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Detailed experimental results of staging two laser-driven, relativistic electron accelerators are presented. During the experiment called STELLA (staged electron laser acceleration), an inverse free-electron laser (IFEL) is used to modulate the electron energy, thereby, causing ∼3 fs microbunches to form separated by the laser wavelength at 10.6 μm (equivalent to a 35 fs period). A second IFEL accelerates the electrons depending upon the phase of the microbunches entering the second IFEL with respect to the laser beam driving the second IFEL. The data presented includes electron energy spectra as a function of the phase delay and laser power driving the first IFEL. Also shown is a comparison with the computer model, which includes space charge and misalignment effects.

Staging of two laser-driven, relativistic electron accelerators has been demonstrated for the first time in a proof-of-principle experiment, whereby two distinct and serial laser accelerators acted on an electron beam in a coherently cumulative manner. Output from a CO₂ laser was split into two beams to drive two inverse free electron lasers (IFEL) separated by 2.3 m. The first IFEL served to bunch the electrons into ∼3 fs microbunches, which were rephased with the laser wave in the second IFEL. This represents a crucial step towards the development of practical laser-driven electron accelerators.

The spin-lattice relaxation time T₁ of liquid ³He and ³He-⁴He mixtures is determined by two parallel relaxation processes: intrinsic relaxation, which is caused by dipolar interaction between the ³He nuclear spins, and surface relaxation, due to interaction of the ³He nuclear spins with the magnetic moments at the walls of the experimental cell. Using a type of torque magnetometer, we have measured T₁ of liquid ³He containing 0.5% ⁴He and ³He-⁴He mixtures with a ³He concentration ranging from 6 to 95%, as a function of magnetic field up to 22 T at temperatures between 40 mK and 1 K. Due to the difference in their magnetic-field dependences, we have been able to separate the intrinsic and surface contributions to T₁. Our measurements reveal a surface relaxation mechanism for liquid ³He, with a relaxation time proportional to the square of the magnetic field, which can be described by the classical relaxation theory of Bloembergen, Purcell, and Pound. We relate the observed classical relaxation mechanism to the dynamics of the ³He atoms in the ⁴He film at the surface. The temperature dependence of the surface relaxation time T_s is consistent with the hypothesis that the surface relaxation is caused by diffusive motion of ³He atoms near the surface. This mechanism would naturally explain the previously unexplained observations that T_s is inversely proportional to the diffusion coefficient, while T_s is clearly larger than the diffusion time. We find the intrinsic relaxation time T_in of the pure liquid ³He in good agreement with existing Fermi-liquid theory, and observe the T_in of the ³He-⁴He mixtures at 1 K to be proportional to the ³He concentration, in agreement with theoretical predictions.

An electron beam microbunched on the optical wavelength scale of ≈2.5 μm by an inverse free electron laser accelerator was observed. The optimum bunching was achieved for a 1% energy modulation of a 32 MeV electron beam with 0.5 GW CO₂ laser power. The microbunching process was investigated by measuring the coherent transition radiation produced by the energy modulated electron beam. A quadratic dependence of the transition radiation signal on the electron beam charge was observed. The observed shortest wavelength of coherent transition radiation is less than 2.5 μm. The debunching process of the microbunched electron beam was experimentally investigated.

Using a low-temperature molecular-beam epitaxy growth procedure, Ga_1-xMn_xAs — a III-V diluted magnetic semiconductor — is obtained with Mn concentrations up to x∼9%. At a critical temperature T_c (T_c≈50 K for x=0.03–0.05), a paramagnetic to ferromagnetic phase transition occurs as the result of the interaction between Mn-h complexes. Hole transport in these compounds is strongly affected by the antiferromagnetic exchange interaction between holes and Mn 3d spins. A model for the transport behavior both above and below T_c is given. Above T_c, all materials exhibit transport behavior which is characteristic for systems near the metal-insulator transition. Below T_c, due to the rising spontaneous magnetization, spin-disorder scattering decreases and the relative position of the Fermi level towards the mobility edge changes. When the magnetization has reached its saturation value (below ∼10 K) variable-range hopping is the main conduction mechanism. The negative magnetoresistance is the result of the expansion of the hole wave functions in an applied magnetic field.

Magnetoresistance measurements of highly underdoped superconducting La_2-xSr_xCuO₄ films with x  =  0.051 and x  =  0.048, performed in dc magnetic fields up to 20 T and at temperatures down to 40 mK, reveal a magnetic-field–induced transition from weak to strong localization in the normal state. The normal-state conductances per CuO₂ plane, measured at different fields in a single specimen, are found to collapse to one curve with the use of a single scaling parameter that is inversely proportional to the localization length. The scaling parameter extrapolates to zero near zero field and possibly at a finite field, suggesting that in the zero-field limit the electronic states may be extended.

Measurements of the nuclear magnetic spin-lattice relaxation time T₁ of liquid ³He containing 0.5% ⁴He and of ³He-⁴He mixtures as a function of magnetic field up to 22 T and at temperatures down to 40 mK show a new surface relaxation mechanism for liquid ³He, which is proportional to the square of the magnetic field and can be described by classical relaxation theory. The intrinsic relaxation time of liquid ³He, obtained from T₁ measurements by eliminating the surface relaxation contribution, is in good agreement with existing Fermi liquid theory.

A 40 MeV electron beam, using the inverse free-electron-laser interaction, has been accelerated by ΔE/E  =  2.5% over a distance of 0.47 m. The electrons interact with a 1–2 GW CO₂ laser beam bounded by a 2.8 mm i.d. sapphire circular waveguide in the presence of a tapered wiggler with B_max≈1 T, and a period 2.89≤λ_w≤3.14 cm. The experimental results of ΔE/E as a function of electron energy E, peak magnetic field B_w, and laser power W_l compare well with analytical and 1D numerical simulations and permit scaling to higher laser power and electron energy.