Biomedical subjects

Joseph Nilsen

Publications and source records attributed to Joseph Nilsen.

6 recordsLinked to original sources

PubMed2006

Prediction and observation of tin and silver plasmas with index of refraction greater than one in the soft x-ray range.

We present the calculated prediction and the experimental confirmation that doubly ionized Ag and Sn plasmas can have an index of refraction greater than one for soft x-ray wavelengths. Interferometry experiments conducted using a capillary discharge soft x-ray laser operating at a wavelength of confirm that in few times ionized laser-created plasmas of these elements the anomalous dispersion from bound electrons can dominate the free electron contribution, making the index of refraction greater than one. The results confirm that bound electrons can strongly influence the index of refraction of numerous plasmas over a broad range of soft x-ray wavelengths confirming recent observations. The understanding of index of refraction at short wavelengths will become even more essential during the next decade as x-ray free electron lasers will become available to probe a wider variety of plasmas at higher densities and shorter wavelengths.

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PubMed2005

Plasma interferometry and how the bound-electron contribution can bend fringes in unexpected ways.

Utilizing a new average atom code, we calculate the index of refraction in C, Al, Ti, and Pd plasmas and show many conditions over which the bound-electron contribution dominates the free electrons as we explore photon energies from the optical to 100 eV (12 nm) soft x rays. For decades measurement of the electron density in plasmas by interferometers has relied on the approximation that the index of refraction in a plasma is due solely to the free electrons and therefore is less than 1. Recent measurements of Al plasmas using x-ray laser interferometers observed fringes bending in the opposite direction than expected due to the bound-electron contribution causing the index of refraction to be larger than 1. During the next decade x-ray free-electron lasers and other sources will be available to probe a wider variety of plasmas at higher densities and shorter wavelengths, so understanding the index of refraction in plasmas is essential.

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PubMed2004

Plasmas with an index of refraction greater than 1.

Over the past decade, x-ray lasers in the wavelength range 14-47 nm have been used for interferometry of plasmas. As in optical interferometry of plasmas, the experimental analysis assumed that the index of refraction is due only to free electrons. This makes the index of refraction less than 1. Recent experiments in A1 plasmas have shown fringe lines bending the wrong way as though the electron density were negative. We show how the bound electrons can dominate the index of refraction in many plasmas and make the index greater than 1 or enhance the index such that one would greatly overestimate the density of the plasma using interferometry.

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PubMed2004

Picosecond-resolution soft-x-ray laser plasma interferometry.

We describe a soft-x-ray laser interferometry technique that allows two-dimensional diagnosis of plasma electron density with picosecond time resolution. It consists of the combination of a robust high-throughput amplitude-division interferometer and a 14.7-nm transient-inversion soft-x-ray laser that produces approximately 5-ps pulses. Because of its picosecond resolution and short-wavelength scalability, this technique has the potential for extending the high inherent precision of soft-x-ray laser interferometry to the study of very dense plasmas of significant fundamental and practical interest, such as those investigated for inertial confinement fusion. Results of its use in the diagnostics of dense large-scale laser-created plasmas are presented.

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PubMed2003

Mo:Y multilayer mirror technology utilized to image the near-field output of a Ni-like Sn laser at 11.9 nm.

Although bright x-ray sources exist at shorter wavelengths, the development of sophisticated diagnostics with x-ray laser sources has been restricted to wavelengths longer than 12.5 nm because of the limitations of the widely used Mo:Si multilayer mirrors. With the novel Mo:Y multilayer mirrors that we present, many x-ray laser applications can be extended to the 7-12-nm range. We demonstrate this new capability by imaging the near-field output of the Ni-like Sn laser at 11.9 nm.

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PubMed2003

Longitudinal coherence measurements of a transient collisional x-ray laser.

We present what is to our knowledge the first longitudinal coherence measurement of a transient inversion collisional x-ray laser. We investigated the picosecond output of a Ni-like Pd x-ray laser at 14.68 nm generated by the COMET laser facility at the Lawrence Livermore National Laboratory. Interference fringes were generated with a Michelson interferometer setup in which a thin multilayer membrane was used as a beam splitter. We determined the longitudinal coherence for the 4d1S0 --> 4p1P1 lasing transition to be approximately 400 microm (1/e half-width) by changing the length of one interferometer arm and measuring the resultant variation in fringe visibility. The inferred gain-narrowed linewidth of approximately 0.29 pm is a factor of 4 less than previously measured in quasi-steady-state x-ray laser schemes.

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