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 interaction of high-current pulse beams with plasma in a magnetic field

We have studied the interaction of an electron beam with currents up to 8.5 A and energies up to 15 keV with a plasma in a magnetic field, the intensity of which varied between 36O and 1320 Oe. We studied the energy distribution of electrons after passage of the electron beam through the plasma as a function of the beam current, residual gas pressure (air), and the intensity of the longitudinal magnetic field. We measured the intensities of the transverse and longitudinal highfrequency fields excited by the beam in the plasma; these reach about 100 V/cm and 1–2 kV/cm respectively. This proves that the high energy losses of the beam due to its passage through the plasma are caused by excitation of high-frequency vibrations.Translated from Atomnaya Énergiya, Vol. 14, No. 3, pp. 249–256, March, 1963     

Programmable electron density patterns induced by the interaction of an array laser and underdense plasma

The spatially modulated electron distribution of plasma is the basis for obtaining programmable electron density patterns. It has an important influence on plasma technology applications. We propose an efficient scheme to realize controllable electron density patterns in underdense plasma based on the array laser–plasma interaction. Theoretical evidence for the realization of programmable electron density patterns and the corresponding electrostatic field is provided analytically, which is confirmed by particle-in-cell simulations. Results show that the spatial distribution of electron density in the propagation and transverse directions of the laser can be highly modulated to obtain rich programmable electron density patterns by adjusting the array pattern code and pulse width of the array laser beam.     

Investigation of electromagnetic radiation from a straight high-current discharge

In this work we investigate the formation of runaway electrons in the electric field of a gas discharge. It is shown that the interaction of the runaway beam with the plasma results in the production of intense coherent electromagnetic radiation at frequencies in the region of the plasma frequency. The power of this radiation is much greater than the thermal noise power.Translated from Atomnaya Énergiya, Vol. 14, No. 4, pp. 349–352, April, 1963     

Tokamak field error measurements with an electron beam in KSTAR

It is possible to detect the presence of small field errors in a tokamak with an electron beam. This was demonstrated earlier on T-15 and TEXTOR. This paper discusses the concept, past experience on these tokamaks, calculations for the Korea Superconducting Tokamak Advanced Research (KSTAR) device, an electron beam source, measurement devices for these measurements, and some results. It is shown that small toroidally averaged field errors can be detected by this method. A low voltage electron beam (e-beam) gun and fluorescent screen were mounted in a vertical port and inserted into the vacuum vessel at the end of the KSTAR 2nd campaign plasma experiments. A camera with a narrow field of view was mounted in midplane port in a tube tangent to the field lines at R ∼ 1.3 m and photographed the beam striking the screen. The poloidal field (PF) currents were held constant during the camera exposure period. Many shots with various PF coils energized were made and the deflections of the e-beam were measured. The measurements were made with a camera integration time of 300 ms because of the low light intensity. The results show that there are large field errors that diminish as the PF currents are raised. There appears to be no significant up-down asymmetry for static fields. Measurements with a 7 PF coil scenario with a calculated field null located at e-beam radial position show much larger fields than calculated. KSTAR was constructed with Incoloy 908 conduit using cable-in-conduit conductors (CICC) in 10 of the 14 PF coils and all 16 of the toroidal field (TF) coils. Incoloy 908 has a relative magnetic permeability, μ, of about 10. The field errors appear to be largely due to Incoloy 908.     

Simulation of Toroidal Rotation Effect on Heat Flux Transport in the Edge Plasma of Small Size Divertor Tokamak

The effect of toroidal rotation on heat flux transport in the edge plasma of small size divertor was simulated by B2SOLP0.5.2D transport code. The main results of simulation shows that, the following: (1) the radial heat flux is strongly influenced by toroidal rotation. (2) The amplification of conduction part of radial heat flux imposes nonresilient profile of ion temperature, under which the effect of toroidal rotation on ion temperature profile is strong. (3) The ion distribution and its gradients are lower for counter-injection neutral beam than for co-injection neutral beam. (4) Reversal of toroidal rotation during using neutral beam injection result in reverses of radial electric field and E × B drift velocity. (5) The toroidal rotation strong influence on the ion temperature scale length of the ion temperature gradient (ITG). (6) Switch on and off all drifts leads to higher change in the ion density distribution in edge plasma of small size divertor tokamak when the unbalance neutral beam injection are considered (7) the comparison between radial heat flux at different momentum input shows that, the radial ion heat flux with larger ion temperature scale length in the case of co-injection neutral beam is larger than the ion heat flux with smaller ion temperature scale length in the case of counter-injection neutral beam.     

Closed magnetic plasma traps with a zero angle of rotational conversion

Toroidal plasma traps with closed magnetic lines of force (zero angle of rotational conversion) are studied by the method of expanding in series with respect to distance from the magnetic axis. The curvature of the magnetic axis of such traps is inevitably modulated. The modulation of the curvature of the axis enables neighboring lines of force to be made shorter; this is equivalent to creating a minimum of the average magnetic field on the axis, which is necessary for the stabilization of the plasma. In principle it is possible to create a minimum of the average magnetic field in a trap with a magnetic axis in the form of a plane curve.Translated from Atomnaya Énergiya, Vol. 22, No. 5, pp. 356–361, May, 1967.     

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.     

Self-focusing of Gaussian Laser Beam Through Collisionless Plasmas and Its Effect on Second Harmonic Generation

This paper presents an investigation of self-focusing of gaussian laser beam in a collisionless plasma and its effect on second harmonic generation. In the presence of gaussian beam, the carriers get redistributed from the high field region to low field region by ponderomotive force and transverse density gradient is established in plasma which in turn generates the plasma wave at pump frequency. This plasma wave interacts with incident laser beam and a second harmonic is generated. Furthermore, if the initial power of input beam is more than the critical power for self-focusing, the beam gets self-focused and hence the generated plasma wave and second harmonic which depend upon the background electron concentration and power of the main beam also get modified accordingly.     

Operation of the ITER Electrostatic Residual Ion Dump with a Perturbed Field

ITER will use a novel electrostatic method to remove the unwanted residual charged component from the neutral beam injectors in place of the usual magnet separation system. This technique has not been tested experimentally and is subject to the additional complication of plasma formation perturbing the electric field. Previous calculations have shown that whilst this is not significant for the 1 MeV heating beam systems, the lower energy diagnostic beam system will be susceptible. An analytical model of the electrostatic dump has been developed that includes the perturbation of the vacuum electrostatic field by both plasma and the separating positive and negatively charged residual beams. An approximate solution of Poisson’s equation is formulated that allows analysis of the space charge field when plasma density is insufficient to ensure zero electric field at the anode. The resulting modified electric field is then incorporated into a particle trajectory code to determine the deposition of the residual ions on the ERID panels. It is shown that the effect of plasma formation is to introduce an asymmetry into the deflecting field and the effect of the separating charges is to weaken the deflection of the residual beams. As a consequence the reference design for the ITER diagnostic beam will not collect all of the residual ions and it is recommended that the deflection voltage be increased by at least 50%.     

Study of a new cusp field for an 18 GHz ECR ion source

A feasibility study was performed to generate new sufficient mirror cusp magnetic field (CMF) by using the coils of the existing room temperature traditional 18 GHz electron cyclotron resonance ion source (ECRIS) at RIKEN. The CMF configuration was chosen because it contains plasma superbly and no multipole magnet is needed to make the contained plasma quiescent with no magneto-hydrodynamic (MHD) instability and to make the system cost-effective. The least magnetic field, 13 kG is achieved at the interior wall of the plasma chamber including the point cusps (PC) on the central axis and the ring cusp (RC) on the mid-plane. The mirror ratio calculation and electron simulation were done in the computed CMF. It was found to contain the electrons for longer time than in traditional field. It is proposed that a powerful CMF ECRIS can be constructed, which is capable of producing intense highly charged ion (HCI) beam for light and heavy elements.     

Combining the “Kinetic Tandem” and the “Kinetic Stabilizer” Concepts

A novel means of MHD stabilizing axially symmetric mirror systems was proposed by Ryutov and demonstrated in the Gas Dynamic Trap at Novosibirsk. It relies on the strongly stabilizing effect exerted by low density plasma on the expanding field lines outside the mirrors. The “Kinetic Stabilizer Tandem Mirror” implements Ryutov’s technique by injecting axially directed ion beams into the expander. The ions, stagnated, and reflected by the converging magnetic field, form the stabilizing plasma. MHD stability code calculations show stabilization at beta values of 40%, with stabilizer beam powers that are small compared to the T-M fusion power output. Implicit in the calculations is the assumption that adequate “communication” exists between the plug plasmas and the stabilizer plasmas. This paper examines one means for enhancing communication: Utilize the stabilizer plasma to create a potential peak that, together with the plug potential, forms a potential well that traps and contains a “bridging” plasma.     

Commissioning of the first KSTAR neutral beam injection system and beam experiments

The neutral beam injection (NBI) system was designed to provide plasma heating and current drive for high performance and long pulse operation of the Korean Superconducting Tokamak Advanced Research (KSTAR) device using two co-current beam injection systems. Each neutral beam injection system was designed to inject three beams using three ion sources and each ion source has been designed to deliver more than 2.0 MW of deuterium neutral beam power for the 100-keV beam energy. Consequently, the final goal of the KSTAR NBI system aims to inject more than 12 MW of deuterium beam power with the two NBI for the long pulse operation of the KSTAR. As an initial step toward the long pulse (～300 s) KSTAR NBI system development, the first neutral beam injection system equipped with one ion source was constructed for the KSTAR 2010 campaign and successfully commissioned. During the KSTAR 2010 campaign, a MW-deuterium neutral beam was successfully injected to the KSTAR plasma with maximum beam energy of 90 keV and the L-H transition was observed with neutral beam heating. In recent 2011 campaign, the beam power of 1.5 MW is injected with the beam energy of 95 keV. With the beam injection, the ion and electron temperatures increased significantly, and increase of the toroidal rotation speed of the plasma was observed as well. This paper describes the design, construction, commissioning results of the first NBI system leading the successful heating experiments carried in the KSTAR 2010 and 2011 campaign and the trial of 300-s long pulse beam extraction.     

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.     

Formation of Electron Internal Transport Barrier in Edge Plasma of Small Size Divertor Tokamak

The formation of electron internal transport barrier (EITB) during using counter-neutral beam injection (NBI) heating in the edge plasma of small size divertor tokamak can be simulated by using fluid transport code B2SOLPS0.5.2D. The results of simulations give us the following: (1) Plasma heating with counter-neutral beam injection leads to, strong, parabola type electron internal transport barrier (EITB) was formed in the edge plasma of small size divertor tokamak. (2) In case of plasma heating by counter-neutral beam injection, the radial electric field shear (E _r –gradient) was increased, while electron transport coefficients were reduced in conjunction with the formation of electron internal transport barrier (EITB). (3) The plasma heating by counter-neutral beam injection play significantly role in redistribution of parallel (toroidal) velocity in edge plasma of small size divertor tokamak.     

Drift-free magnetic equilibrium reconstruction using the response to plasma position modulation

Inductive magnetic sensors are widely used for plasma equilibrium reconstruction and control. However, their measurements involve electronic integration and can therefore experience drift leading to inaccurate plasma positioning. For this reason, we have studied an original drift-free approach to estimate the plasma equilibrium. The principle of this correction is based on modulation of the plasma position and current at three independent frequencies and analysing the modulated magnetic signals to provide additional estimation on the equilibrium. Using a plasma model based on current wires, the accuracy of such a method is assessed for Tore Supra in terms of the signal to noise ratio. The plasma position is recovered within a precision of 5 mm for a signal to noise ratio better than 80 dB. Applying our approach to dedicated experiments performed on the Tore Supra tokamak, we confirm the quality of the result and find that we can estimate the radial and vertical positions of the plasma to 1 mm with a one standard deviation confidence interval.     

Operational characteristics of the high flux plasma generator Magnum-PSI

In Magnum-PSI (MAgnetized plasma Generator and NUMerical modeling for Plasma Surface Interactions), the high density, low temperature plasma of a wall stabilized dc cascaded arc is confined to a magnetized plasma beam by a quasi-steady state axial magnetic field up to 1.3 T. It aims at conditions that enable fundamental studies of plasma–surface interactions in the regime relevant for fusion reactors such as ITER: 10²³–10²⁵ m⁻² s⁻¹ hydrogen plasma flux densities at 1–5 eV. To study the effects of transient heat loads on a plasma-facing surface, a high power pulsed magnetized arc discharge has been developed. Additionally, the target surface can be transiently heated with a pulsed laser system during plasma exposure. In this contribution, the current status, capabilities and performance of Magnum-PSI are presented.     

AC operation of large titanium sublimation pumps in a magnetic field: Results of the test stand for the W7-X neutral beam injectors

A neutral beam injection (NBI) system is being built for the Stellarator experiment Wendelstein 7-X (W7-X) currently under construction at IPP Greifswald. The NBI system consists of two injectors which are essentially a replica of the system present in the Tokamak experiment ASDEX-Upgrade at IPP Garching. A vacuum system with high pumping speed and large capacity is required to ensure proper vacuum conditions in the neutral beam line. For this purpose, large titanium sublimation pumps (TSP) are installed inside the NBI boxes, consisting of 4 m long hanging wires containing Ti and the surrounding condensation walls. The wires are DC ohmically heated up with 142 A to Ti sublimation temperature. A TSP system has been operated since many years in the AUG-NBI system, sublimating Ti in the pauses between the plasma discharges, when no magnetic field is present. However, at W7-X the superconducting coils generate a magnetic field permanently during experimental campaigns, whose stray B field with a maximum of 30 mT, affects the TSPs. Operated with DC, the wires would be deflected against the surrounding panels due to the Lorentz force. A simple possible solution is heating with AC, which reduces the wire deflection amplitude, inducing a risky wire oscillation. The feasibility of the AC operation in an equivalently strong B field such as the stray B field around W7-X has been demonstrated in a test stand for different AC waveforms and frequencies. Several test campaigns have shown no qualitative difference in the pumping properties between AC and DC operation of the TSP and no critical dynamic behaviour of the wires.     

Deuteron Beam Source Based on Mather Type Plasma Focus

A 3 kJ Mather type plasma focus system filled with deuterium gas is operated at pressure lower than 1 mbar. Operating the plasma focus in a low pressure regime gives a consistent ion beam which can make the plasma focus a reliable ion beam source. In our case, this makes a good deuteron beam source, which can be utilized for neutron generation by coupling a suitable target. This paper reports ion beam measurements obtained at the filling pressure of 0.05–0.5 mbar. Deuteron beam energy is measured by time of flight technique using three biased ion collectors. The ion beam energy variation with the filling pressure is investigated. Deuteron beam of up to 170 keV are obtained with the strongest deuteron beam measured at 0.1 mbar, with an average energy of 80 keV. The total number of deuterons per shot is in the order of 10¹⁸ cm^?2.     

托卡马克电场漂移电子注入的物理分析研究

对电场漂移电子注入中的电子束物理,电子注入情况下手 马克等离子的平衡,电子注入器中的物理问题等进行了分析研究。结果表明,漂移电子束的稳定条件是外加电场必须大于电子束本身所产生的电场;为了保持等离子体环的平衡,外加电压愈低愈好;