CREATING the stuff of stars-this was the goal of a team of experimental researchers from Lawrence Livermore and Texas A&M University.Using the Laboratory's electron-beam ion trap (EBIT) as an ion source and a cryogenic Penning ion trap (RETRAP) to capture, confine, and cool the ions, these experimenters produced a form of matter that is the thermodynamic analog of the matter found in white dwarf stars. "This development has exciting astrophysical ramifications," says Dieter Schneider, EBIT program leader. "Understanding the cooling process of white dwarf stars will help us determine their age and the age of the universe."1





Making Ions to Order
Originally developed and built at Lawrence Livermore by physicists Mort Levine and Ross Marrs in 1985, EBIT uses a tightly focused and energy-tunable electron beam to create and trap highly charged ions. (An ion is an atom or molecule that has become charged by gaining or losing one or more electrons. A completely ionized atom is one stripped of all of its electrons.) Virtually any charge state of any element in the periodic table can be studied using EBIT. It is the only ion source in the world that can create the highest charged ions at rest; other sources able to produce such highly charged ions involve accelerators that increase the velocity of the ions to extremely high energies.
The Livermore EBIT consists of a high-current-density electron beam (up to 5,000 amperes per square centimeter) passing through a series of three drift tubes that hold in place ions of the element being studied. These positively charged ions are confined radially by being attracted to the center of the electron beam and are trapped axially by voltages applied to the end drift tubes. As the electrons in the beam collide with an ion, they strip electrons off the ion until the energy required to remove the next electron is higher than the beam energy.
The original EBIT had a peak electron-beam energy of about 30 kiloelectronvolts, enough to make uranium ions with the same number of electrons as neon (U82+). The Super-EBIT can achieve an electron beam energy of 220 kiloelectronvolts, enough to make bare uranium (U92+). Uranium has the highest atomic number among the naturally occurring elements, and therefore, Super-EBIT is sufficiently powerful to serve as the ion source for all of these elements.
Once the highly charged ions are created in EBIT, they are extracted to RETRAP, where they are cooled, stored, and studied. (One of several types of ion traps, the Penning trap uses static electric and magnetic fields to hold the ions.) RETRAP allows researchers to control the temperature and the relative position of the ions. In particular, it allows cooling the ions (reducing their kinetic energy by slowing down their random motion) to the near-zero temperatures needed to create strongly coupled, crystallized plasmas.2
The cooling is accomplished in a two-step cooling scheme developed at Lawrence Livermore. (See the figure below.) First, the cloud of light, singly charged beryllium ions (Be+) in the ion trap is illuminated with laser beams whose frequency has been selected so that only those ions moving away from the beam absorb the laser light. As the ions reemit the light in a random direction and return to their ground state, they (on average) lose kinetic energy. The process cools the ion cloud to temperatures of a few kelvins. Highly charged xenon ions (Xe44+) are extracted from EBIT and moved to the RETRAP. The beryllium ions, which continue to be cooled by the laser, sympathetically cool the xenon ions, slowing them down. The temperature (energy) of the xenon ions drops until both ion species come to a thermal equilibrium, which takes about 20 seconds. As the temperature of the mixture drops to about 9 kelvins, the mixed plasma splits, with the highly charged xenon ions making up a microplasma trapped in the center of a surrounding cloud of low-charged beryllium ions. (The xenon ion cloud is nonfluorescing and therefore cannot be seen.) The research team has trapped microplasmas for as long as 1,000 seconds.
The densities of the microplasmas created with EBIT and RETRAP reach about 100 million (108) ions per cubic centimeter, with the distance between the ions being a few micrometers. (Normal, room-temperature liquids and solids have densities of about 1023 atoms per cubic centimeter and distances between atoms of a few nanometers.)





Cool and Thin Equals Hot and Dense
At these ultracold temperatures and at certain densities, the microplasmas condense to form an ionic crystal in which the individual ions lock into place relative to each other yet retain their individual identities. "Unlike a noncharged or neutral plasma," says Schneider, "these highly charged plasmas can exist at thermal equilibrium."
What's more, these microplasmas are thermodynamically equivalent to certain exotic high-density plasmas found in white dwarf stars. That is, these two plasmas, at opposite ends of the temperature and density spectrums, have the same thermodynamic properties-for example, specific heat and phase transitions. The reason for this strange parallelism lies in the definition of a single, dimensionless parameter called the Coulomb coupling parameter. This parameter is determined by the density and temperature of the plasma as well as the amount of attraction or repulsion felt by neighboring ions because of their charge (the Coulomb force). Schneider says, "In the EBIT-RETRAP system, we create a strongly coupled, highly charged plasma that crystallizes and has the same Coulomb coupling parameters as those found in the plasmas of white dwarf stars. As long as the parameter is the same, the thermodynamic properties of the plasmas are analogous, even though, in the trap, the ion densities are 20 orders of magnitude and the temperatures 9 orders of magnitude lower than those found in white dwarf stars."
The extreme conditions in white dwarf stars lead to highly ionized plasmas that are essentially bare nuclei of mostly carbon and oxygen, stripped of all their electrons. The electrons form a uniform background of negative charge, confining the star plasma much as the microplasma is confined by electric and magnetic fields in the RETRAP. Another big plus for researchers who seek to understand these plasmas better is that the EBIT-RETRAP system can be used to create microplasmas consisting of a mix of ion species, just like those in the stars themselves. The system is unique because not only can researchers choose the concentration of different ion species they want in the plasma mix, but they also can control the density and the temperature of the plasma. "This capability exists nowhere else in the world," says Schneider.





Future in the Stars
Studies of these exotic plasmas are helping researchers understand and model the cooling of hot, dense stars and the evolution of our galaxy.
Other intriguing research directions are possible, notes Schneider, including the possibility of creating quantum-computing gates based on ions in crystals stored in traps. A quantum computer could exponentially reduce the time required to complete a complex computation.
—Ann Parker

References:
1. Stringfellow, G. S., et al., "Equation of State of the One-Component Plasma Derived from Precision Monte Carlo Calculations," Physical Review A, 41(2), 1105-11 (1990).
2. Steiger, J., et al., "Coulomb Clusters in RETRAP," AIP Conference Proceedings (No. 457), Trapped Charged Particles and Fundamental Physics, Asilomar, CA, 31 August-4 September 1998, (American Institute of Physics, 1999), pp. 284-289. Key Words: Coulomb coupling parameter, cryogenic Penning ion trap (RETRAP), electron-beam ion trap (EBIT), highly ionized plasmas, microplasmas, white dwarf stars.

For more information contact Dieter Schneider (925) 423-5940 (schneider2@llnl.gov).


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