Coherent Control and Manipulation of Natural and Un-Natural Parity Contributions to Electron Impact Ionization from Laser-Excited Atoms.

Plasmas are ubiquitous in nature, with more than 99% of the universe being in this fourth state of matter. Plasmas can consist of a mixture of ions, electrons and neutral atoms or molecules, depending upon their temperature. At very high temperatures (e.g. at the centre of the sun) all atoms are ionized, so only ions and electrons are present in these regions. At lower temperatures (such as at the surface of the sun and in the stellar atmosphere, in fluorescent and neon lights, in interstellar space, in ion lasers or in the earths ionosphere) a plasma consists of a mixture of charged and neutral particles. The neutral particles are atoms or molecules that are either in their ground state, or they may be in an excited state. Electrons and ions in the plasma can then collide with these neutral particles, leading to an exchange of energy resulting in further ionization or excitation. The most common collisions are with electrons, since they move most easily within the plasma. The interactions are complex in nature, and lead to many of the features observed from a plasma, such as the production of light in neon tubes and fluorescent lights, in lightning and in auroras.

Physicists need to understand the dynamics of these collisions to allow them to accurately describe the plasma. This is important in a wide range of areas, from optimising the energy of plasmas used in science and industry, through to understanding and predicting the solar wind that affects our climate. It is hence important to provide accurate models of the collisions that can occur.

In the research proposed here we will experimentally and theoretically study ionizing collisions with excited atoms for the first time. This will be a combined international effort, with experiments being conducted in Manchester, and theory being developed in the USA. Understanding collisions with excited atoms is important, as the collision probability is usually greater than for ground-state atoms. This is because their cross-section (which effectively defines their 'area') is often much larger than when they are in the ground state. This occurs since the excited electron is in a higher orbital, and so is effectively at a larger distance from the nucleus. Even if the density of excited atoms in the plasma is lower than for ground-state targets, they may hence be of equal or greater importance when describing the dynamics.

Little is known about collisions with excited targets, since it is very difficult to produce a high density of excited atoms in the laboratory. This has recently changed, since we now have tuneable lasers that can create excited atoms in sufficient quantities to carry out the experiments. The University of Manchester has invested in a suite of lasers that allow these difficult experiments to be performed. To accumulate data of sufficient accuracy the laser wavelength has to be controlled to better than 1 part in 1 billion for long periods of time, which is extremely challenging. We have demonstrated that this is possible in a set of pioneering experiments, and as part of this new work we will build control systems to allow this precision and stability to be achieved for periods of up to several weeks.

A significant advantage of exciting atoms using lasers is that we can 'shape' them prior to the collision occurring. We have discovered that this allows us to probe the collision in a unique way, so that different contributions to quantum models of the interaction can be rigorously tested. In particular, we found that so-called 'un-natural parity' terms are very important, as they can contribute up to 50% of the cross-section. These terms are not included in current plasma models, so we believe they are badly underestimating the effect of excited atoms within the plasma. The experiments proposed here provide a unique way to study these terms, and they will allow new and precise models to be developed as part of this international collaboration.

Efficient non-polluting energy generation is a goal that fusion reactors may deliver in the future, and so a large international effort is underway to understand and control these reactions. The UK is one of the main collaborators in this research, and is investing significant funding to achieve these goals. Controlled fusion usually occurs in a Tokamak, which is a large vacuum chamber that uses magnetic fields to confine a plasma that is heated to very high temperatures. If the plasma density and stability can be controlled so that the reaction is self-sustaining (i.e. more energy is released than is needed to maintain the reaction) then a net energy gain becomes possible. Instabilities and losses from the plasma must be minimized to achieve these goals. These occur through turbulence and through contamination of the plasma by atoms ejected from the vacuum walls. Subsequent collisions with these ejected atoms leads to further instabilities and further losses, and so it is important to understand these processes so that their effects can be minimized.

The research proposed here will study collisions from excited atoms, which may be created in the plasma through photon absorption or through ion or electron collisions. The ionization cross-sections from excited targets are usually much larger than for those in the ground state, and so collisions with excited atoms can lead to significant losses. The work proposed here will provide new data on these interactions, and will develop new models of the collision process. The results from these calculations can then be fed into kinetic models of the plasma to understand, control and minimize the effects of these collisions. This research will hence deliver benefits to the UK economy, by providing information that will allow more efficient energy generation from the fusion reactors of the future.

The combined experimental and theoretical research programme proposed here is an international effort that will deliver the first comprehensive measurements of excited atom ionization by electron impact. A series of experiments will be carried out using the unique facilities at the University of Manchester, so that new interaction models are fully tested. Results from the models will then impact on future areas of science and industry, ranging from fusion energy generation as described above, to optimizing the ionization cross-sections in ion lasers and lighting.

The results will also have impact in astrophysics, since they will be used in solar and planetary atmospheric models to better understand their dynamics. Dense stellar atmospheres have recently been predicted to contain a majority of excited atoms, and so accurate quantum calculations of collisions in these atmospheres are essential to predict the behavior of the sun and of other stars.

The current theories of excited-target collisions that are presently used in these areas have never been tested, due to an absence of experimental data. Recent work in Manchester and at the Los Alamos Laboratory in the USA has shown that these models do not include essential terms in the excited-state cross-section, and so are expected to be inaccurate. The new experiments proposed here will be the first to deliver a comprehensive set of data for these interactions, and precise and fully tested quantum calculations will be developed as part of this work. The combined experimental and theoretical research programme proposed here will hence deliver significant impact in all areas of science and in industry where plasmas are used or studied.