored by photoconducting detectors (PCDs). Three different types of higher energy x-ray bursts were observed. The first type appears to be produced by electrons generated starting at the moment of, or immediately after, the first thermal x-ray burst (typically 40–50 ns after the current start), and Si detector signals last 1–2 ns. The second type of harder x-ray burst occurs 50–80 ns after the current start, lasts 2.5–10 ns, and is typically not correlated with a thermal x-ray burst. These two types of bursts were generated near the cross-wire region. The third type of x-ray burst occurs 70–100 ns after the current start, and is also uncorrelated with PCD signals. The energetic electrons responsible for these x rays are generated for 10–30 ns, and the radiation is produced in the anode region.

The X pinch as an x-ray source for point-projection radiography

Two methods of using the X pinch as a source of X-ray radiation for the radiography of dense plasmas and other objects are presented. These methods do not use pinholes, instead taking advantage of the small source size (<1 mm, and in some cases <1 pm) and short X-ray emission duration (< 2 ns , and < 20 ps in some cases) of the X pinch radiation. Detailed measurements of the emission characteristics of X pinches made using different wire materials and in different energy ranges using a set of X-ray diagnostics with high temporal and spatial resolution are presented. Several applications of the X pinch are discussed.

X-pinch source size measurements

The X pinch plasma emits subnanosecond bursts of x-rays in the 3 - 10 keV energy range from a very small source. As such, it has been used for high-resolution point-projection imaging of small, dense, rapidly changing plasmas, as well as submillimeter thick biological samples. The very small x-ray source size of the X pinch provides high spatial coherence of the x-rays, enabling the X pinch to be used for imaging low absorption, low contrast objects with excellent spatial resolution by incorporating wave-optics effects. The reverse procedure has been used to determine the X pinch x-ray source size: well-defined micro-fabricated slits were imaged by point-projection radiography, and the detailed patterns were compared with wave-optics calculations of the expected image patterns on film as a function of x-ray source size and energy band. In addition, an x-ray streak camera was used to study the X pinch source size as a function of time. Dynamic shadow images of a boron fiber with a tungsten core and glass fiber sheathed in plastic were compared with a time-integrated radiographic image. Source sizes as small as 1.2 μm (full width at half maximum, assuming a Gaussian spatial intensity profile for the source) have been inferred.

Focusing x-ray spectrograph with crossed dispersion

We present a focusing spectrograph configuration, the Focusing Spectrograph with Crossed Dispersion (FSCD), that enables us to solve the problem in a crystal spectrograph of reflected radiation in many orders. In the FSCD, the x-ray radiation from a small source is dispersed by a transmission grating placed in front of the Bragg reflector (mica crystal). This configuration retains the advantages of a spherically bent mica crystal spectrograph while enabling the analysis of the radiation in a number of spectral bands simultaneously and independently. Results using a 150 mm radius spherically bent mica crystal combined with 0.5 μm period gold transmission gratings in X pinch experiments are presented.

X-pinch source characteristics for x-rays above 10 keV

X pinch radiation produced by electron beams accelerated in the X pinch minidiode ranging in energy from 10 to 100 keV has been studied and used to image a variety of different objects. The experiments have been carried out using the XP pulser (470 kA, 100 ns) at Cornell University and the BIN pulser (280 kA, 120 ns) at the P.N. Lebedev Physical Institute. This electron-beam-generated x-ray sources geometric, temporal and spectral properties have been studied over different energy ranges between 10 and 100 keV. The imaging was carried out in a low magnification scheme, and spatial resolution of a few tens of μm was demonstrated.

Investigation of the radiation properties of L- and M-shell X-pinch plasma x-ray sources using a transmission grating spectrometer

A transmission grating spectrometer (TGS) with one dimensional spatial resolution was used to investigate the radiative properties of X-pinch plasmas produced using wires made of NiCr and CoNiCr alloys, and Mo. Experiments were carried out on the Cornell XP machine. The TGS was used to estimate of the energy yields of L-shell Cr and Ni, and M-shell Mo radiation. The absolute energy yields in the wavelength range 10 42 A. The results are compared with data obtained previously with the TGS on a 1 MA...

X pinch source characteristics for X-rays above 10 keV

Summary form only given, as follows. Experiments have been carried out using the XP pulser (470 kA peak current, 100 ns fwhm pulse duration) at Cornell University to investigate the geometric and spectral parameters of the X pinch x-ray source for photon energy E > 10 keV. The X pinch is known to emit sub-ns bursts of x-ray radiation in the 1.5-5 keV energy range from /spl sim/1 micron dense plasma sources. Using a pinhole camera with different filters to image a variety of objects on each pulse has shown that X pinches radiated up to 100 keV, and that the source size and duration of radiation vary with the photon energy range. For example, several mm thick metal objects have been imaged with 30 keV x-rays in a low magnification scheme, and the /spl sim/10 micron scale image resolution implies a source size for this radiation of at most a few tens of microns. This radiation is believed to be due to electron beams accelerated across the minidiode that forms just as an X pinch emits its soft x-ray burst.

Application of the focusing x-ray spectrograph with crossed dispersion to investigations of X pinch plasmas

A new focusing spectrograph configuration with crossed dispersion (FSCD) has been developed for use in X pinch experiments. This device solves the problem in a crystal spectrograph of the overlapping of reflected radiation in many orders. The x-ray radiation from a small source is dispersed by a transmission grating before it reaches the Bragg reflector (mica crystal). A large aperture (10 mm×10 mm), 0.5 μm period gold transmission grating was made on a 0.5 μm thick Si3N4 substrate and combined with spherically bent mica crystals with radii of curvature of 100 mm or 186 mm. Results of FSCD application to plasma experiments with X pinch x-ray sources is presented.

Determination of the size and geometry of the X-pinch X-ray source using microfabricated structures, and applications of this source to high-resolution imaging of a variety of objects

Summary form only given, as follows. The X-pinch is a very small radiation source in the 3-10 keV X-ray band. It has been used successfully to obtain micron scale resolution point-projection images of rapidly varying plasmas with subnanosecond temporal resolution, such as other X-pinches and exploding wire arrays, as well as to image /spl les/2 mm thick biological samples, such as small insects and beet seeds.