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Active stub tuning with a fast ferrite tuner (FFT) allows for the system to respond dynamically to changes in the plasma impedance such as during the L-H transition or edge localized modes (ELMs), and has greatly increased the effectiveness of fusion ion cyclotron range of frequency systems. A high power waveguide double-stub tuner is under development for use with the Alcator C-Mod lower hybrid current drive (LHCD) system. Exact impedance matching with a double-stub is possible for a single radiating element under most load conditions, with the reflection coefficient reduced from G to G² in the "forbidden region." The relative phase shift between adjacent columns of a LHCD antenna is critical for control of the launched n|| spectrum. Adding a double-stub tuning network will perturb the phase of the forward wave particularly if the unmatched reflection coefficient is high. This effect can be compensated by adjusting the phase of the low power microwave drive for each klystron amplifier. Cross-coupling of the reflected power between columns of the launcher must also be considered. The problem is simulated by cascading a scattering matrix for the plasma provided by a linear coupling model with the measured launcher scattering matrix and that of the FFTs. The solution is advanced in an iterative manner similar to the time-dependent behavior of the real system. System performance is presented under a range of edge density conditions from under-dense to over-dense and a range of launched n||. [ABSTRACT FROM AUTHOR]

The plasma afterburner has been proposed as a possible advanced acceleration scheme for a future linear collider. In this concept, a high energy electron(or positron) drive beam from an existing linac such as the SLC will propagate in a plasma section of density about one order of magnitude lower than the peak beam density. The particle beam generates a strong plasma wave wakefield which has a phase velocity equal to the velocity of the beam and this wakefield can be used to accelerate part of the drive beam or a trailing beam. Several issues such as the efficient transfer of energy and the stable propagation of the particle beam in the plasma are critical to the afterburner concept. We investigate the nonlinear beam-plasma interactions in such scenario using a new 3D particle-in-cell code called QuickPIC. Preliminary simulation results for electron acceleration, beam-loading and hosing instability will be presented. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The amount of spectral energy released by the plasma as visible light is proportional to the amount of energy loss of the beam, a simple beam independent diagnostic which is an ideal tool to help understand wake field plasma interaction is presented here. Continuum light emitted by particles through Cerenkov and/or optical transition processes also indicate the appearance of plasma particles trapped by the large amplitude plasma wake of the PWFA. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

An expression for the maximum accelerating field in the narrow bunch limit (kpσr<<1) for a bi-Gaussian drive beam is derived from linear plasma wakefield theory. The optimal plasma density for a given beam (N, σz, σr) is found to be kpσz = [square_root_of_2] in the narrow beam limit. We also show that the previously obtained scaling law in which the wake amplitude increases with the inverse square of the bunch length must be modified by a weak logarithmic function of the beam spot size σr. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

In current plasma-based accelerator experiments, very short bunches (100–150μm for E164 and 10–20 μm for E164X experiment at Stanford Linear Accelerator Center (SLAC)) are used to drive plasma wakes and achieve high accelerating gradients, on the order of 10–100GV/m. The self-fields of such intense bunches can tunnel ionize neutral gases and create the plasma. This may completely change the physics of plasma wakes. A 3-D object-oriented fully parallel PIC code OSIRIS is used to simulate various gas types, beam parameters, etc. to support the design of the experiments. The simulation results for real experiment parameters are presented. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

A series of plasma wakefield acceleration (PWFA) experiments are being conducted with a 30 GeV drive beam from the Stanford Linear Accelerator Center (SLAC). These experiments continue to address the application of meter-scale plasmas to focus and accelerate electrons and positrons in the context of future applications to high-energy accelerators. [ABSTRACT FROM AUTHOR]