X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2000-01-12
2001-12-25
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Source
C378S121000, C378S122000
Reexamination Certificate
active
06333966
ABSTRACT:
The concept and feasibility of using the extremely high electro-magnetic fields achievable in the Rayleigh region around the focal plane of a multi-Joule short pulse laser to accelerate electrons to multi-MeV energies was first developed by Schoen in the mid 1980s, and experimental confirmation followed with the advent of femtosecond regime high power lasers in the early 1990s. Details of this work appear in the cited references. These developments could lead to the design and fabrication of table-top electron accelerators, with concomitant low costs when compared with synchrotrons that currently are necessary to provide low emittance high energy electron beams.
With the advent of these compact low cost electron sources, it should now be possible to develop the technology for using these multi-MeV electrons to produce extremely short pulse x-rays, which can be used for several applications, including x-ray crystallographic and absorption techniques to study the structure and reaction dynamics of biological molecules, for high resolution medical x-rays, and for structural studies and process techniques in the semiconductor manufacturing area. The integration of several novel techniques to accomplish the above objectives, and performance estimates, appears in the following sections.
BACKGROUND
X-ray sources have been in use for medical and physics research applications since the discovery of x-rays by William Roentgen at the turn of the century. Early sources were based on acceleration of electrons in an x-ray vacuum tube (using high voltage power supplies) and their subsequent collision with a cooled metal anode, which produced K- and L-edge x-ray line emission from the metal (usually copper) atoms in addition to continuum radiation from Bremsstrahlung radiation. The shape and duration of the x-ray pulse was limited by the inductive rise time of the electron tube circuitry (in the microsecond range) for the pulse leading edge, and the power supply and cooling capability for the pulse duration.
With the advent of the electron beam accelerators in the 1930s and 1940s, a new shorter pulse time domain opened up as a result of the “bunched” nature of electron beams in cyclotron and synchrotron electron accelerators, due to the time and phase restrictions of repetitive geometry accelerating structures. Electron bunches of the order of picoseconds are now routine in the current generation of electron accelerators. However, these machines are usually very large and expensive (>$10 million for low energy machines), and are currently confined to government laboratories for basic research applications.
The introduction of lasers (in the 1960s) has led to several techniques to produce x-ray pulses, based on the ability to focus laser beams to very high powers over small areas (microns). One recent technique involves short-pulse lasers to create a plasma by focusing the light onto a thin foil, which is heated to several thousand or more degrees over a region of several microns in a time very short compared to the thermal diffusivity of the foil. This plasma then radiates as a black body, producing an x-ray spectrum with energy spread dependent on the temperature of the plasma. However, this technique results in x-ray emission over 4 pi steradians, or isotropic radiation, and thus the x-ray flux drops rapidly with distance from the source (roughly as 1/r
2
). More importantly, the x-ray pulse length depends on the plasma thermodynamic properties, which are not easily controlled, and this results in x-ray pulses significantly longer than that of the laser driver, as demonstrated by the Umstadter, et. al. patent.
The use of laser accelerator electrons combined with novel x-ray conversion in this invention should provide advantages in the intensities of the x-rays produced and the angular distribution of x-rays. These features, coupled with the very short pulse duration will open up new capabilities for research and materials processing. For example, reaction kinetics and related molecular structure changes for a variety of important biological molecules could be studied, since many of these processes occur on picosecond time scales. Photosynthesis reactions in particular involve electron transfers at molecular sites that are known to be sub-picosecond processes. The very short x-ray pulses also can “freeze” molecular structures for x-ray diffraction studies. Another potential application is the study of the disordering and re-ordering (annealing) of semiconductor surface layers after rapid “melting” by short laser pulses followed by x-ray probes of the layer behavior. The effects of “hot electrons” injection in semiconductor devices could also be studied using the above laser techniques. Finally, changes in the above-K-edge absorption of x-rays (EXAFS) can be used to examine changes in short range structure at molecular sites, even for “amorphous” collections of molecules (as is the case in fluids, as opposed to crystalline structures).
SUMMARY OF THE INVENTION
The invention described herein is a novel integration of several individual technologies developed originally for the high energy particle physics community, to provide low cost fast x-ray sources not presently available from commercial or laboratory organizations. The laser accelerator femtosecond x-ray source (LAFXS) is based on an electron acceleration technique developed by the inventor, and recently confirmed experimentally in the U.S. and Europe. The use of a high peak power short pulse laser, focused to diffraction limited size, provides extremely high electric and magnetic fields which can accelerate electrons via the ponderomotive force to energies of the order of 100 MeV in axial distances of the order of the Rayleigh range, as indicated in the Schoen patent cited reference. A description of the basic components of the LAFXS is as follows.
In order to accelerate electrons to 50 MeV or higher energies, laser peak powers approaching 10
21
Watts/cm
2
may be necessary. The development of chirped pulse amplification ( see article by D. Strickland and G. Mourou, Optical Communications, Vol. 56, 219, (1985) for example) now enable these high peak powers to be achieved in a Rayleigh region focus, with durations of the order of several hundred femtoseconds down to as few as 50 femtoseconds. The laser used for accelerating electrons consists of a mode-locked glass laser (e. g., NdYAG) which feeds pulses into a chirped pulse amplifier (CPA). The CPA is a pulse stretching optical cavity, between two frequency gratings, with a filtering mask within the cavity to modify the phase and/or amplitude of each frequency component of the input pulse spatially separated by the grating. The pulse components exiting the grating enter an amplifier and then a pulse re-compressor to produce the required femtoseconds duration, high intensity focal spot. A gas jet can provide the source of electrons, via ionization by the intense laser pulse. The electrons are accelerated to high energies by the ponderomotive force of the laser beam; those with the highest initial axial velocity reach the highest energies, as described in the previous references and figures in the following section. Electron acceleration can also take place via a plasma wake field or beat wave process, although indications from recent experiments imply that only the lower energy electrons (e.g., those up to a few MeV) are created by this mechanism. It is also possible that these lower energy electrons produced by plasma waves serve as a source for the ponderomotive acceleration to high energies, since electrons with high initial axial velocities will be accelerated to the multi-MeV (of the order of 50 MeV) energies observed, as calculated via computer simulation in the referenced patent.
The electrons produced can be transported to an x-ray conversion region via standard electron optics elements, including focusing quadrupole magnets and dipoles for energy spectrometry/filtering. Electron beam optics designs from synchrotrons could also be used to accommodate the momentum spread of
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