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Detectors
Published in C. R. Kitchin, Astrophysical Techniques, 2020
and the electron is observed via its Čerenkov radiation. This mechanism is called the ‘charged current reaction’ because the charged W boson mediates it. The second mechanism is mediated by the neutral Z boson and is, thus, called the ‘neutral current reaction’. In it, a neutrino of any type simply splits the deuterium nucleus into its constituent proton and neutron. The neutron is thermalised in the heavy water and eventually combines with another nucleus with the emission of gamma rays. The gamma rays in turn produce relativistic electrons via Compton scattering, and these are finally detected from their Čerenkov radiation. Although the neutron can combine with a deuterium nucleus, the capture efficiency is low so that 75% of the neutrons will escape from the detector. The SNO detector could, therefore, add two tonnes of salt to the heavy water to enable the neutron to be captured more easily by a C1735l nucleus (converting it to C1736l) and reducing to 17% the number of escaping neutrons. The thermalised neutrons can also be detected using H23e proportional counters (Section 1.3). The neutron combines with the H23e to produce a proton and tritium and these then ionise some of the remaining gas to give an output pulse.
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.
Modeling of Ablatant Deposition from Electromagnetically Driven Radiative Pellets for Disruption Mitigation Studies
Published in Fusion Science and Technology, 2019
Robert Lunsford, Roger Raman, A. Brooks, R. A. Ellis, W.-S. Lay
Mitigation of disruption events is a critical need for ITER as well as any other future burning plasma device. Disruptions can lead to reduced component lifetime and possible first-wall failure due to damages incurred through impact of relativistic electrons, extreme localized heating, and large electromechanical forces.1 Present disruption mitigation strategies involve the rapid injection of large quantities of impurities ensuring that the resultant thermal quench is dominated by radiative rather than conductive heat losses. In order to provide sufficient preemptive mitigation of the disruption event, the radiative payload must be delivered to the discharge with a warning time that could be as small as 10 ms (Ref. 2). In addition, for the mitigation strategy to be maximally effective, the impurity source must rapidly penetrate the H-mode transport barrier to concentrate deposition within the core of the discharge. As discharge stored energy and confinement increase, these timescale and penetration requirements challenge the response of presently employed disruption mitigation strategies.3–5