Quantum Electrodynamics, Strong Fields, and Ion-Matter Interactions

Physics Motivation

The new facility at GSI has key features that offer a range of new opportunities in atomic physics and related fields. First, high-charge state ions moving at velocities close to the speed of light generate electric and magnetic fields of exceptional strength. Second, at those relativistic velocities, the energies of optical transitions, such as for lasers, are boosted to the x-ray region. The strong fields carried by heavy, highly-charged ions are their outstanding attributes for atomic and applied physics research. Together with anticipated high beam intensities a range of important experiments is envisioned.

In relativistic, high-Z ion-atom collisions, extremely intense photon fields arise due to both, the high nuclear charges and the extremely high velocities. This will even lead to the creation of real particle-antiparticle pairs (e.g. e+-e).

For the heaviest ions, Quantum ElectroDynamics (QED), the ‘Standard Model’ of electromagnetism and a basis of modern physics, will be probed near the critical field limit associated with the extreme conditions of high charge states and high velocities. The fields present in highly relativistic collisions are strong enough to produce real e+-e- pairs directly out of the vacuum (Figure 1.). Precision studies of QED in bound states will be possible through the large Doppler shifts of highly relativistic ions which generate extreme energy shifts for photons in the ion rest frame. As a consequence, even the heaviest few-electron ions can now be studied in precision QED experiments by using state of the art laser systems. The Doppler effect will also be used for the first time for laser cooling of heavy, highly-charged ions, promising beams at relativistic energies and brilliances that are suited for unique precision studies in atomic and nuclear physics. Moreover, the interaction of relativistic, highly-charged heavy ions with matter provides new possibilities in applications, in particular in material modifications and tests as well as in biophysics and space research.

Atomic Physics Research

At SIS200, and the associated fixed target area, the elementary atomic interaction processes with matter will be studied at high values of the relativistic Lorentz factor (. In this case the electric and magnetic fields increase dramatically and are strongly deformed. In contrast to lower energies where magnetic forces are generally of minor importance, they start here to equal the electric ones. This high-relativistic region could not be addressed in any detail up to now. For example, at high ( the magnetic terms will completely change the elementary photon-electron interaction reflected by photo-ionization and radiative recombination. Moreover, new and fundamental recombination, excitation and ionization processes involving pair creation will come into play. The high Lorentz boost for photons will allow precision laser spectroscopy of highly-charged– especially Li-like– heavy ions using standard laser techniques at normal photon energies. Laser cooling of relativistic heavy ions will be investigated.

At the New Experimental Storage Ring (NESR) both, atomic structure and ion-atom collisions of highly-charged ions will be studied background-free. The excellent qualities of the heavy ion beams allow, in the strong field limit, the study of subtle higher order effects for elementary interaction processes as well as important tests of fundamental symmetries. The electron-electron interaction, manifested in auto-ionization and dielectronic recombination, will be studied at the new electron target by means of cooled heavy-ion beams, decelerated in special cases for improved sensitivity. There, even the innermost electrons of high-Z ions will be probed with drastic increases of interaction strengths due to magnetic effects. The collision of counter-propagating laserpulses with the electron bunches from the attached electron collider will create intense x-ray pulses.

Additionally, behind the NESR, a dedicated experimental area for extracted highly-charged, heavy ions at low energies opens the area of adiabatic collisions far off charge-state equilibrium, of exotic multi-excited states formed by multi-electron capture and their stabilization, as well as the field of precision spectroscopy and QED tests in bare and few-electron heavy atomic nuclei. In this connection the deceleration of these heavy ionic species with subsequent capture in an ion-trap and ion cooling will provide for a new regime of sensitivity. A corresponding trapping facility has been considered and technical developments are currently being supported by an EU RTD project, with the hope for construction in the future.

The unique features of the new facility together with powerful experimental tools also pave the way to highly sensitive tests of fundamental symmetry principles, such as parity conservation or time-reversal invariance. Moreover, stored, cooled and polarized nuclei would allow one to search for a nuclear electric dipole moment, caused by a simultaneous violation of both, parity and time-reversal symmetry. By measuring beta-neutrino correlations of trapped and cooled radioactive nuclei the Standard Model of weak interaction can be tested with high sensitivity. This is also discussed in the section on rare-isotope beams.

The advanced studies at the new facility will strongly benefit from the experience gained at the excisting SIS/ESR. In particular, the heavy-ion storage ring ESR has played a pioneering role in opening unexplored fields of research with energetic heavy ions. Both, atomic structure and atomic collision dynamics under the extreme conditions of the strongest fields available were investigated at SIS/ESR in a regime where relativistic effects only begin to become important. QED studies for the strong-field case have successfully been started. For elementary processes, like photon-electron or electron-electron interactions, unexpected effects of the magnetic part in the strong-field cases have been already seen at low ?. In parallel, atomic structure investigations have already reached an impressive level of precision. By means of collinear laser spectroscopy the ground state hyperfine-splitting of very heavy, hydrogen-like atoms has been probed with high accuracy. Based on this experience, nuclear properties like radii, spins, magnetic dipole moments and higher electromagnetic moments of nuclei very far off stability will be addressed at the new facility by experimental techniques of atomic physics. Many of the research topics mentioned – from collision studies to spectroscopy – that were started successfully at the ESR, will be expanded into new regimes and under much better and advanced experimental conditions at the NESR.

Instrumentation

The experimental areas at the new facility provide a range of novel instrumentation for atomic and applied research. From SIS200 the unprecedented combination of high intensities and of acceleration up to (= 23 will be available. The new multi-purpose experimental atomic physics area will be supplied by ion beams both from SIS18 and SIS200. A target station for high-energy ions will also serve for the irradiation of samples for biological and materials research and will be equipped with a raster scan system.

In addition, laser installations are planned for various experimental areas of the new GSI facility. In particullar, the high power PHELIX facility will allow the study of interactions of the most intense laser fields with heavy ions.

The NESR will be the workhorse for atomic physics experiments. Compared to all other heavy-ion storage rings currently in operation or under construction, the NESR will be the most flexible one, providing the most intense beams up to bare uranium. Moreover, new instrumentations will be available, such as an internal gas jet, an electron target, and electron bunches provided by the electron collider for interactions with collinear, high-intensity laser pulses. The intense beams of highly-charged radioactive ions make novel experiments possible at the interface of atomic and nuclear physics.

For a wide range of ion velocities, the interaction of highly-charged ions with matter will be investigated in fixed target experiments. This experimental area will be located next to the NESR and is devoted to experiments with fast- or slow-extracted ions. An important feature of the NESR is its capability to decelerate heavy ions. The highly-charged ions can, after extraction, be actively slowed down further, even to rest, for ultra-precision studies in the heavy ion trap facility HITRAP. Compared to the EBIT facilities (electron-beam ion traps), the HITRAP facility will allow the capture of highly-charged ions of any element up to uranium with substantially higher yields. The high intensity of secondary beams, produced at the SFRS and decelerated in the NESR, will allow the trapping even of exotic nuclei.