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  • 1
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 4779-4784 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Plasmas are generated in a low ionization potential gas, tetrakis(dimethylamino)ethylene (TMAE) vapor through a one-photon ionization process by an ultraviolet laser beam at a 193 nm wavelength. The TMAE plasma characteristics are studied by means of a Langmuir probe and microwave scattering. A new method is used to measure the 193 nm ultraviolet (UV) photon absorption cross section in TMAE. It is determined to be 1.1±0.3×10−17 cm2 from the axial profile of electron density. The temporal evolution of electron temperature is measured. A peak electron density of ne=5×1013/cm3 and peak electron temperature of Te=1 eV are measured at 500 mTorr TMAE pressure. The plasma decay process is studied, and the electron-ion recombination coefficient is measured to be 5.4±0.5×10−6 cm3/s. A theoretical model is derived to describe the photon flux and the one photon ionization process. An application of a TMAE plasma as a mirror for microwave reflections is proposed.
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 92 (2002), S. 698-709 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The feasibility of using a photoionized, low-ionization potential organic seed gas to initiate a high pressure plasma discharge is examined and compared to radio frequency breakdown of high pressure argon alone. The seed gas, tetrakis(dimethylamino)ethylene, which has an ionization potential of 6.1 eV is ionized by an ultraviolet laser through 6.4 eV photon absorption, and forms a plasma column inside a vacuum chamber. The plasma absorbs additional power through inductive coupling of 13.56 MHz helical antenna radio frequency wave fields to the plasma through electron acceleration, ionization, and collisional damping. Laser initiation of 2–6 mTorr of the seed gas in 1–150 Torr of argon is accomplished and produces steady-state line-average plasma densities of ne(approximate)4×1012 cm−3 in a volume of 300 cm3. The two-body recombination coefficient of the organic seed gas and its optimum partial pressure when mixed with argon are experimentally determined and analyzed. Particle loss and power requirements for maintaining the discharge are evaluated by examining ionization, diffusion, and recombination processes. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 4253-4261 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A two-dimensional (2-D), finite-difference computer code is developed to examine helicon antenna coupling, wave propagation, collisionless Landau, and collisional heating mechanisms. The code calculates the electromagnetic wave fields and power absorption in an inhomogeneous, cold, collisional plasma. The current distribution of the launching antenna, which provides the full antenna spectra, is included in the model. An iterative solution that incorporates warm plasma thermal effects has been added to the code to examine the contribution of collisionless (Landau) wave absorption by electrons. Detailed studies of the wave fields and electron heating profiles at low magnetic fields (B0〈100 G), where both Trivelpiece–Gould (TG) and helicon (H) modes are present, are discussed. The effects of the applied uniform magnetic field (B0=10–1000 G), 2-D (r,z) density profiles (ne0=1011–1013 cm−3), neutral gas pressures of 1–10 mTorr and the antenna spectrum on collisional and collisionless wave field solutions and power absorption are investigated. Cases in which the primarily electrostatic (TG) surface wave dominates the heating and the power is absorbed near the edge region and cases in which the propagating helicon wave transports and deposits its energy in the core plasma region are examined. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2797-2805 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The shape of the gain curve and physics of interaction between microwave fields and the electron beam for a periodic rectangular grating slow-wave Traveling Wave Tube (TWT) amplifier are investigated. This research focuses on the collective (ωpτ(very-much-greater-than)1) low-voltage (≤10 kV) interaction in the backward wave regime for the Ku-band (12.5–17.5 GHz) range of frequencies. For amplifier experiments, a round "probe" beam (10 kV, 0.25 A, 1 mm diameter) confined by a 1 kG solenoidal focusing magnetic field was utilized. Simultaneous single-pass collective gain and electron-beam velocity spread measurements are performed by means of a Faraday cup repeller energy analyzer configured as the beam collector. Experiments near 13.0 GHz show the maximum normalized single-pass gain to be in agreement with the theoretical prediction for several axial velocity spreads (δv||/v||). Collective single-pass experimental results are presented and compared with theory. The effect of an increased interaction length which is accomplished by partial end reflections on the enhanced convective growth is measured and discussed. Narrowing of the gain curve as a function of beam voltage and frequency is shown to result from critical details of the electron-beam radial current density distribution and is verified experimentally by varying the beam-grating separation. Experimental results for several beam-grating separations and the effect of a nonuniform peaked radial current density on the interaction physics are also discussed. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2707-2715 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A periodic, rectangular grating slow-wave-structure is considered for forward and backward wave low-voltage (≤10 kV) Ku-band traveling wave tube (TWT) amplifiers. For forward wave operation, it is required that the ratio of groove depth, d, to grating period, p, be large (i.e. d/p≥5) while small values ofd/p allow backward wave operation. For large d/p, skin effect losses in the grating slots are large and can substantially reduce the growth rate produced by the beam-slow wave interaction. Phase and amplitude measurements of the grating structures utilizing a slotted line and a fast Fourier transform (FFT) analysis have been carried out. The results show that the measured dispersion relations for both shallow (d/p= 0.446) and deep groove gratings (d/p= 7.43) agree very well with the theoretical dispersion relations. For amplifier experiments, a round "probe" beam (10 kV, 0.25 A, 1 mm radius) from a Litton Pierce electron gun (model M707) is utilized. The beam is confined by means of a 1 kG focusing solenoidal magnetic field. Tuning curve and single particle backward wave gain measurements are presented and discussed. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 63-68 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A pulsed, 193 nm excimer laser is utilized to photoionize the organic gas tetrakis- dimethylamino-ethylene (TMAE). The laser ionizes a plasma sheet with a width of 7.8 cm and an adjustable thickness of 0.7–1.4 cm. The axial scale length of the plasma density is a function of TMAE neutral pressure and is typically 50 cm. X-band (10 GHz) microwaves are incident on the plasma with the electric field polarized parallel to the laser beam axis. The power reflection coefficient and the phase of the reflected signal are studied as a function of time. A monostatic homodyne detection system with a response time of 10 ns is utilized to determine the amplitude and phase of the reflected wave. The peak plasma density is ne(approximate)4×1013 cm−3, sufficiently above the critical density (ncrit=1.2×1012 cm−3) to produce reflections comparable to a conducting sheet placed in the same position as the plasma. A computer model is developed to interpret and optimize the plasma conditions which provide the highest backscatter and phase-stable reflection coefficient for the longest period of time. The presence of axial density gradients causes the reflected wave to be scattered through a wide angle. As the gradients relax, the backscatter reflection coefficient increases to a value of nearly 100%. The plasma density and two-body recombination coefficient are measured by means of microwave backscatter plasma reflectivity and Langmuir probes. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 6974-6979 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A 193 nm excimer laser and a custom fabricated cylindrical lens system is used to produce a plasma sheet of 8 cm×30 cm×0.4 cm in tetrakis(dimethylamino)ethylene (TMAE), a low ionization energy organic gas. Plasma density variation due to photon absorption is studied by scanning the filling pressure of TMAE between 12 and 150 mTorr. A high density (n≥2.0×1013 cm−3), low temperature (Te≈0.8 eV) plasma sheet of 4 mm thickness is obtained with less than 50% spatial density variation over the 30 cm axial length. Charge recombination is found to be the dominant process for t≤1.2 μs with the plasma diffusion playing a perturbational role. A one-dimensional plasma model is utilized to model the experimental plasma data by treating the diffusion as a perturbation. This study shows that the recombination coefficient is 1.8±0.1×10−7 cm3 s−1 and the diffusion coefficient is 2.8±0.4×104 cm2 s−1. The plasma sheet has attractive properties for a microwave agile mirror. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 3018-3023 
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The propagation of a directly launched ion-Bernstein wave is considered in the tokamak edge plasma region in the frequency range Ωi 〈ω〈 2Ωi, and in the limit of small k⊥0ρi. Employing standard perturbation techniques, the wave scattering from a Gaussian spectrum of density fluctuations and the self-modulation resulting from the ponderomotive force nonlinearity are investigated. The results for the typical parameters of the PLT tokamak [Phys. Rev. Lett. 54, 2339 (1985)] and the ALCATOR-C tokamak [Phys. Rev. Lett. 59, 1201 (1987)] indicate that the scattering effect becomes important in the edge region as the fluctuation level is increased from 1% to 30%. Also, it is found that at the power levels currently being used in the experiments, large ponderomotive modifications in the plasma density are produced in the near zone of the launcher. In the weak nonlinearity limit, the ion-Bernstein wave is found to be modulationally unstable, thus implying the possibility of localized heating.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3887-3892 
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Electromagnetic wave scattering in a plasma half-space from a propagating ion acoustic wave density perturbation with a transverse conducting boundary condition is analyzed. The analysis shows that there are two different types of scattered waves. One propagates only in the region where the plasma density is perturbed, its frequency and wave vector satisfies the conditions for a three-wave interaction process. The other propagates as an unperturbed electron cyclotron mode with either an upshifted or downshifted frequency. Experiments are performed to examine electron cyclotron wave scattering by an ion acoustic wave excited by means of a grid along the magnetic field line in a mirror machine. The backscattered waves propagating away from the density perturbation region are measured and their dependence on the ion acoustic wavelength is examined. The scattered wave in the region of ion acoustic wave propagation is examined by measuring the scattered wave field pattern when the ion acoustic wave number is much larger than that of the incident wave. The measurements demonstrate that the scattered waves indeed propagate as two different modes.
    Materialart: Digitale Medien
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  • 10
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 4224-4230 
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Large shifts in the equilibrium position of the plasma column in the Phaedrus tandem mirror [Nucl. Fusion 25, 1213 (1985)] are observed when a 25 kW, 1 msec, 15.5 GHz microwave heating pulse (ω∼2ωce) is injected into one end cell of the device. The shift in the plasma column axial position occurs in all regions of the device, and is comparable in size to the initial plasma radius. The change in plasma position is a result of nonuniform electron heating produced by azimuthally asymmetric illumination of the plasma column by the microwave beam.
    Materialart: Digitale Medien
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