Layer, BrianUsing the unique properties of the interaction between intense, short-pulse lasers and nanometer scale van-der-Waals bonded aggregates (or `clusters'), modulated waveguides in hydrogen, argon and nitrogen plasmas were produced and extreme ultraviolet (EUV) light was generated in deeply ionized nitrogen plasmas. A jet of clusters behaves as an array of mass-limited, solid-density targets with the average density of a gas. Two highly versatile experimental techniques are demonstrated for making preformed plasma waveguides with periodic structure within a laser-ionized cluster jet. The propagation of ultra-intense femtosecond laser pulses with intensities up to 2x10<super>17<super> W/cm<super>2<super> has been experimentally demonstrated in waveguides generated using both methods, limited by available laser energy. The first uses a `ring grating' to impose radial intensity modulations on the channel-generating laser pulse, which leads to axial intensity modulations at the laser focus within the cluster jet target. This creates a waveguide with axial modulations in diameter with a period between 35 &mu;m and 2 mm, determined by the choice of ring grating. The second method creates modulated waveguides by focusing a uniform laser pulse within a jet of clusters with flow that has been modulated by periodically spaced wire obstructions. These wires make sharp, stable voids as short as 50 &mu;m with a period as small as 200 &mu;m within waveguides of hydrogen, nitrogen, and argon plasma. The gaps persist as the plasma expands for the full lifetime of the waveguide. This technique is useful for quasi-phase matching applications where index-modulated guides are superior to diameter modulated guides. Simulations show that these `slow wave' guiding structures could allow direct laser acceleration of electrons, achieving gradients of 80 MV/cm and 10 MV/cm for laser pulse powers of 1.9 TW and 30 GW, respectively. Results are also presented from experiments in which a nitrogen cluster jet from a cryogenically cooled gas valve was irradiated with relativistically intense (up to 2x10<super>18<super> W/cm<super>2<super>) femtosecond laser pulses. The original purpose of these experiments was to create a transient recombination-pumped nitrogen soft x-ray laser on the 2p<sub>3/2<sub>&rarr;1s<sub>1/2<sub> (&lambda; = 24.779 &Aring;) and 2p<sub>1/2<sub>&rarr;1s<sub>1/2<sub> (&lambda; = 24.785 &Aring;) transitions in H-like nitrogen (N<super>6+<super>). Although no amplification was observed, trends in EUV emission from H-like, He-like and Li-like nitrogen ions in the 15 - 150 &Aring; spectral range were measured as a function of laser intensity and cluster size. These results were compared with calculations run in a 1-D fluid laser-cluster interaction code to study the time-dependent ionization, recombination, and evolution of nitrogen cluster plasmas.DissertationPhysicsAcceleratorLaserPlasmaWaveguidesX-ray