China
Africa
Explore chapters and articles related to this topic
Microwave Power Tubes
Published in Jerry C. Whitaker, Power Vacuum Tubes, 2017
One device based on this principle is the free electron laser or the ubitron [13]. A schematic diagram of the device is given in Figure 6.87. A high-speed relativistic electron beam is emitted from an electron gun focused by a longitudinally applied dc magnetic flux density B and is periodically deflected by magnetic means. The repetitious deflections create among relativistic electrons high-energy states and low-energy states. If the high-energy electrons in the deflection waveguides are stimulated by the resonance frequency of the waveguide resonator, then the downward transition (the transition of electrons in a high-energy state to a low-energy state) occurs. Microwave emissions result because of the energy transitions at the stimulation frequency.
Electron Beam Plasmas
Published in Alexander Fridman, Lawrence A. Kennedy, Plasma Physics and Engineering, 2021
Alexander Fridman, Lawrence A. Kennedy
Electron beam plasma is formed by injection into a neutral gas a cylindrical or plane electron beam with electron energies usually ranging from 10 keV to 1 MeV. A general scheme of the electron beam plasma generation is illustrated in Figure 12.1 (V.L. Bykov, M.N. Vasiliev, A.S. Koroteev, 1993). A thin cylindrical electron beam is formed by an electron gun located in a high-vacuum chamber. The beam is injected through a window into a gas-filled discharge chamber, where the high-energy electrons generate plasma and transfer energy into the gas by various mechanisms depending on pressure, beam current density, electron energy, and plasma parameters. Total energy losses of the electron beam in a dense gas due to interaction with neutrals along its trajectory (axis z) are: −1n0dEdz=LE, where n0 is the neutral gas density; E is the electron energy; L(E) is the electron energy loss function, which is well known for different gases and mixtures. As an example, the L(E) function of nitrogen is shown in Figure 12.2 (M.N. Vasiliev, 2000). For calculations of the electron energy loss function L(E), it is convenient to use the numerical Bethe formula, which can be applied even for relativistic electron beams (although not for the strong relativistic case): LE=AE1+E/mc21+E/2mc2ln1.17⋅EIex. In this relation, m is an electron mass; c is the speed of light; Iex is the characteristic excitation energy (which is equal to 87 eV for air, 18 eV for hydrogen, 41 eV for helium, 87 eV for nitrogen, 102 eV for oxygen, 190 eV for argon, 85 eV for carbon dioxide, and 72 eV for water vapor); the factor A also depends only on gas composition, for example, in air A = 1.3 ⋅ 10−12 eV2 cm2. If the kinetic energy of electrons exceeds mc2 (which numerically is about 0.5 MeV), such electrons are called relativistic, and their beam is usually referred to as the relativistic electron beam. Dynamic analysis of such beams definitely requires taking into account the special effects of relativistic mechanics.
Temporal Profiling of Electron Temperatures Using the Hα–Hβ Line Emission and Triple Langmuir Probe Array in the Pre-Ionization Discharge of the MT-I Spherical Tokamak
Published in Fusion Science and Technology, 2020
M. Usman Naseer, F. Deeba, S. I. W. Shah, S. Hussain, A. Qayyum
The plasma characterization in the pre-ionization phase gives a lot of useful information regarding system optimization for the better confined and long-duration pulse of plasma current. In spherical tokamaks there is a limitation on the size of an ohmic heating central solenoid due to the relatively low space available in the central bore. Due to the small size of a central solenoid, its capability for flux swing gets insufficient for breakdown and the subsequent plasma current startup. Pre-ionization is used to supplement the ohmic heating of a central solenoid in the plasma current startup. It reduces the requirement of loop voltage by producing excess free electrons before the initiation of the toroidal current. Various pre-ionization techniques are currently being employed, including relativistic electron beams, microwaves, or injection of neutral beams. However, plasma heating with microwaves [electron cyclotron resonance heating (ECRH)] at the electron cyclotron resonant frequency is a highly effective and practical technique for the heating of cold plasmas.11,12 It reduces the requirement of loop voltage, and subsequently, the cost of the poloidal field power supply system. The reduction in ohmic heating flux also supports the discharge to sustain for longer pulse lengths with improved plasma parameters.