Robert Bradford and Dr. Steven Turley, Physics and Astronomy
I. Introduction
Due to the high energy of soft x-ray radiation, coherent x-rays are rather difficult to produce. The most common source of x-rays is bremsstrahlung, or breaking radiation. Brehmsstahlung is produced by accelerating an electron across a potential, and then rapidly decelerating the particle, giving off an x-ray. Although the use of bremsstrahlung is widespread, it is not an adequate xray source for many potential applications because it is broadband in wavelength and incoherent. The shorter wavelength radiation of x-rays could be used to print smaller, more efficient electronic circuits through lithography, or could be used in medical imaging. Hence, the search for an x-ray laser is of great interest.
The Department of Physics and Astronomy has been involved in producing a gain source for an x-ray laser for several years. The principle of the experiment is to pass a high voltage pulse, produced by a Marx generator, through a gas filled capillary. This pulse produces a plasma in the capillary. As the electrons in the atoms of this plasma are de-excited, they emit x-rays. Although much remains to be done, my work has been involved in determining how varying the physical parameters (ie. pulse voltage, of the capillary discharge affect the behavior of the plasma, in an attempt to maximize the gain achieved in a discharge through an argon-filled capillary.
II. Progress Made
As Chris Crawford completed his Master’s Thesis in December, 1997, it became evident that we would have to increase the temperature of our plasmas in order to achieve the desired excitation states of the argon gas. Chris measured the electron temperature as being 20-30 eV [1], giving us a population of the desired ionization state, Argon IV, but leaving these ions in their ground state. If the voltage of the pulse discharged through the capillary were increased, we will be able to achieve a higher electron temperature. In order to increase our pulse voltage, I worked to remove sharp edges from our Marx generator.
Just before the pulse discharges through the capillary, it travels down a water-filled transmission line. This transmission line has been a constant scene of arcing. Maintaining a high resistivity in the transmission-line water is key to prohibiting arcing. Upon examination, it was discovered that several iron surfaces exposed to the water were corroding. The corrosion was leeching into the water, and decreasing the resistivity. To lessen arcing from the transmission line, Josh Arel and I coated all exposed iron surfaces with a special epoxy mastic paint. This paint controls the rust problem while not adding ions to the water.
III. Current Status of the Experiment
Despite the improvements to the design of the generator, we are still not able to run efficiently enough to complete a thorough analysis of the variation of physical parameters of the experiment. The main challenges facing the experiment are:
1. A large leak in one of our vacuum lines has made it difficult to maintain the low pressures needed to work with soft x-rays. Currently, our best pumps require two hours to reach a pressure of 10-3 millitorr.
2. The paint required to control the rust problem has loosened the transmission line mounting. The line has a tendency to fall off after discharges. Remounting the line by immersing my hand in the transmission line tank requires another two hours to circulate the water through ion exchangers so that the resistivity of the water reaches an acceptably high level.
3. A measurement of the current through the capillary needs to be obtained. The capillary discharge itself produces an abundance of electrical noise, so we have difficulty finding the signal which is related to our voltage pulse.
The vacuum lines will be improved as soon as new parts can be ordered. I have already identified the location of the leak and know which part is at fault.
I am also redesigning the water tank and transmission line mounting to maintain high resistivity in the water. The transmission line will be more securely mounted to eliminate contact with the water that occurs as the line is remounted.. A smaller tank will be used so that the water circulates faster through the ion exchangers. The new tank will also be completely enclosed to lessen humidity in the Marx generator. A drop in humidity will reduce arcing from the Marx generator and premature discharges.
Finally, the difficulties in the current measurement will be resolved by better shielding. We are in the process of constructing a screen room to shield our oscilloscope and diagnostic equipment. I have examined the use of a Rogowski coil, constructed a test probe, and performed some simple diagnostics with this coil. I am also investigating alternate probes that may be more noise-immune.
IV. Future Direction
As the new semester begins, work on this project will continue. In the immediate future, we will complete the improvements outlined in the previous section. After a current measurement is obtained, our experimental collaboration will publish two papers detailing work completed by Alexander Shevelko and Chris Crawford. Once these improvements are completed , we will move ahead with my work on determining the effect of varying the physical parameters of the experiment. I spent time with Alexander Shevelko this summer learning the spectroscopic techniques that will be necessary for this analysis.
V. Acknowledgments
I would like to thank Dr. R. Steven Turley for his consistent guidance in this project as my research advisor. The chance to work with Chris Crawford was a privilege. Alexander Shevelko, of the Lebdev Institute in Moscow, was patient in teaching me the spectroscopic methods needed for our upcoming analysis. Richard Miller has offered advice with our high voltage systems. I also appreciate the help of other undergraduates Malcolm Hicken, Matt Squires, and Josh Arel, a 33 research intern from Elon College. Joseph Young has solved several problems and Scott Daniel has also been helpful. My wife, Tammy Lynn Bradford, is always a great support.
This work has been funded by an ORCA undergraduate grant, as well as the BYU Department of Physics and Astronomy.
VI. References
- C. Crawford. Spectroscopic Investigations of Soft X-Ray Amplification in a Capillary Discharge. 1997, p. 102. Unpublished master’s thesis.