106 research outputs found
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Global ICRF modeling in large non-circular tokamak plasmas with finite temperature
No full textFull wave ICRF coupling calculations in two- and three dimensions have been extended to treat tokamaks with non-circular flux surfaces and conducting boundaries. The magnetic field configuration is derived from a Solovev equilibriium with finite poloidal magnetic fields. The conducting boundary may be of arbitrary shape. The mode conversion model is that of Colestock et al., in which the fourth order finite temperature wave equation is reduced to a second order equation which describes the effects of mode conversion on the fast wave but neglects the detailed structure of the ion Bernstein wave. Results show the effect of non-circular cross section on excitation, wave propagation, and absorption in Doublet III-D and JET. Also, in the limit of circular cross section, toroidal phasing of the resonant double loop antenna design for TFTR is studied
Effect of surface modes on coupling to fast waves in the LHRF
No full textThe effect of surface modes of propagation on coupling to fast waves in the LHRF is studied theoretically and experimentally. The previously reported up-down' poloidal phasing asymmetry for coupling to a uniform plasma is shown to be due to the properties of a mode which carries energy along the plasma-conducting wall interface. Comparison of the theory with coupling experiments performed on the PLT tokamak with a phased array of twelve dielectric-loaded waveguides at 800 MHz shows that the observed dependence of the net reflection coefficient on toroidal phase angle can be explained only if the surface wave is taken into account. 43 refs., 10 figs
Trapped Ions and Beam Coherent Instability
No full textIn accelerators with negatively charged beams, ions generated from the residual gas molecules may be trapped by the beam. Trapped ions may interact resonantly with the beam and cause a beam-ion coherent instability. This coherent instability bears many similarities to the resistive wall instability and can present important limitations to those machines operation. A description of this effect requires a treatment of the beam coherent instability including both the normal machine wake field and the interaction with ions. They present a linear approach incorporating contributions from the machine impedance as well as ion forces. it also includes spreads in beam and ion frequencies and thus Landau damping. The analysis results in a modified stability diagram which will be used together with physical arguments to explain experimental observations in the Fermilab antiproton accumulator
Excitation and propagation of lower‐hybrid waves in a bounded, inhomogeneous plasma
No full textThe excitation and propagation of lower‐hybrid waves in an inhomogeneous, cylindrical plasma is studied theoretically for finite‐length electrostatic sources. The boundary‐value problem for the electrostatic potential in a cold, inhomogeneous plasma is solved numerically as a superposition of the radial eigenmodes excited by a finite‐length source. Radial eigenmodes are found numerically by an algorithm which includes the case where the lower‐hybrid resonance layer occurs in the plasma. The eigenmode superposition is carried out for several phased‐ring sources. The plasma response is found to be composed of resonance‐cone surfaces along which the potential is a maximum. When the resonance layer does not occur in the plasma, the resonance‐cone surfaces reflect from the column axis and at the plasma boundary. For the case when the resonance layer does occur, the resonance‐cone surfaces become asymptotic to the resonance layer and do not penetrate to the center. The presence of damping causes the resonance‐cone singularities to dissolve axially leaving the lowest‐order radial mode excited by the source.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70156/2/PFLDAS-19-8-1229-1.pd
Measurement of Escaping Ions in the Fermilab Antiproton Accumulator
No full textPositively charged ions trapped in the negatively charged beam of the Fermilab antiproton accumulator pose a limit to beam stability and density. To better understand the dynamics and the consequences of the beam-ion interaction, they have built and installed a low energy ion detector and energy analyzer in the Fermilab accumulator. This analyzer is capable of energy analysis of the escaping ions using a probe with energy retarding grids and may also be scanned in the pitch angle of the escaping ions. Measurements have been made in both longitudinal and transverse planes under a variety of machine operating conditions. The experimental measurement results are presented together with attempts to model the ion dynamics and explain observations
Direct Measurement of Diffusion Rates in High Energy Synchrotrons Using Longitudinal Beam Echoes
No full textParasitic excitation of ion Bernstein waves from a Faraday shielded fast wave loop antenna
No full textParasitic excitation of ion Bernstein waves is observed from a Faraday shielded fast wave loop antenna in the ion cyclotron frequency range. Local analysis of the Vlasov-Maxwell equations demonstrates the role of plasma density gradient in the coupling process. The effects of plasma density and of parallel wave number on the excitation process are investigated
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