Interaction of intense electron beams with a plasma

In this work an experimental determination has been made of the energy losses of an initially unmodulated electron beam passing through a plasma (with no magnetic field). These losses amount to 12% of the beam energy for a beam current of 8 amp, a beam voltage of 26 kev and a plasma density of 7–9·10¹⁰ cm^–3.It is shown that these high losses are due to the coherent interaction of the beam with the plasma.     

The effect of a strong magnetic field on the magnetohydrodynamic stability of plasma and the containment of charged particles in the “Tokamak”

Results of an investigation of plasma in the toroidal apparatus TM-2 in a strong longitudinal magnetic field (up to 22 kOe) are described. It is shown that increasing the magnetic field sharply decreases the low frequency oscillations in the oscillograms of the loop voltage and discharge current derivative, and also weakens the interaction between the plasma and the wails of the discharge chamber.For a large enough ratio of the longitudinal field intensity to the intensity of the current selffield, oscillations are not observed. According to radiointerferometric measurements, the mean electron density in this case hardly alters during the course of the operations. The conductivity reaches a value of about 10¹⁶ cgs.Translated from Atomnaya Énergiya, Vol. 15, No. 5, pp. 363–369, November, 1963     

Interaction of modulated heavy-current electron-pulse beams with plasma in a longitudinal magnetic field

Results of experiments on the interaction of modulated heavy-current electron-pulse beams with plasma in a longitudinal magnetic field are presented. The plasma is formed by the beam itself. It is shown that under certain conditions the modulated electron beam interacts much more strongly with the plasma than an unmodulated beam. Longitudinal waves with a considerably greater electric field strength (some seven times) than in the absence of initial modulation are excited in the beam and the plasma. An explanation of the results is offered.Translated from Atomnaya Énergiya, Vol. 18, No. 4, pp. 315–322, April, 1965     

Low-Energy Positron Injector

The LEPTA low-energy positron accumulator, which is to be used for producing directed fluxes of positronium and antihydrogen atoms, is under development at the Joint Institute of Nuclear Research. The monochromatic positron injector, operating in the pulsed mode, in the accumulator must generate a positron beam with intensity 10⁸–10⁹ particles in a pulse with duration less than 300 nsec, the positron energy is 10 keV, the relative energy spread in the beam is less than 2·10^–3, and the beam radius is 0.5 cm.Radioactive ²²Na serves as a positron source. The positrons at the exit from the source are decelerated in a solid target and enter the magnetic trap. There they are once again decelerated in a gas to thermal velocity and accumulate in ~100 sec. For injection into the accumulator, the positrons are pulled out of the trap by a pulsed electric field and acclerated up to the required energy.     

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.     

Plasma jet deflection in magnetic fields

We investigated the motion of plasma jets in a quadripole magnetic field produced by four current conductors whose axial lines were bent through 90° (the curvature radius was 30 cm). The maximum strength of the magnetic field in the slit between the current conductors was 6 kOe. The plasma jet, which was produced by means of a coaxial gun, was injected along the axis of the magnetic system. The magnetic system was adequate for defecting the plasma jet, which had an initial velocity of 8×10⁶ cm/sec and a maximum concentration before deflection of 2×10¹⁵ cm^–3. The jet velocity was equal to 7×10⁶ cm/sec. In spite of the considerable loss of particles (due to the presence of slits in the magnetic system), the ion concentration in the jet beyond the turn attained 2×10¹⁴ cm^–3, while the over-all number of particles was as large as 10¹⁷.As a result of deflection, it was possible to eliminate completely the neutral gas accompanying the jet and to obtain virtually totally ionized plasma. The optimum value of the magnetic field's strength was 8 kOe.Translated from Atomnaya Énergiya, Vol. 19, No. 4, pp. 329–335, October, 1965     

Effect of a transverse magnetic field on a toroidal discharge in a strong longitudinal magnetic field

The effect of a transverse magnetic field on a toroidal discharge in a strong longitudinal magnetic field was studied. It was found that, for a definite value of the pinch displacement caused by the 1/c [I,B], force, the oscillations on the oscillograms of the electrical characteristics of the discharge had minimum amplitude, while the mean plasma conductivity reached a maximum. It was shown that the effect of the transverse component of the magnetic field could, in general, be explained from the concept of the equilibrium of the plasma pinch inside the conducting sheath.Translated from Atomnaya Énergiya, Vol. 17, No. 3, pp. 177–184, September, 1964.     

Interaction of a straight plasma pinch with a varying magnetic field of quadrupole configuration

The interaction of a straight plasma pinch (current up to 4 kA) with a high-frequency (~1.3 Me/see) quadrupolar magnetic field (~100 Oe) is studied by very simple methods.Translated from Atomnaya Énergiya, Vol. 18, No. 4, pp. 323–329, April, 1965     

Simulation study on electron heating characteristics in magnetic enhancement capacitively coupled plasmas with a longitudinal magnetic field

The electron heating characteristics of magnetic enhancement capacitively coupled argon plasmas in presence of both longitudinal and transverse uniform magnetic field have been explored through both theoretical and numerical calculations. It is found that the longitudinal magnetic field can affect the heating by changing the level of the pressure heating along the longitudinal direction and that of the Ohmic heating along the direction which is perpendicular to both driving electric field and the applied transverse magnetic field, and a continuously increased longitudinal magnetic field can induce pressure heating to become dominant. Moreover, the electron temperature as well as proportion of some low energy electrons will increase if a small longitudinal magnetic field is introduced, which is attributed to the increased average electron energy. We believe that the research will provide guidance for optimizing the magnetic field configuration of some discharge systems having both transverse and longitudinal magnetic field.     

Plasma scenarios, equilibrium configurations and control in the design of FAST

The Fusion Advanced Studies Torus (FAST) conceptual study has been proposed [A. Pizzuto on behalf of the Italian Association, The Fusion Advanced Studies Torus (FAST): a proposal for an ITER Satellite facility in support of the development of fusion energy, in: Proceedings of 22nd IAEA Fusion Energy Conference, Geneva, Switzerland, October 13–18, 2008; Nucl. Fusion, submitted for publication] as possible European ITER Satellite facility with the aim of preparing ITER operation scenarios and helping DEMO design and R&D. Insights into ITER regimes of operation in deuterium plasmas can be obtained from investigations of non linear dynamics that are relevant for the understanding of alpha particle behaviours in burning plasmas by using fast ions accelerated by heating and current drive systems.FAST equilibrium configurations have been designed in order to reproduce those of ITER with scaled plasma current, but still suitable to fulfil plasma conditions for studying burning plasma physics issues in an integrated framework. In this paper we report the plasma scenarios that can be studied on FAST, with emphasis on the aspect of its flexibility in terms of both performance and physics that can be investigated. All plasma equilibria satisfy the following constraints: (a) minimum distance of 3 energy e-folding length (assumed to be 1 cm on the equatorial plane) between plasma and first wall to avoid interaction between plasma and main chamber; (b) maximum current density in the poloidal field coils, transiently, up to around 30 MA/m². The discharge duration is always limited by the heating of the toroidal field coils that are inertially cooled by helium gas at 30 K. The location of the poloidal field coils has been optimized in order to: minimize the magnetic energy; produce enough magnetic flux (up to 35 Wb stored) for the formation and sustainment of each scenario; produce a good field null at the plasma break-down (B_P/B_T < 2 × 10⁻⁴ at low field, i.e. B_T = 4 T and E_T = 2 V/m for at least 40 ms).Plasma position and shape control studies will also be presented. The optimization of the passive shell position slows the vertical stability growth time down to 100 ms.     

Neutral beam deposition in large,finite-beta noncircular tokamak plasmas

A parametric pencil beam model is introduced for describing the attenuation of an energetic neutral beam moving through a tokamak plasma. The nonnegligible effects of a finite beam cross-section and noncircular shifted plasma cross-sections are accounted for in a simple way by using a smoothing algorithm dependent linearly on beam radius and by including information on the plasma flux surface geometry explicitly. The model is bench-marked against more complete and more time-consuming two-dimensional Monte Carlo calculations for the case of a large D-shaped tokamak plasma with minor radiusa=120 cm and elongationb/a=1.6. Deposition profiles are compared for deuterium beam energies of 120–150 keV, central plasma densities of 8×10¹³ to 2×10¹⁴ cm^–3, and beam orientation ranging from perpendicular to tangential to the inside wall.     

The use of an electrothermal plasma gun to simulate the extremely high heat flux conditions of a tokamak disruption

Disruption damage conditions for future large tokamaks like ITER are nearly impossible to simulate on current tokamaks. The electrothermal plasma source SIRENS has been designed, constructed, and operated to produce high density (> 10²⁵/m³), low temperature (1–3 eV) plasma formed by the ablation of the insulator with currents of up to 100 kA (100 s pulse length) and energies up to 15 kJ. The source heat fluence (variable from 0.2 to 7 MJ/m²) is adequate for simulation of the thermal quench phase of plasma disruption in future fusion tokamaks. Different materials have been exposed to the high heat flux in SIRENS, where comparative erosion behavior was obtained. Vapor shield phenomena has been characterized for different materials, and the energy transmission factor through the shielding layer is obtained. The device is also equipped with a magnet capable of producing a parallel magnetic field (up to 16 T) over a 8 msec pulse length. The magnetic field is produced to decrease the turbulent energy transport through the vapor shield, which provides further reduction of surface erosion (magnetic vapor shield effect).     

Investigation of stimulated Raman scattering in longitudinal magnetized plasma by theory and kinetic simulation

Stimulated Raman scattering(SRS)in a longitudinal magnetized plasma is studied by theoretical analysis and kinetic simulation.The linear growth rate derived via one-dimensional fluid theory shows the dependence on the plasma density,electron temperature,and magnetic field intensity.One-dimensional particle-in-cell simulations are carried out to examine the kinetic evolution of SRS under low magnetic intensity of ωc/ω0＜0.01.There are two density regions distinguished in which the absolute growth of enveloped electrostatic waves and spectrum present quite different characteristics.In a relatively low-density plasma(ne～0.20nc),the plasma wave presents typical absolute growth and the magnetic field alleviates linear SRS.While in the plasma whose density is near the cut-off point(ne～0.23nc),the magnetic field induces a spectral splitting of the backscattering and forward-scattering waves.It has been observed in simulations and verified by theoretical analysis.Due to this effect,the onset of reflectivity delays,and the plasma waves form high-frequency oscillation and periodic envelope structure.The split wavenumber Δk/k0 is proportional to the magnetic field intensity and plasma density.These studies provide novel insight into the kinetic behavior of SRS in magnetized plasmas.     

Cyclotron instability in ogra

Conclusions The experimental results reported here indicate that a cyclotron instability can and does develop in Ogra. Furthermore, at the present time, as far as we know there is no other possible explanation for the anomalous magnitude and the dependence of electric field (at the cyclotron frequency) on plasma density observed experimentally. The presence of density waves with different phase velocities can cause electron heating and electron loss. In this regard, the fact that the electrons can interact with the electric waves seems to be indicated by experiments with an electron beam carried out by Yu. A. Kucheryaev and D. A. Panov [9]; these experiments indicate that an electron beam passing through a plasma along the magnetic field loses or gains energy by virtue of interaction with waves at the cyclotron frequencies corresponding to H ₂ ⁺ and H ₁ ⁺ ions.On the one hand, the effect of the cyclotron instability can cause ions to form bunches as a result of nonlinear effects, and these can lead to a more effective interaction, with the dissipation and exchange of energy. On the other hand, the existence of electric fields perpendicular to the magnetic field can cause ion drift across the magnetic field when the phase velocity of these waves is approximately equal to the ion velocity. As is evident from the table, this situation can arise in certain modes of operation. For a more detailed explanation of the effect of the cyclotron instability on ion loss and electron loss, it will be necessary to carry out further investigations. The author wishes to take this opportunity to thank I. N. Golovin for his continued interest in this work and for a number of valuable comments offered in discussions of the experimental results. E. P. Velikhov for help in carrying out the calculations, and A. N. Karkhov and V. F. Nefedov for help in carrying out the measurements with Ogra. Fruitful discussions of the experiments and the results of the calculations with colleagues working with Ogra were very helpful in determining the physical pattern of these effects.Translated from Atomnaya Énergiya, Vol. 14, No. 1, pp. 72–81, January, 1963     

Experimental results from a beam direct converter at 100 kV

A direct-energy converter was developed for use on neutral-beam injectors. The purpose of the converter is to raise the efficiency of the injector by recovering the portion of the ion beam not converted to neutrals. In addition to increasing the power efficiency, direct conversion reduces the requirements on power supplies and eases the beam dump problem. The converter was tested at Lawrence Berkeley Laboratory on a reduced-area version of a neutral-beam injector developed for use on the Tokamak Fusion Test Reactor at Princeton. The conversion efficiency of the total ion power was 65 ±7% at the beginning of the pulse, decaying to just over 50% by the end of the 0.6-s pulse. Once the electrode surfaces were conditioned, the decay was due to the rise in pressure of only the beam gas and not to outgassing. The direct converter was tested with 1.7 A of hydrogen ions and with 1.5 A of helium ions through the aperture with similar efficiencies. At the midplane through the beam, the line power density was 0.7 MW/m, for comparison with our calculations of slab beams and the prediction of 2–4 MW/m in some reactor studies. Over 98 kV was developed at the ion collector when the beam energy was 100 keV. When electrons were suppressed magnetically, rather than electrostatically, the efficiency dropped to 40%. However, a better designed electron catcher could improve this efficiency. New electrode material released gas (mostly H₂ and CO) in amounts that exceeded the input of primary gas from the beam. The electrodes were all made of 0.51-mm-thick molybdenum cooled only by radiation. This allowed the heating by the beam to outgas the electrodes and for them to stay hot enough to avoid the reabsorption of gas between shots. By minor redesign of the electrodes, adding cryopanels near the electrodes, and grounding the ion source, these results extrapolate with high confidence to an efficiency of 70–80% at a power density of 2–4 MW/m. Higher power may be possible with magnetic electron suppression.     

Simulation of the Particle Dynamics in Storage Rings with Coupled Transverse Motion

The toroidal low-energy particle accumulator is intended for confining and electronic cooling of a circulating positron beam and generating antihydrogen and positronium in flight. Its special features are a longitudinal magnetic field, a spiral particle winding, and a sectional structure of the optical components, which serve to form a closed orbit. The longitudinal magnetic field simultaneously magnetizes the positron beam and, as a consequence, ensures a long lifetime for the circulating beam. However, such a field results in strong coupling of the horizontal and vertical degrees of freedom and the appearance of additional resonances. The particle dynamics is simulated using the specially developed BETATRON computer code, based on the BOLIDE program package (for producing interfaces for computational and simulation programs). In the present work, the results of the simulation of particle dynamics and a calculation of the structure functions for a positron storage ring are presented.__________Translated from Atomnaya Energiya, Vol. 98, No. 4, pp. 300–306, April, 2005.     

Experiments on producing intensive proton beams by means of the method of charge-exchange injection

The present article provides the results of experiments on accumulating protons along a track with a constant magnetic field by means of the charge-exchange method under resonance conditions. The coherent effects connected with the high intensity were detected and investigated. It was established that the limitation of the orbital current is connected with the action of the longitudinal component of the beamTs electric field.Translated from Atomnaya Énergiya, Vol. 22, No. 5, pp. 348–356, May, 1967.     

Individual Injection, Cooling, and Accumulation of Rare Radioactive Ions

A scheme for accumulating radioactive ions, which is geared toward a quasicontinuous low-intensity flux, is discussed. It is based on individual correction of the trajectory and momentum deflection of each ion in the transport channel and individual ion injection into the accumulator ring. The advantages of this scheme are low accumulator acceptance 5–10 ·mm·mrad, high ion accumulation rate – up to 10³ sec^–1 – with beam intensity after the fragment separator 10³–10⁴ sec^–1, and a 10⁴–10⁵-fold decrease of the pulse intensity of the primary beam on the productive target.     

Race-Track Microtron–Recirculator with Superconducting RF Cavities

The importance of designing and building a modern continuous electron beam accelerator for medium energy and possible directions for the design and implementation of the basic systems of such an accelerator are examined. Substantiation is given for the type of accelerator – a race-track microtorn with energy up to 200 MeV using superconducting accelerating cavities operating at liquid-helium temperature. A novel scheme is proposed for the microtron–recirculator. In this scheme, recirculation with individual deflection and focusing systems is orgnized for the first few orbits; further acceleration occurs in a standard microtron regime.     

Heavy-current accelerator based on a transformer

The operating principles of a direct-action accelerator designed to acceIerate electrons to an energy of 1.5 MeV with a mean beam power of tens of kilowatts and an efficiency of around 90% are described. The electron-current pulse length can be varied from 0 to g msec, and the repetition frequency up to 50 times per sec. The mean current im may reach 1/6 of the maximum current in the pulse. Magnetic lenses are installed in order to focus electron currents of up to 100 mA into a beam a few mm in diameter in the accelerating tube. Heavy-metal screens are placed close to the axis of the tube in order to protect the gas gaps and other electrically-stressed parts of the accelerator from radiation arising inside the tube.The construction of a system for producing an electron beam with an energy of 1.5 MeV and a mean power of 25 kW (i_m = 17 mA) is described.Translated from Atomnaya Énergiya, Vol. 20, No. 5, pp. 385–392, May, 1966.