The Integrable Optics Test Accelerator (IOTA) is a storage ring for advanced beam physics research currently being built and commissioned at Fermilab. It will operate with protons and electrons using injectors with momenta of 70 and 150 MeV/c, respectively. The research program includes the study of nonlinear focusing integrable optical beam lattices based on special magnets and electron lenses, beam dynamics of space-charge effects and their compensation, optical stochastic cooling, and several other experiments. In this article, we present the design and main parameters of the facility, outline progress to date and provide the timeline of the construction, commissioning and research. The physical principles, design, and hardware implementation plans for the major IOTA experiments are also discussed.

We report on the experimental generation of a train of subpicosecond electron bunches. The bunch train generation is accomplished using a beamline capable of exchanging the coordinates between the horizontal and longitudinal degrees of freedom. An initial beam consisting of a set of horizontally-separated beamlets is converted into a train of bunches temporally separated with tunable bunch duration and separation. The experiment reported in this Letter unambiguously demonstrates the conversion process and its versatility.

We studied the conditions under which optical gain is measured in nanocrystalline silicon (nc-Si) using the variable stripe length method. Waveguide samples have been produced by magnetron sputtering of alternating layers of Si and SiO2, followed by high temperature annealing. No optical gain was observed under continuous wave pumping conditions. Under high intensity pulsed excitation, a superlinear fast (10 ns) recombination component yielding an optical gain up to 50 cm−1 has been independently measured in two different laboratories. A control experiment confirmed that the presence of nc-Si is necessary to achieve gain in our structures.

We report on the tunneling density of states (DOS) in ultrathin and strongly disordered Be films quench-condensed at 20 K. Above 5 K, the DOS shows the well-known logarithmic anomaly at the Fermi level. Only in a narrow temperature range near 2 K is the DOS linearly dependent on energy, as predicted by Efros and Shklovskii. However, both the zero-bias conductance and the slope of the linear DOS are found to decrease drastically with decreasing temperature. Tunneling measurements at mK temperatures have revealed conclusively that a hard correlation gap opens up in the DOS.PACS numbers: 73.40. Gk, 72.15.Rn, 71.30.+h, 74.40+k It is known that electron-electron (e-e) Coulomb interactions can drastically alter the density of states (DOS) near the Fermi energy in disordered electronic systems. In the weakly disordered limit, Altshuler et al.[1] have predicted that interactions lead to a singular depletion of the DOS with a |ǫ| 1/2 dependence in three dimensions (3D) and a ln|ǫ| dependence [2] in two dimensions(2D), where ǫ is the energy measured from the Fermi level. These corrections have been observed in tunneling studies of the DOS in disordered metals in 3D [3] and 2D [4,5]. In the strongly insulating regime, Efros and Shklovskii (ES) have predicted [6,7] that Coulomb interactions lead to a soft Coulomb gap in the single-particle DOS, with a vanishing DOS at the Fermi level. This soft gap is quadratic in energy in 3D and linear in energy in 2D. In both 2D and 3D, the Coulomb gap is predicted [7] to lead to a variable-range hopping resistance of R ✷ (T) = R 0 exp [(T 0 /T ) ν ], where ν = 1/2 and T is the temperature.Although it was predicted over two decades ago, the Coulomb gap is by no means an understood subject. The existence of the ES Coulomb gap had mainly been inferred from transport experiments such as glassy electronic relaxation [8] and hopping conduction [9,10]. The ES Coulomb gap in 3D was directly observed a few years ago by tunneling in Si:B [11]. Direct evidence for the ES Coulomb gap in 2D has been reported only during the past year by Butko et al. [12], but no temperature dependence and magnetic field dependence have been reported. The Coulomb gap predicted by Efros and Shklovskii [6,7] describes the DOS for adding an extra electron to the ground state without allowing relaxation. Later theoretical studies [13] of the Coulomb gap, taking into consideration multi-electron processes, have found a further reduction of the DOS near the Fermi energy, leading to a much harder gap with effectively no states within a narrow but finite range of energy. In fact, a change in the hopping exponent with decreasing temperature from ν = 1/2 to ν = 1 was reported a few years ago in Si:B[10], suggesting that a hard gap might exist at low temperatures. Most recently, the Coulomb gap in 2D has become a subject of renewed interest [14] with the unexpected discovery of a metal-insulator transition in the 2D electron gas in semiconductor devices [15].

We experimentally demonstrate the production of narrow-band (df =f % 20% at f % 0:5 THz) transition radiation with tunable frequency over [0.37, 0.86] THz. The radiation is produced as a train of sub-picosecond relativistic electron bunches transits at the vacuum-aluminum interface of an aluminum converter screen. The bunch train is generated via a transverse-to-longitudinal phase space exchange technique. We also show a possible application of modulated beams to extend the dynamical range of a popular bunch length diagnostic technique based on the spectral analysis of coherent radiation.