X-ray Plasma Physics and Imaging: General Overview
Complex diagnostic systems, integrating state-of-the-art detectors and X-ray optics, have been developed for leading MFE experiments, like the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory and the C-Mod tokamak at MIT. The spherical torus represents a promising path toward economical fusion energy, relying on the achievement of near unity beta (plasma pressure to magnetic pressure ratio), in a tight aspect-ratio configuration. The Johns Hopkins systems enable experiments that cannot be performed by conventional instrumentation, like imaging of peripheral magnetic islands, or determination of the hot plasma resistivity. Our X-ray Plasma Physics Group also plays an active role in the National NSTX Research Team, whose mission is to advance the spherical torus concept toward its assessment as a viable fusion reactor.
In the domain of high-density, inertial fusion research, we develop phase-contrast imaging Talbot-Lau interferometers in order to obtain refraction-based estimates of the electron density gradients and identify regions of hydrodynamic instabilities, which are not observable in conventional radiography due to small scale and limited spatial resolution, line integration, or lack of attenuation contrast. This research will be extended to the measurement of a demonstration experiment on the Titan laser at the Lawrence Livermore National Laboratory’s Jupiter Laser Facility, in which a simple test object such as the Be/CH target will be imaged and the density profile measured using a pinhole-aperture backlighter and a high-magnification Talbot interferometer.
The above-mentioned differential phase-contrast imaging (DPC) relies on the refraction of the X-rays passing through an object; because this process is sensitive to density gradients in the measured object—rather than to its bulk X-ray absorption—DPC represents a potentially efficient tool in medical research. Indeed, refraction has a contrast-enhancing effect at tissue boundaries, which enables the detection of soft tissue otherwise invisible in conventional X-ray imaging. In addition, the ultra-small angle scattering occurring in micro-structured soft tissue such as cartilage, tendon, ligament, or muscle also has a volume contrast-enhancing effect. We develop, in the framework of an NIH Grant, Talbot-Lau interferometers based on the same principles as those used in ICF diagnostics, for operation in the mean energy range for soft tissue imaging purposes. Similar methods are explored for applications in the study of novel materials and nanostructures.
Magnetically Confined Fusion Plasma Science
The research develops Far Ultraviolet and soft X-ray spectroscopic instrumentation for the diagnostic of Magnetic Fusion Energy (MFE) experiments and applies it to the study of high temperature plasmas. It covers topics central to the fusion plasma physics, like magneto-hydrodynamic stability, particle and energy transport, as well as atomic physics topics, like the spectroscopy of the highly ionized species relevant to these plasmas.Complex diagnostic systems, integrating state-of-the-art detectors and X-ray optics, have been developed for leading MFE experiments, like the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory and the C-Mod tokamak at MIT. The spherical torus represents a promising path toward economical fusion energy, relying on the achievement of near unity beta (plasma pressure to magnetic pressure ratio), in a tight aspect-ratio configuration. The Johns Hopkins systems enable experiments that cannot be performed by conventional instrumentation, like imaging of peripheral magnetic islands, or determination of the hot plasma resistivity. Our Plasma Spectroscopy Group has also an active role in the National NSTX Research Team, which has the mission of advancing the spherical torus concept toward its assessment as a viable fusion reactor.
Recent research topics of the group include the development of 2-D and 3-D ultrafast imaging techniques in the soft X-ray range, for the study of localized MHD perturbations, such as the neo-classical tearing modes. These perturbations seem to have a profound effect on the stability and confinement properties of high beta plasmas. New research subject could also be the study of turbulence in fusion plasma using focusing, soft X-ray telescopes non magnetic mapping of the field structure in the core and control of the MHD activities at the plasma edge and inside the divertor.
The atomic physics packages necessary for retrieving the plasma parameters from the spectroscopic data are developed in collaboration with researchers at the Lawrence Livermore National Laboratory and benchmarked on various fusion experiments in the U.S. and Europe. International collaborations included the development of a 2-D Far Ultraviolet imaging system for the measurement of local particle transport in the Large Helical Device, the largest fusion experiment in Japan, particle transport studies on TFU in Frascati (ENEA, Italy) and in the future, 2D soft X-ray arrays on the EAST superconducting tokamak in Hefei, China.
High Energy Density Plasma Diagnostics
The diagnostic of High Energy Density Laboratory Plasmas (HEDLP) is challenging due to the small spatial scales involved (μm-mm), short time scales (ps-few ns), extreme densities (many times the solid density), and extreme radiation, as is the case of imploding Inertial Fusion Energy (IFE) capsules. Due to high densities and sharp gradients a main challenge in HEDLP diagnostics is measuring with high spatial resolution the areal or line integrated density of the plasma. To penetrate high density plasmas, hard X-rays in the ~5-100 keV range are needed. Thus, X-ray Thomson scattering and X-ray radiography are being developed for density diagnostic in HED plasmas. X-ray radiography is a simple density diagnostic technique based on the attenuation in the target plasma through photo absorption and Compton scattering of hard X-rays produced by backlighters, such as pinhole-apertured point backlighting and foil or wire backlighting. Attenuation based radiography has however fairly low contrast in low-Z HED plasmas since hard X-rays attenuation coefficients are small. Recently, it was observed that refraction may offer a stronger contrast mechanism for HEDLP radiography than attenuation, since for low-Z matter the real part of the complex index of refraction δ, is much larger than the imaginary part, β.Our current research on HED plasmas focuses on the development of a refraction based radiographic density diagnostic using the Talbot-Lau differential phase-contrast method. The Talbot-Lau method has been studied in recent years for medical phase-contrast imaging and has recently been integrated to the research efforts of the X-ray Plasma Physics and Imaging group. Laboratory experiments are performed at Johns Hopkins University, department of Physics and Astronomy. Simulations are done with the XWFP code developed for synchrotron research, which includes X-ray absorption, refraction, diffraction, and free space propagation [T. Weitkamp, Proc. of SPIE Vol. 5536 (2004)].
The Talbot-Lau method uses three micro-periodic gratings in a shearing interferometer configuration (source, beam-splitter, and analyzer), to measure the small angular deviations of an X-ray beam as it traverses an object having gradients in the index of refraction. Since for X-ray energies far from absorption edges the index of refraction is proportional to the electron density, the Talbot method offers first a simple diagnostic for the density gradient in HED plasmas; with simple boundary conditions this gradient can be integrated to obtain the absolute density. In addition, since the X-rays are also small angle scattered on microscopic density gradients, the Talbot-Lau method may also offer a diagnostic for small scale hydrodynamic instabilities. The attractiveness of the Talbot-Lau method for HEDLP diagnostic comes from the capability to work with incoherent and polychromatic X-ray sources such as the backlighters used in HEDLP radiography. In addition, the spectral response of the Talbot-Lau interferometer can be easily adapted to various source spectra by changing the parameters of the beam-splitter grating. The high optical transmission of the Talbot interferometer (≤50%) enables also making efficient use of the limited number of photons available from the powerful, but briefly emitting backlighters used in HEDLP radiography.
There are two main strengths of the Talbot-Lau method for X-ray HEDLP diagnostic:
(i) The capability for sensitive and quantitative density gradient measurements: For density gradient measurements with Talbot-Lau interferometers two techniques have been exploited in the laboratory. The first is Moiré deflectometry, in which one of the gratings is rotated by a small angle θ to produce Moiré fringes of period g/θ >> g. The Moiré effect converts to a macroscopic scale the changes in the microscopic Talbot pattern, allowing the use low resolution but high sensitivity and fast detectors for the refraction measurement. One the limitation of the technique is that the spatial resolution of the density measurement is limited to the Moiré fringe spacing, which is typically ≥100 μm. Nevertheless, can be easily translated to the HEDLP experimental conditions and has shown to be little affected by grating misalignment. Another technique for refraction angle measurement is called Phase Scanning. It consists in obtaining Moiré free images while laterally scanning one of the gratings through its period, which gives raise to intensity modulations in each pixel of the images. The ‘dark field’ and ‘bright field’ images correspond to the minima and the maxima of the intensity modulation, while the images at mid-height of the intensity modulation have maximal refraction contrast enhancement. The modulations in the phase-scan curve can be Fourier analyzed to separate the attenuation and refraction contribution to the images.
(ii) The capability to detect micro-inhomogeneities having spatial scale below the spatial resolution of the radiographic system. Together with the phase gradient and the attenuation images, the Talbot technique obtains also a third image, corresponding to the amplitude of the phase-scan oscillation in each pixel. This amplitude measures the angular broadening of the X-ray beam due to ultra-small angle scattering (USAXS) in the object and it is a direct measure of the density of micro-inhomogeneities with characteristic size λ/W, where W is the interferometer angular width, which for typical interferometer angular widths and high X-ray wavelengths, corresponds to spatial scales of a few μm. USAXS imaging can evidence and localize regions of micro-inhomogeneities in an object that are much smaller than the spatial resolution of the radiographic system and that have also negligible attenuation contrast. In addition, USAXS scatter measurements using the Moiré technique have been demonstrated recently. The USAXS measurement could thus be of interest for diagnostic of R-T micro-turbulence, which has comparable spatial scale in HEDLP experiments such as IFE.
The research efforts will enable:
- Refraction-based diagnostic of the electron density in HEDLP experiments without requiring high energy X-ray lasers, focusing optics, or highly coherent backlighters
- Discriminating and quantifying the refraction and attenuation contributions to the HEDLP radiographic images and thus improving the accuracy of the attenuation based density diagnostic
- Diagnosing regions of hydrodynamic instabilities which are not observable in conventional radiography due to small scale and limited spatial resolution, line integration, or lack of attenuation contrast.
Phase Contrast Imaging for Bio-medical and Material Science Applications
X-ray differential phase-contrast (DPC) or refraction based imaging with Talbot-Lau grating interferometers has the potential to become a new medical imaging modality, offering higher soft tissue contrast than conventional attenuation based imaging and higher spatial resolution than magnetic resonance imaging (MRI). For instance, DPC computed tomography (DPC-CT) could enable detection of small lesions in soft tissue, such as from early stage tumors, which is not possible with any other imaging modality. New bone imaging modalities may also be possible.
The Talbot-Lau interferometer consists of three micro-period gratings: ‘source’, ‘beam-splitter’, and ‘analyzer’ [Pfeiffer F., Weitkamp T., Bunk O. and David C, Nature Physics 2, 258, 2006; Momose A., Yashiro W., Takeda Y., Suzuki Y. and Hattori T., Japanese J. Appl. Phys. 45, 5254, 2006]. The source and analyzer are absorption gratings typically made of Au, while the beam-splitter is a thin phase grating typically made of Si or Ni.
To enable imaging of the human body the interferometer must first work at high enough energy. For instance we are developing DPC imaging for the knee, for which conventional radiography is done at >60 kVp (>40 keV mean spectral energy) and CT at >80 kVp (>55 keV mean energy).
In addition, for medical applications the interferometer must be very sensitive to small X-ray angular changes to enable refraction imaging with acceptable dose. The sensitivity is determined by two parameters: the interferometer contrast or ‘visibility’ V, and the angular resolution, W. High contrast (>20%) is essential for medical DPC imaging, because the signal-to-noise-ratio in the DPC images increases proportional to V2. Good angular resolution (W < several µ-radian) is also necessary, because the X-ray refraction angles in soft tissue are <
In summary, for clinical DPC imaging of large body parts the Talbot-Lau interferometer must have >20% contrast at mean spectral energies >40 keV, while using gratings with few µm period. This is not possible however with the conventional normal incidence Talbot-Lau interferometer, because the thickness of a few micron period absorption gratings is technologically limited to around100 µm. To overcome this limitation we developed a glancing angle Talbot-Lau interferometer (GAI), using gratings with the bars inclined at an angle α∼10−30° along the beam direction. This increases the effective absorber thickness from the normal incidence value t, to t/sin(α), which enables achieving high contrast at high energy using the existing gratings. Further on, the gratings can be ’tiled’ on a single substrate to achieve the large field of view needed for imaging of large body parts. Studies performed in collaboration with researchers at the JHU Department of Biomedical Engineering and with physicians at JHU Department of Radiology show that the tiled grating GAI offers a direct path towards the development of high energy medical DPC systems.
To further improve the sensitivity of the Talbot-Lau interferometer for biomedical applications we study ways to improve also the angular resolution, W. This can be done by operating the interferometer in high Talbot orders, which in turn narrows the spectral region of high contrast. To select this narrow region of high contrast from the broad tube spectrum we study spectral filtering with K-edge absorbers and with grazing incidence X-ray mirrors. For instance, using a total reflection Au mirror at 2.7 mrad incidence in conjunction with a Mo tube at 50 kVp we obtained up to >30% contrast in the fifth Talbot order. The combination of simultaneous high angular sensitivity and contrast achieved with mirror filtered interferometers enables single-exposure refraction imaging of soft tissues. Studies in collaboration with researchers at JHU Radiology show for instance that quasi-monochromatic interferometry has high potential for the detection of cancerous tumors in small-animal disease models. The spectral filtering mirror can be a micro-periodic mirror which replaces the source grating. The mirror filtering offers also a path for further increasing the interferometer sensitivity at high X-ray energy.
Lastly, DPC imaging with GAIs and with mirror filtered interferometers has high potential for material science applications. Phase and ultra small angle scatter (USAXS) X-ray imaging on very small spatial scales and at energies from several keV to several tens of keV is possible, because dose is no longer a limiting factor. For instance, our measurements show that DPC imaging with the GAI can discriminate nano-dispersed low-Z materials from homogenous ones at energies in the several tens of keV range. This capability is also of interest for security and industrial applications.