Plasma Pulse Duration Research Articles

Analysis of the Effects of the Nuclear Heat Load on the ITER TF Magnets Temperature Margin

In the International Thermonuclear Experimental Reactor, the nuclear radiation escaping from the vacuum vessel reaches the superconducting toroidal field (TF) magnets, affecting the temperature margin ΔTmar, that is, the difference between the current sharing temperature and the operating temperature. The TF magnets are designed to operate at a minimum margin ΔTmarmin = 0.7 K. Recent design activity on in-vessel components, for example, blanket, in-vessel coils for plasma stability, suggests a potential enhancement of the nuclear heat load, leading to a reduction of ΔTmar, which is accurately assessed in the paper using the validated 4C code. For the case when the margin goes below the minimum, different possible mitigation strategies are investigated: the first considers the possible reduction of the He bath temperature from the nominal 4.3 K down to 3.8 K, and is proven to be successful. The others consider the possible increase of the dwell time between plasma pulses, and is shown to be inadequate, or the decrease of plasma pulse duration, which turns out to be effective below 300 s.

Effect of Compressed Plasma Flow on Tantalum-Titanium Thin Layer Deposited on Silicon Substrate

The interaction of a dense compressed nitrogen plasma flow with a system of Ta-Ti layers on a silicon substrate has been investigated. This plasma flow was generated in a quasi-stationary pulsed plasma accelerator that can provide a supersonic high energy plasma flux. The plasma pulse duration, discharge current, concentration, and energy densities absorbed by the target were 100 µs, 80 kA, 1018 cm−3, and 3–13 J/cm2 respectively. The samples were exposed to a single plasma pulseor to a series of pulses and were characterized by scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analyses. The result showed that the changes in the elemental and structure phase compositions depends on energy and number of plasma pulses. Formation of Ti5Si3 and Ta2N, are the main results of the current research.

Phase transformations in Ta-Si system induced by compression plasma flow

In this study, the effect of a dense compression nitrogen plasma flow on a microstructure and the elemental and phase compositions of a “tantalum layer-silicon substrate” system is investigated. Predeposited tantalum thin films (∼2 μ m thick) on Si(1 1 1) and Si(1 0 0) substrates were exposed to a single plasma pulse or to a series of pulses. The power density absorbed by the target, plasma pulse duration and discharge current were 1.2 GW/m 2 , 100 μ s and 80 kA, respectively. The temperature field distribution and its time evolution were approximately simulated. SEM and EDX analyses revealed the presence of tantalumrich domains consisting of nano-sized particles and spherical multilevel structures. X-ray diffraction analysis showed the formation of crystalline hexagonal tantalum-rich silicides and tantalum nitride. A mechanism explaining the structural changes and phase transformation of the Ta/Si system is proposed.

Role of stochastic heating in wakefield acceleration monitored by optical injection

Abstract It is shown that stochastic heating can play an important role in Laser Wake Field Acceleration. When considering low density plasma interacting with a high intensity wave perturbed by a low intensity counterpropagating wave, stochastic heating can provide electrons with the right momentum for trapping in the wake field. The influence of stochastic acceleration on the trapping of electrons is compared to the one of cold injection by considering several polarizations of the colliding pulses. For some value of the plasma density and pulse duration, a transition from an injection due to stochastic acceleration to a cold injection dominated regime – regarding the trapped charge – has been observed from PIC code simulations. When the plasma density exceeds some value, stochastic heating becomes important and is necessary in some circumstances to get electrons trapped into the wakefield.

Measurements of suprathermal hydrogen flux in CASTOR tokamak

A study of the non-equilibrium absorption of hydrogen particles by a plasma-facing absorption probe (AP) made of Group V metals in the CASTOR tokamak environment has been undertaken with the main purposes: (1) to investigate the role of nonmetallic coatings upon plasma-facing materials in D/T inventory and recycling and (2) to develop a method of the registration and diagnostic of the flux of suprathermal hydrogen. Two series of experiments were performed: with AP of Nb and of V. The absorption of suprathermal hydrogen particles coming from the tokamak plasma was investigated as a function of AP temperature, AP distance from the plasma, AP bias and of plasma pulse duration. The possibility of a reliable registration of suprathermal hydrogen was demonstrated in spite of the short plasma pulse duration and a relatively high background H 2 pressure. The composition of the hydrogen flux (molecules, atoms, ions) impinging on the tokamak walls was analyzed, including ion energy distribution.

ANALYSIS OF SPARK ERODED CRATER FORMED UNDER GROWING PLASMA CHANNEL IN ELECTRO-DISCHARGE MACHINING

ABSTRACT In electro-discharge machining, the occurrence of sparks cause material removal in the form of craters. These craters are due to melting and vaporization of workpiece over a localized area under the spark, which acts as the heat source. The crater under a single spark has been predicted by theoretical models adopting different approaches in solving the transient heat conduction equation, considering suitable assumptions with appropriate initial and boundary conditions. In the present work, a transient thermal model for a very large solid cylinder has been used to predict the size of the crater obtained under a single spark by determining the melting isotherm in both the axial and radial directions. An analytical study of the effect of plasma channel radius, heat flux, and pulse duration on the size of the crater has been made. Experiments are conducted using a commercial electro-discharge machine. The craters are measured under microscope and a comparison with theoretical results is presented. Subsequently, the nature of variation of crater diameter, crater depth and volume of material removed with respect to different machining parameters such as ‘ON’ time, ‘OFF’ time, and current have been explained by the theoretical results and it has been concluded that the plasma channel grows with respect to pulse duration such that at the end of the pulse the plasma channel radius becomes equal to the crater radius.

Thomson scattering diagnostic on the ETE tokamak: status and progress

In order to measure the plasma temperature and density in the spherical tokamak ETE, a one-channel Thomson scattering system was implemented. During the upgrade of capacitor banks and optimization since the beginning of operation, the plasma pulse duration has increased from 1.5 ms up to 12 ms with plasma currents varying from 10 kA to 60 kA. During this phase, the electron temperature was increased from 20 eV to 160 eV with densities as high as 3.5×10(19) m-3. Presently, the Thomson scattering diagnostic is being upgraded based on the time-delay technique, that consists in usingfi bers of different lengths to transmit the scattered light signals to the same polychromator. This system will allow measurements of electron temperature and density profi les with ten spatial points per laser shot and per polychromator. This work describes in details the Thomson scattering system, presents a selection of results obtained by this system since the initial phase of operation, and shows details of the proposed upgrade of the Thomson scattering system.

Processing considerations with plasma-based ion implantation of polymers: theoretical aspects, limitations, and experimental results

Processing of polymers using plasma-based ion implantation techniques (PBII) has general implications in terms of plasma specifications and pulse characteristics. In particular, the different aspects of the processing of polymer layers are discussed as functions of plasma density, pulse duration, and layer characteristics (thickness and permittivity). Clearly, severe limitations (true implantation energy, arcing) may appear for high-density plasmas as well as for long pulse durations, when processing polymer layers with thickness in the mm range. A review of the experimental results of ion implantation in polymeric materials via PBII processing is presented. The experimental results demonstrate the possibility of processing polymer layers with the PBII technique, but with severe limitations resulting from the process itself.

Plasma formation and first OH experiments in the Globus-M tokamak

The article reports the results of experimental campaigns on plasma ohmic heating performed during 1999-2000 on the spherical tokamak Globus-M. Later experimental results with the tokamak fed by thyristor rectifiers are presented in detail. The toroidal magnetic field and plasma pulse duration in these experiments were significantly increased. The method of stray magnetic field compensation is described. The technology of vacuum vessel conditioning, includingboronization of the vessel performed at the end of the experiments, is briefly discussed. Specific features of neutral gas column breakdown inspherical tokamaks by applied inductive voltage are outlined. Also discussed is the influence of ECR preionization on the breakdown conditions. Experimental data on plasma column formation and current ramp-up in different regimes of operation with the magnetic flux of the central solenoid limited to ∼100 mWb are presented. A significant reduction of the background density after boronization (below 2 × 1018m-3) allows the density to be completely controlled with external gas puffing and makesthe influence of the wall negligible. The magnetic flux consumption efficiency is discussed. The results of magnetic equilibrium simulations are presented and compared with experiment. Ramp-up of the plasma current of 0.25 MA for a time interval of about 0.03 s with about 0.02 s flat-top at a toroidal field strength of 0.35 T allows the conclusion that the target design parameters of Globus-M could be achieved in a double swing regime.

PBII processing of dielectric layers: physical aspects limitations and experimental results

Processing of dielectric layers using a plasma-based ion implantation (PBII) technique has general implications in terms of plasma specifications and pulse characteristics. In particular, the different aspects of the processing of dielectric layers are discussed as functions of plasma density, pulse duration, and layer characteristics (thickness and permittivity). Clearly, severe limitations (true implantation energy, arcing) may appear for high-density plasmas as well as for long pulse durations when processing dielectric layers with thicknesses in the millimeter range. Typical examples of ion implantation in dielectric materials are presented, e.g. oxygen ion implantation in polymer sheets (for hydrophilic or adhesion treatments) and nitrogen implantation of polysilicon films on glass. The experimental results demonstrate the feasibility of processing dielectric layers with the PBII technique, but with severe limitations resulting from the process itself.

Damages of hot-pressed boron carbide during solid target boronization in Uragan-3M torsatron

The investigation of hot-pressed boron carbide erosion caused by the arc-regime of a solid target boronization (STB) procedure was carried out. The sample had a high density (2.44 g/cm 3, B 78.2 a.m.%, C 21.5 a.m.%) and was used as a head plate of the movable limiter during the pulsed discharge cleaning campaign in the Uragan-3M (U-3M) torsatron. The total number of pulses during STB was about 2 × 10 4 ( n e=2 × 10 12 cm −3, T e=10–15 eV, B=0.035 T, the plasma pulse duration t=50 ms, the negative pulsed biasing of B 4C-plate with an amplitude of up to 200 V had a time duration of 10–50 ms). As was shown by an optical examination of the surface morphology of the exposed sample, the character and intensity of damages of the limiter surface strongly varied depending on the distance from the plasma column axis. The most damaged sites were covered with many craters of 0.1–1.2 mm in diameter and spallings with a size up to 5 mm. The possible physical mechanisms of these damages and erosion behavior of hot-pressed in vacuum boron carbide caused by the arc-regime of STB are discussed.

Exploration of the afterglow of the pulsed electron cyclotron resonance source under the injection of neutral fluxes

The results of numerical simulation of highly charged ion production in the electron cyclotron resonance ion source (ECRIS) during the afterglow of the pulsed operation mode are presented in this article. The influence of the short neutral flux injected into the plasma on the parameters of afterglow is studied. It is shown that a pulse of neutral atoms reduces the duration and increases the amplitude of afterglow. The free expansion of laser ablation plasma was experimentally studied to find the ways for neutral flux production. A Q-switched YAG:Nd+3 laser of 60 mJ focused onto the surface of a Zn target was used for this purpose. A neutral density up to 1013 cm−3 at a distance of 5 cm from the target was obtained. The plasma pulse duration is 10 μs, the ionization degree in the flux is about 5%, and the velocity of neutrals is in the range of 106 cm/s. These parameters of the neutral flux are quite enough to test the afterglow enhancement as well as for the load of metal or solid elements into the ECRIS.

The multichannel triple-laser interferometer/polarimeter system at RTP

A 19-channel combined interferometer and polarimeter system has recently become operational at the Rijnhuizen Tokamak Project (a=0.164 m, R=0.72 m, Btor⩽2.5 T, Ip⩽150 kA, plasma pulse duration ⩽500 ms), in order to determine the poloidal magnetic field and plasma density distribution simultaneously. The method is based on the separate propagation of two counter-rotating, circularly polarized probing waves, and enables high temporal resolution measurements of both the interferometric and the polarimetric phase change using only one detector per line of sight. In this article a thorough discussion of the (polarization-related) optical, electronic, and computational aspects of the system will be presented.

The multichannel triple-laser interferometer/polarimeter system at RTP (abstract)

A 19-channel combined interferometer and polarimeter system has recently become operational at the Rijnhuizen Tokamak Project (a=0.164 m, R=0.72 m, Btor⩽2.5 T, Ip⩽150 kA, plasma pulse duration ⩽500 ms), in order to determine the poloidal magnetic field and plasma density distribution simultaneously. The method is based on the separate propagation of two counter-rotating, circularly polarized probing waves, and enables high temporal resolution measurements of both the interferometric and the polarimetric phase change using only one detector per line of sight. In this article a thorough discussion of the (polarization-related) optical, electronic, and computational aspects of the system will be presented.

Production of very hard surface layers by repetitive use of high energy plasma pulses

At the Fachgebiet Raumfahrttechnik of the Technical University at Munich, electrothermal and electromagnetic launchers are used to accelerate projectiles to velocities of 5–15 km s −1 to simulate the impact of micrometeorites on structure of spacecraft. The discharge of stored electrical energy (in the order of some kJ) across selected materials generates a plasma armature, which accelerates the projectile. Without a projectile a high pressure (in the order of 10–50 kbar) plasma pulse (duration 10–50 μs, temperature approximately 1–10 4 K, 10 6–10 7 W cm −2) is accelerated to velocities of up to 20 km s −2. This pulse, to which selected materials (e.g., Cr, C) can be deliberately added, is directed towards the surface of a target for treatment. Experiments with steel targets show that an extremely hard surface layeer of average 30 μm thickness is created by a single pulse [1]. The surface layer thickness could be increased to approximately 400 μm by repetitive impacting. After 10 process steps, where the target is placed with a small offset after each “shot”, an adhesive surface layer with a fairly high surface roughness is produced. The surface layer has then to be ground to a thickness of 200 μm in a subsequent step to achieve a smooth surface. The surface layers are investigated by means of metallography, scanning electron microscopy (SEM) and X-ray diffraction. The layer consists of two different zones with a measured microhardness up to 1300 HV 0.01. There is a very hard white etching zone immediately below the surface, and a dark etching zone between the white zone and the core workpiece which has a needle-shaped martensitic microstructure. Preliminary results indicate that the white zone, which is created after every new impact, transforms into the dark zone in dependence on temperature during subsequent treatments. The production of very hard adhesive surface layers with thicknesses of minimum 200 μm is a further step towards industrial application of this method.

Treatment of the surface layer of steel with high energy plasma pulses

At the Lehrstuhl für Raumfahrttechnik of the Technical University at Munich various types of electrothermal and electromagnetic launchers are used to accelerate projectiles to velocities of 5–15 km s −1 to simulate the impact of micrometeoroids on structures of spacecraft. The discharge of stored electrical energy (of the order of kilojoules) across selected materials generates a plasma armature, which accelerates the projectile. Without a projectile a high pressure (of the order of kilobars) plasma pulse (duration, 10–50 μs; temperature about 10000 K) is accelerated to velocities of the order of 20 km s −1. This pulse, to which selected materials (e.g. Cr, C) can be deliberately added, can be directed towards the surface of a target for treatment. Experiments with Cr-Ni steel showed that an extremely hard surface layer of 20–30 μm thickness was generated. This layer is currently investigated by means of metallography (including scanning electron microscopy), X-ray diffractometry and analytical electron microscopy. Preliminary results indicate that the layer consists of structures with dimensions of the order of several nanometres. Experimental and theoretical results reveal a dependence of the hardness and the dimensions of the structure of the layer on the cooling rate (of the order of 10 6 K s −1). X-ray diffraction and electron diffraction suggest that by addition of graphite (or chromium) to the plasma pulse carbide phases may be formed, which together with the nanocrystallinity can explain the observed hardness of the layers.

Treatment of the surface layer of steel with high energy plasma pulses

At the Lehrstuhl für Raumfahrttechnik of the Technical University at Munich various types of electrothermal and electromagnetic launchers are used to accelerate projectiles to velocities of 5–15 km s −1 to simulate the impact of micrometeoroids on structures of spacecraft. The discharge of stored electrical energy (of the order of kilojoules) across selected materials generates a plasma armature, which accelerates the projectile. Without a projectile a high pressure (of the order of kilobars) plasma pulse (duration, 10–50 μs; temperature about 10000 K) is accelerated to velocities of the order of 20 km s −1 . This pulse, to which selected materials (e.g. Cr, C) can be deliberately added, can be directed towards the surface of a target for treatment. Experiments with Cr-Ni steel showed that an extremely hard surface layer of 20–30 μm thickness was generated. This layer is currently investigated by means of metallography (including scanning electron microscopy), X-ray diffractometry and analytical electron microscopy. Preliminary results indicate that the layer consists of structures with dimensions of the order of several nanometres. Experimental and theoretical results reveal a dependence of the hardness and the dimensions of the structure of the layer on the cooling rate (of the order of 10 6 K s −1 ). X-ray diffraction and electron diffraction suggest that by addition of graphite (or chromium) to the plasma pulse carbide phases may be formed, which together with the nanocrystallinity can explain the observed hardness of the layers.

The plasma system of RFX

The “plasma system” of RFX includes the first wall, the vacuum vessel, the stabilizing shell, the vacuum and gas injection system, the remote handling system and the relevant auxiliary plants (cooling, baking, glow discharge cleaning, etc.). Since no limiters are provided, the full energy and particle flux from the plasma must be absorbed by the first wall. Taking into account the significant power losses from the plasma in nominal conditions, the high peaking factors due to “dynamo” effects, field errors and plasma shift, and possible sudden losses of configuration, the first wall was designed to be able to withstand high thermal loads. Also the need to use a low Z plasma-facing material led to the choice of pure, microcrystalline graphite. The wall is subdivided into 2016 tiles, individually clamped to the vacuum vessel by means of a special bayonet system suitable for remote handling. The remote handling system, which was designed to facilitate the replacement and inspection of all 2016 graphite tiles, comprises an arm with five degrees of freedom, two grippers for different tasks, two television cameras, and a control system for automatic or teleoperated motions. The vacuum vessel is a stiff sandwich structure with 72 solid poloidal rings bearing the first wall tile supports. It is cooled by CO 2 flowing through the interspace; baking up to 350°C is also provided by means of heating cables attached to the outer surface. The vessel is closely surrounded by a shell, giving passive stabilization against magnetohydrodynamic modes; the shell is made of aluminium alloy, 65 mm thick, and, to allow fast field penetration, it is electrically subdivided into four sections, by means of epoxy-glass layers. The RFX vacuum system is a combination of 12 cryopumps and 12 turbomolecular pumps, giving a total pumping speed at the vessel of 4500 1 s −1 for nitrogen. Because of the short plasma pulse duration, the gas injection system must produce high gas throughputs (up to 700 Pa m 3 s −1) with very short response times (some milliseconds). The glow discharge cleaning system utilizes four anodes toroidally equispaced and located at the centre of the vessel minor cross-section, is r.f. assisted and allows wall conditioning in hydrogen, helium or methane. In this paper we present the basic concepts underlying the main design choices and then, for the main components and plants, we describe the significant design features, as well as the methods and results of the structural, thermal and electromagnetic calculations. We also deal with preliminary tests on prototypes and with the most important manufacturing technologies, production cycles, and assembly procedures. Finally, the preliminary design of the control system for the plasma main electromagnetic parameters is presented.

Multichannel interferometer/polarimeter system for the RTP tokamak

A nine-channel interferometer/polarimeter system is designed for the RTP tokamak (Rijnhuizen Tokamak Project, a =0.16 m, R =0.72 m, BT =2.5 T, Ip ≤ 200 kA, and plasma pulse duration 200 ms). A CO2-pumped dual-cavity FIR laser system is used to obtain two FIR laser beams of λ =432 μm with a frequency difference of 1 MHz. The FIR beams are expanded by a set of parabolic mirrors. Corner-cube mixers with Schottky diodes are used as detectors. The mixers are mounted on a linear rail system, such that their positions can be easily changed. The interferometer can be operated with a maximum of 19 channels, and will be extended with a polarimeter to measure the poloidal magnetic field distribution. It will be tried to increase the accuracy of the polarimeter by modulating the polarization of the incoming beam. A scheme for polarization modulation which is based on the idea of Dodel and Kunz will be presented in this paper. This scheme has the advantage that only one set of detectors is needed to measure the electron density and current density. The complete system will be presented along with some pilot experiments concerning the polarization modulation.

Equilibrium evolution on the resistive time scale in a tokamak reactor

In the Engineering Test Facility (ETF), the plasma pulse duration is expected to be hundreds of seconds, which is comparable to the resistive time scale that governs the resistive diffusion of the equilibrium. The resistive evolution of the safety factorq profile may, for MHD stability reasons, limit the duration of the plasma burn in a tokamak reactor. It may be possible to control this evolution and extend the plasma burn time through proper profile tailoring. We study the evolution of theq profile on the resistive time scale numerically using a one- and-one-half-dimensional (1 1/2-D) single fluid transport code. Two high beta (βT ∼ 7–16%) cases are considered: (a) a beam-driven hydrogen plasma with no nuclear alpha heating for which the beam energy is used as a device to control the temperature profile, and (b) an ignited D-T plasma in which the neutral injection has been turned off. For the beam-driven plasma, it is shown that low beam energy heating profiles lead to resistive steady states having broad temperature profiles and flatq profiles, while high beam energy heating profiles lead to resistive steady states having peaked temperature profiles and deepq profiles. The centralized nuclear heating in an ignited D-T plasma causes the evolution of theq profile for this case to behave much like that in the high energy, beam-driven case: namely, theq values near the plasma center decrease on the resistive time scale until a deep, resistive, steady-stateq profile is reached.