Beam Injection Energy Research Articles

Design facts in the axial monotron

Upon reexamining the electron beam radio frequency (RF)-field interaction of the monotron, which is the simplest transit-time microwave tube, it is found that a 20% maximum conversion efficiency can be attained at weakly relativistic beam energies (/spl sim/200 keV). It is shown that the conversion efficiency can be cast as a function of three parameters, namely, injection beam energy, resonant frequency, and electric field strength. From this fact, a design procedure of how the optimum operating parameters should be selected is presented. In support of the analytic study, 2.5-D particle-in-cell simulation of a transverse magnetic (TM)/sub 020/, 6.7 GHz axial monotron operating at 10 keV, 70 A beam parameters has given a tube efficiency of 15.4%, while one-dimensional (1-D) analysis assuming a strictly monoenergetic beam has predicted a maximum theoretical efficiency of 18.5% at a beam energy of 10 keV.

Tracking study of the new injection bump system of the HLS ring

In this paper, a summary of the existing three-kicker injection bump system of the HLS ring is presented and a new lattice-independent, four-kicker bump system which has been proposed for the second phase project of NSRL is described. Conditions for generating a bump orbit by four kickers are discussed. A theory of multiturn injection based on single-particle dynamics is introduced to analyse the conditions to achieve multiturn injection. Tracking study of the new system are carried out to verify the correctness of the scheme. Parameters such as dimension of the injected beam phase space ellipse, tolerance of the injected beam energy dispersion, timing jitter allowance of the kickers, etc., are calculated.

First operation of a wiggler-focused, sheet beam free electron laser amplifier*

A wiggler-focused, sheet beam free electron laser (FEL) amplifier utilizing a short-period wiggler magnet has been proposed as a millimeter-wave source for current profile modification and/or electron cyclotron resonance heating of tokamak plasmas. As proposed, such an amplifier would operate at a frequency of approximately 100–200 GHz with an output power of 1–10 MW CW. Electron beam energy would be in the range 500–1000 keV. To test important aspects of this concept, an initial sheet beam FEL amplifier experiment has been performed using a 1 mm×2 cm sheet beam produced by a pulse line accelerator with a pulse duration of 100 ns. The 500–570 keV, 4–18 A sheet beam is propagated through a 56 period uniform wiggler (λw=9.6 mm) with a peak wiggler amplitude of 2–5 kG. Linear amplification of a 5–10 W, 94 GHz signal injected in the TE01 rectangular mode is observed. All features of the amplified signal, including pulse shape and duration, are in accordance with the predictions of numerical simulation. Amplified signal gain has been measured as a function of injected beam energy, current, and wiggler field amplitude and is also in good agreement with simulation results. Continuation of this experiment will involve studying nonlinear amplifier operation and adding a section of tapered wiggler.

Effective Ion Tail Formation during Startup Neutral Beam Heating in D-3He Plasmas

Formation of an effective ion tail due to neutral beam injection heating during startup in D-[sup 3]He plasmas is investigated. The main idea is to reduce the energy input required for startup heating as well as the 14-MeV neutron yield by creating an effective tail. The optimal beam injection energy and beam species are first estimated by solving the steady-state Fokker-Planck equations for the injected species and for tritons. The startup of D-[sup 3]He plasma is simulated by simultaneously solving the time-dependent power balance and particle conservation equations together with the Fokker-Planck equations. As a result of tail formation in the fuel ion distribution, both the total input energy and the 14-MeV neutron yield during the startup phase are reduced by [approximately]20% from the values for Maxwellian plasma. 12 refs., 10 figs., 3 tabs.

Synthesis of Diamond-Like Carbon (a-C:H) Films by Hybrid Plasma CVD

Diamond-like carbon films have been deposited by hybrid plasma decomposition of CH4 diluted with H2 gas. A hybrid plasma was generated by microwave and RF discharges, and an ion beam was produced by applying do voltage between these two plasmas. The ion beam energy was controllable by adjusting the bias voltage. The property of the films strongly depended on the bias voltage corresponding to ion beam injection energy for deposition. The infrared bands had a main peak at 2955cm-1 and a broad band around 2860cm-1, corresponding to tetrahedral (sp3) bonding structure. The deposition rate was decreased from 50A/min to 25A/min by increasing the bias voltage. The optical gap was measured with a UV-BIS absorption spectrometer for the films deposited on a quartz plate, and estimated to be 2.15-2.45eV; its value declined with increased bias voltage during deposition. The microvickers hardness of the films was increased from 200kg/mm2 to 600kg/mm2 by increasing the bias voltage during deposition. Raman spectra showed two broad peaks at approximately 1300cm-1 and 1600cm-1 at a bias voltage of 100V, identified with the peaks of microcrystalline glass-like carbon.

Improvement of fusion reactivity and fusion power multiplication factor in the presence of fast ions

Improvement of fusion reactivity (〈σv〉) and fusion power multiplication factor ( Q) in the presence of fast ions has been investigated with the analytical solution of the velocity distribution function. Fast ions created by only NBI heating, only ICRF heating and a combination of NBI and ICRF heating are considered in a D-T plasma. The dependences of 〈σv〉 on the plasma temperature, the deposited power density, the electron density and the injected beam energy are clarified. Calculations of evaluating Q employ a simple plasma model in which the radial profiles of the plasma temperature and density are the same as those observed in recent experiments. 〈σv〉 and Q are enhanced by only NBI heating, the combined NBI and ICRF heating and only ICRF heating in this order under the same heating power. However, the increase in 〈σv〉 and Q for the former two methods is almost the same. In the presence of fast ions, the enhancement of 〈σv〉 and Q is larger than the increase in the plasma pressure.

Superconducting magnet of Aurora

The AURORA superconducting magnet system is composed of a cylindrical single-body magnet and a refrigeration system for superconducting coils. The magnet generates Bz=1 T on the central orbit at the 150 MeV electron beam injection energy and Bz=4.3 T at the 650 MeV storage energy. The diameter of the central orbit is 1 m. Iron poles and yokes are used for shielding the magnetic field, reducing the electromagnetic force between superconducting coils, and making the magnetic field distribution adequate for beam injection and storage.

Simultaneous heating by high power lower hybrid waves and neutral beams in the JT-60 tokamak

By injection of high power lower hybrid (LH) waves of up to 6.0 MW into neutral beam heated (20 MW) plasmas in the medium electron density regime (n̄e ≤ 3.5 × 1019 m-3) the plasma energy content is increased at the same incremental energy confinement time as it is increased with neutral beam heating alone. When LH heating is used in addition to thermal electron and ion heating, the beam ions are accelerated to energies considerably higher than the beam injection energy. In contrast to LH injection into ohmically heated plasmas in the same density regime, where substantial high energy electron production is observed, there are much less high energy electrons in the case of combined heating. The heating efficiency of LH waves during combined heating tends to decrease as the electron density is increased. Ray tracing analysis suggests that the accessibility condition prevents effective heating in a high density plasma. Calculation of wave damping, taking into account the beam component of the ion velocity distribution function, indicates that the waves are absorbed by beam ions before they are absorbed by electrons.

Ion temperature from tangential charge exchange neutral analysis on the Tokamak Fusion Test Reactor

Fokker-Planck simulations of the Tokamak Fusion Test Reactor (TFTR) energetic ion mode discharges were performed to evaluate the utility of deriving the central ion temperature Ti from deuterium neutral beam charge exchange spectra above the neutral beam injection energy. The Ti values obtained from fitting the calculated spectra obtained from sightlines nearly tangent to the neutral beam injection radius reproduce the central ion temperature within ±10% over the full range of TFTR energetic ion mode parameters. The code simulations demonstrate that the ion temperature obtained from the high energy tangential deuterium charge exchange spectrum is insensitive to variations in plasma density, Zeff, plasma current, loop voltage, and injected neutral beam power and energy. The use of this method to reduce charge exchange data from TFTR energetic ion mode plasmas is demonstrated.

Discharge threshold with electron beam injection in a weakly ionized plasma

The discharge threshold is investigated both experimentally and theoretically with a model that a positive DC potential is applied to a metal plate with an electron beam injected into a weakly ionized plasma. The experiments are performed systematically by controlling the beam injection energy and the plasma density. In the absence of the plasma, beam injection with an energy larger than the ionization voltage reduces the discharge threshold strongly, whereas the injection has remarkably little influence due to the plasma supply which reduces the threshold. Space charge distributions are also calculated by solving Poisson's equation at a pre-discharge stage. The threshold as functions of the beam energy, the plasma density and the gas pressure is numerically obtained. It is assumed in the calculation that an additional ionization in the plasma begins to occur just at the moment when the electric field becomes zero in the region close to the plate. This implies that the threshold is given when a V-shaped potential profile formed at a pre-discharge stage collapses into a flat profile.

Fast-time-resolution charge-exchange measurements during the ‘fishbone’ instability in the poloidal divertor experiment

Measurements of fast-ion losses due to the ‘fishbone’ instability during high-βTq neutral-beam-heated discharges in the Poloidal Divertor Experiment have been made using two new vertical-viewing charge-exchange analysers. The measurements show that the instability has an n = 1 toroidal mode number, and that it ejects beam ions in a toroidally rotating bacon directed outward along a major radius. Observations of ejected ions with energies up to twice the beam injection energy at R ≈ Ro + a indicate the presence of a non-μ-conserving acceleration mechanism.

Requirements for D-D tandem mirror reactors

Requirements for D-D barrier tandem mirror reactors are calculated from an equilibrium power balance model. To obtain adequate plasmaQ and reasonable power density, axisymmetric configurations are required to decrease barrier length and radial transport and to increase central cell beta. We find that for a reactor producing 900 MW net electric power from aQ=6.5 plasma, a central cell length of 225 m, maximumB of 15 T, and neutral beam injection energy of 700 keV are necessary. In addition to high central cell beta (∼70%), high barrier beta (∼40%) is needed to allow the ECRH power required to reduce the barrier potential. Using too much barrier ECRH power results in a decrease inQ. Nuclear elastic scattering of fusion products plays an important role in the overall plasma power balance. When nuclear scattering and coulomb scattering are included, the plasmaQ value is increased by more than 40% compared to the case when coulomb scattering alone is considered.

Tandem mirror reactor with thermal barriers

A new invention — the thermal barrier — promises to improve the tandem mirror fusion reactor. The thermal barrier consists of a region of reduced magnetic field strength, plasma density, and plasma potential between each end plug and the central cell of a tandem mirror. The depressed plasma potential serves to thermally insulate the plug electrons from the central cell electrons. With barriers and auxiliary electron heating in the plugs, the central cell confining potential can be generated with a lower plug plasma density, magnetic field strength, and beam injection energy than for the case without barriers. This paper summarizes the status of the rapidly evolving physics knowledge concerning tandem mirrors with thermal barriers, describes end plug components typical for tandem mirror reactors — yin-yang magnets, neutral beams, and ECRH heating systems, and discusses central cell design.

Quasi-linear model for ion cyclotron heating of tokamaks and mirrors

A quasi-linear numerical model for RF heating is developed in a form so as to be applicable both to tokamak and to magnetic mirror plasma confinement devices. It is then combined with a two-dimensional multi-species Fokker-Planck code to yield detailed information on the distortion of the ion distribution function due to an imposed level of RF. For a two-component tokamak, it is found that while “fast-wave” ion heating will not increase the fusion output, as much as 40% of the beam power may be replaced by RF in a strongly driven two-component tokamak without significantly degrading the fusion output. For magnetic mirror devices, results indicate that RF in combination with relatively low-energy beams may be expected to heat the ions to well above the beam injection energy as well as to increase confinement.

Parametric dependence of the ion cyclotron instability in a two-energy-component system

The excitation of the ion cyclotron wave and the harmonic ion cyclotron waves by injection of a neutral beam is studied as a function of beam injection energy and angle. The fundamental mode is found to be stable for parameters similar to the current and future neutrally injected tokamaks. The harmonic cyclotron waves are found to have growth rates of a few percent of the ion cyclotron frequency when the thermal ion temperature is below a critical level. The results of these calculations are compared to several current experiments.