Radiative Collapse Research Articles

Matter–antimatter gigaelectron volt gamma ray laser rocket propulsion

It is shown that the idea of a photon rocket through the complete annihilation of matter with antimatter, first proposed by Sänger, is not a utopian scheme as it is widely believed. Its feasibility appears to be possible by the radiative collapse of a relativistic high current pinch discharge in a hydrogen–antihydrogen ambiplasma down to a radius determined by Heisenberg's uncertainty principle. Through this collapse to ultrahigh densities the proton–antiproton pairs in the center of the pinch can become the upper gigaelectron volt laser level for the transition into a coherent gamma ray beam by proton–antiproton annihilation, with the magnetic field of the collapsed pinch discharge absorbing the recoil momentum of the beam and transmitting it by the Moessbauer effect to the spacecraft. The gamma ray laser beam is launched as a photon avalanche from one end of the pinch discharge channel. Because of the enormous technical problems to produce and store large amounts of anti-matter, such a propulsion concept may find its first realization in small unmanned space probes to explore nearby solar systems. The laboratory demonstration of a gigaelectron volt gamma ray laser by comparison requiring small amounts of anti-matter may be much closer.

Non-adiabatic radiative collapse of a relativistic star under different initial conditions

We examine the role of space-time geometry in the non-adiabatic collapse of a star dissipating energy in the form of radial heat flow, studying its evolution under different initial conditions. The collapse of a star filled with a homogeneous perfect fluid is compared with that of a star filled with inhomogeneous imperfect fluid under anisotropic pressure. Both the configurations are spherically symmetric. However, in the latter case, the physical space t = constant of the configurations endowed with spheroidal or pseudospheroidal geometry is assumed to be inhomogeneous. It is observed that as long as the collapse is shear-free, its evolution depends only on the mass and size of the star at the onset of collapse.

Radiative Collapse in Plasma Focus Operated with Heavy Noble Gases

Numerical experiments were carried out using 5-phase Lee model on different plasma focus devices with different filling gases. The relation of radiation power gain/loss terms as a function of temperature has been demonstrated. Variation of radial trajectories versus pressure with 0.1 Torr step has been investigated. Radiative collapse phenomena in plasma focus devices have been observed for heavy noble gases (Ar, Kr, Xe). Obtained results showed that the line radiation emission and tube voltage have huge values near the radiative collapse regime.

Shearing radiative collapse with expansion and acceleration

We investigate the behaviour of a relativistic spherically symmetric radiative star with an accelerating, expanding and shearing interior matter distribution in the presence of anisotropic pressures. The junction condition can be written in standard form in three cases: linear, Bernoulli, and Riccati equations. We can integrate the boundary condition in each case and three classes of new solutions are generated. For particular choices of the metric we investigate the physical properties and consider the limiting behaviour for large values of time. The causal temperature can also be found explicitly.

Numerical Experiments on Radiative Cooling and Collapse in Plasma Focus Operated in Krypton

The Plasma Focus has wide-ranging applications due to its intense radiation of SXR, XR, electron and ion beams and fusion neutrons when operated in deuterium. The 5-phase Lee Model code has been developed for the focus operated in various gases including D, D–T, He, Ne, N, O, Ar, Kr and Xe. Radiation-coupled motion is included in the modelling. In this paper we look at the effect of radiation cooling and radiation collapse in krypton. The Pease–Braginskii current is that current flowing in a hydrogen pinch which is just large enough for the Bremsstrahlung to balance Joule heating. This radiation-cooled threshold current for a hydrogen pinch is 1.6 MA. It is known that in gases undergoing line radiation strongly the radiation-cooled threshold current is considerably lowered. We show that the equations of the Lee Model code may be used to compute this lowering. The code also shows the effect of radiation cooling leading to radiative collapse. Numerical experiments based on experimentally fitted model parameters are run to demonstrate a regime in which radiation collapse is observed in Kr at a pinch current of 50–100 kA.

Overview of TJ-II experiments

This paper presents an overview of experimental results and progress made in investigating density control using Li-coating, transport and L–H transitions in TJ-II. The Li-coating changes drastically the plasma–wall interaction, decreasing the recycling, and enlarges substantially the operational range of the device delaying the appearance of radiative collapse that happens for higher densities, which permits confinement properties of much denser plasmas to be studied. Moreover, L–H mode transition has only been achieved after Li-coating in TJ-II. The effect of rational surfaces on heat transport is studied showing a decrease in heat diffusivity close to their position, and it is also seen that rational surfaces located in the edge make L–H transition easier. TJ-II findings provide a new guideline for understanding the trigger mechanism of the L–H transition pointing out the importance of low frequency fluctuating sheared E × B flows. The properties of fast-ion confinement are described as well as the effects of impurities on radiation profiles, showing two types of profiles the bell and the dome shape, the latter being more prone to radiative collapse.

Gravitational collapse with decaying vacuum energy

The effect of dark energy on the end state of spherical radiation collapse is considered within the context of the cosmic censorship hypothesis. It is found that it is possible to have both black holes as well as naked singularities.

Extension of the ECRH operational space with O2 and X3 heating schemes to control tungsten accumulation in ASDEX Upgrade

ASDEX Upgrade has been operated with tungsten-coated plasma-facing components for several years. H-mode operation with good confinement has been demonstrated. Nevertheless, purely neutral beam injection-heated H-modes with reduced gas puff, moderate heating power or/and increased triangularity tend to accumulate tungsten, followed by a radiative collapse. Under these conditions, central electron heating with electron cyclotron resonance heating (ECRH), usually in X2 polarization, changes the impurity transport in the plasma centre, reducing the central tungsten concentration and, in many cases, stabilizing the plasma. In order to extend the applicability of central ECRH to a wider range of magnetic field and plasma current additional ECRH schemes with reduced single-pass absorption have been implemented: X3 heating allows us to reduce the magnetic field by 30%, such that the first H-modes with an ITER-like value of the safety factor of q95 = 3 could be run in the tungsten-coated device. O2 heating increases the cutoff density by a factor of 2 allowing higher currents and triangularities to be addressed. For both schemes, scenarios have been developed to cope with the associated reduced absorption. In the case of central X3 heating, the X2 resonance lies close to the pedestal top at the high-field side of the plasma, serving as a beam dump. For O2, holographic mirrors have been developed which guarantee a second pass through the plasma centre. The beam position on these reflectors is controlled by fast thermocouples. Stray-radiation protection has been implemented using sniffer probes.

Radiation Self Absorption Effect in Ar Gas NX2 Mather Type Plasma Focus

The effect of self absorption becomes significant when plasma is dense enough to behave as optically thick. It is essential to account for the effect of self-absorption in that case. The variation of radial trajectory is investigated in this paper. In order to analyze the effect of self absorption of line radiation is investigated in argon plasma by observing the influence of pressure variation of gas. On comparison of the results of numerical experiment considering both aspects i.e. by including and excluding the self absorption term in Lee code an obvious deviation between the trajectories is observed. Last few fractions of seconds (200-300ns) corroborated the slow compression phase. Results with self absorption showed the compression levels off while without self absorption radiative collapse was observed due to radiative cooling. Due to self absorption the absorbed radiation kept the plasma from radiative collapse turning the results to be more realistic.

Dynamic Simulation of the Electron Bernstein Wave Heating Under NBI Conditions in TJ–II Plasmas

AbstractIn this work, we have calculated the expected properties of the Electron Bernstein Wave (EBW) heating using the O–X–B double mode conversion scenario in a plasma that evolves from Electron Cyclotron Resonance (ECR) to Neutral Beam Injection (NBI) heating in the TJ–II device. For this purpose, a transport simulation that reproduces the time evolution of a typical collapsing plasma heated by a combination of ECR and NBI power has been used. It is seen that the predicted EBW absorption depends strongly on the plasma characteristics, whose time evolution depends in turn on the heating properties. Therefore, the need of consistently computing the ray tracing and the plasma evolution is underlined. The fraction of the absorbed EBW heating power becomes very high as soon as the O mode cutoff layer appears. This guarantees the overlapping of both EBW and ECR heating, thus avoiding excessive plasma cooling when the wave cutoff is reached. The EBW power deposition profile evolves from off‐axis to a much more centred shape that persists until the radiative collapse quenches the plasma (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Tungsten transport and accumulation in JT-60U

Absolute density and radiative power of W ions in the core plasma are determined, from previously published relative data (Nakano et al., 2009 [1]), with an absolutely calibrated VUV spectrometer using atomic data calculated with FAC code. With increasing toroidal rotation velocity in the direction opposed to the plasma current, the W accumulation becomes significant. In the most significant accumulation case, the W density inside a q=1 layer increases up to ∼0.1% of the electron density and the radiative power increases up to 2MW, at which time radiative collapse occurs. In contrast, even with W accumulation, at an electron temperature maintained higher than 5keV, radiative collapse does not occur and the H-mode is preserved. The W sputtering yield of the W-coating decreased from 0.003 to 0.001 of the D flux with decreasing ELM energy loss.

Disruption mitigation studies on the Mega Amp Spherical Tokamak (MAST)

Disruptions pose a significant challenge in future devices where the increased stored energy can lead to unacceptably large transient heat loads on plasma facing components (PFCs). One means of mitigating disruptions is that of massive gas injection (MGI), which produces a radiative collapse of the plasma discharge through the injection of impurity gases. The MAST disruption mitigation system is capable of injecting up to 1.95bar litres into the MAST vacuum vessel over a timescale of 1–2ms, corresponding to a particle inventory of 5×1022, around 100 times the plasma particle inventory. High speed infrared thermography, offering full divertor coverage, has shown a 60–70% reduction in divertor power loads during mitigation. A combination of high temporal (0.2ms) and spatial resolution (1cm) Thomson scattering and soft X-ray camera array data show evidence for a cooling front associated with the inward propagation of the injected impurities.

Mixed potentials in radiative stellar collapse

We study the behavior of a radiating star when the interior expanding, shearing fluid particles are traveling in geodesic motion. We demonstrate that it is possible to obtain new classes of exact solutions in terms of elementary functions without assuming a separable form for the gravitational potentials or initially fixing the temporal evolution of the model unlike earlier treatments. A systematic approach enables us to write the junction condition as a Riccati equation which under particular conditions may be transformed into a separable equation. New classes of solutions are generated which allow for mixed spatial and temporal dependence in the metric functions. We regain particular models found previously from our general classes of solutions.

Rotation and transport in Alcator C-Mod ITB plasmas

Internal transport barriers (ITBs) are seen under a number of conditions in Alcator C-Mod plasmas. Most typically, radio frequency power in the ion cyclotron range of frequencies (ICRFs) is injected with the second harmonic of the resonant frequency for minority hydrogen ions positioned off-axis at r/a > 0.5 to initiate the ITBs. They can also arise spontaneously in ohmic H-mode plasmas. These ITBs typically persist tens of energy confinement times until the plasma terminates in radiative collapse or a disruption occurs. All C-Mod core barriers exhibit strongly peaked density and pressure profiles, static or peaking temperature profiles, peaking impurity density profiles and thermal transport coefficients that approach neoclassical values in the core. The strongly co-current intrinsic central plasma rotation that is observed following the H-mode transition has a profile that is peaked in the centre of the plasma and decreases towards the edge if the ICRF power deposition is in the plasma centre. When the ICRF resonance is placed off-axis, the rotation develops a well in the core region. The central rotation continues to decrease as long as the central density peaks when an ITB develops. This rotation profile is flat in the centre (0 < r/a < 0.4) but rises steeply in the region where the foot in the ITB density profile is observed (0.5 < r/a < 0.7). A correspondingly strong E × B shear is seen at the location of the ITB foot that is sufficiently large to stabilize ion temperature gradient instabilities that dominate transport in C-Mod high density plasmas.

Analysis of EUV spectra of Sn XIX–XXII observed in low-density plasmas in the Large Helical Device

We have measured extreme ultraviolet (EUV) spectra from highly charged tin ions in low-density and high-temperature plasmas produced in the Large Helical Device at the National Institute for Fusion Science. The EUV spectra emitted after injection of a tin pellet into a hydrogen plasma were monitored by a grazing incidence spectrometer whose wavelength resolution is about 0.01 nm. Two different types of spectral feature were measured in the 13–17 nm region depending on whether the discharge was stably sustained or underwent radiation collapse. The measured EUV spectra were analyzed by considering the difference in dominant charge states observed in the two types of spectra. Apart from the complex quasi-continuum structure around 13.5 nm, several strong lines occurring at wavelengths longer than 14 nm were found to originate from the transitions between excited states of Sn XXI and Sn XXII by comparison with the other experimental data including the charge exchange collisions experiments. Most of the strong spectral lines which were not identified as Sn XXI or Sn XXII were assigned to the resonance transitions of Sn XIX and Sn XX from comparison with the results of theoretical calculations.

X-ray spatial distribution of a low-inductance vacuum spark discharge

The paper presents the results of X-ray source spatial distribution measurements of a low-inductance vacuum spark discharge based on the analysis of helium-like ions spectral lines structure. The intensities of spectral lines of FeXXV ions and dielectronic satellites with spatial resolution were measured. The polarization of satellite line 1s2p22D5/2-1s22p2P3/2 was found. The obtained dependence of maximum plasma parameters during micropinch events from initial plasma line density in discharge corresponds to the radiative collapse model.

On impurity handling in high performance stellarator/heliotron plasmas

The Large Helical Device (LHD) and Wendelstein 7-X (W7-X, under construction) are experiments specially designed to demonstrate long-pulse (quasi steady state) operation, which is an intrinsic property of stellarators and heliotrons. Significant progress has been made in establishing high performance plasmas. A crucial point is the increasing impurity confinement at high density observed at several machines (TJ-II, W7-AS, LHD) which can lead to impurity accumulation and early pulse termination by radiation collapse. In addition, theoretical predictions for non-axisymmetric configurations predict the absence of impurity screening by ion temperature gradients in standard ion-root plasmas. Nevertheless, scenarios were found where impurity accumulation was successfully avoided in LHD and W7-AS due to the onset of friction forces in the (high density and low temperature) scrape-off-layer (SOL), the generation of magnetic islands at the plasma boundary and to a certain degree also by edge localized modes, flushing out impurities and reducing the net impurity influx into the core. In both the W7-AS high density H-mode regime and in the case of application of sufficient electron cyclotron radiation heating power a reduction in impurity core confinement was observed. The exploration of such purification mechanisms is a demanding task for successful steady-state operation. Impurity transport at the plasma edge/SOL was identified to play a major role for the global impurity behaviour in addition to the core confinement.

Behavior of carbon impurity ions in radiation collapse in the Large Helical Device

We measured time-dependent EUV spectra to make a quantitative study of impurity spectral lines of C III, C IV and C V during radiative collapse in the Large Helical Device (LHD) at the National Institute for Fusion Science. We calculated the intensity ratios of spectral lines of C V and C III using our collisional-radiative model and compared to observed time-dependent intensity ratios. We find that intensity ratios of C V are affected by recombination at the end of plasma for an experiment showing radiation collapse; the population density of 2s3d 3P is increased by recombination.

EUV spectra from highly charged tin ions observed in low density plasmas in LHD

In order to meet the increasing needs for experimental databases of tin (Sn) extreme ultraviolet (EUV) spectra relevant to the next generation EUV lithography, we have measured EUV spectra from highly charged tin ions in low density plasmas produced in the Large Helical Device (LHD) at the National Institute for Fusion Science. A small amount of tin was introduced into a background high temperature and low density hydrogen plasma by injecting a pellet. The EUV spectra were monitored by a grazing incidence spectrometer whose wavelength resolution is about 0.01 nm. Two different types of spectral features were found in the LHD depending on the discharge condition. The well known dense spectral structure around 13.5 nm is measured when the plasma is rapidly cooled and approaching radiative collapse after the pellet injection, while the sparse spectrum with several unidentified discrete lines from 13.8-14.6 nm is observed if the plasma is cooled more slowly and higher temperature is maintained for a while. According to comparison with other charge-separated experimental data, the dominant charge states in the former case are Sn11+-Sn14+. The latter case may be explained by considering the spectral lines from charge states higher than Sn19+.

High density physics in reversed field pinches: comparison with tokamaks and stellarators

Reversed field pinches (RFPs) share with tokamaks and stellarators the experimental evidence of an upper limit for the maximum value of the electron density at which they can operate. Above a certain density level, well described by the Greenwald law for tokamaks and RFPs, a radiative collapse with strong plasma cooling is observed, predominantly due to processes occurring at the plasma boundary. In the RFX-mod RFP close to the density limit a radiating belt, poloidally symmetric and toroidally localized, develops in the region where the plasma is shrunk as an effect of the m = 0 tearing modes. The phenomenology recalls that of MARFES or plasma detachment, though, unlike tokamaks, the appearance of the radiating belt is associated with a soft landing of the plasma discharge.The paper reports the experimental pattern of the RFX-mod plasmas close to the density limit, including density and radiation profiles, plasma flow and turbulence. Particles are toroidally conveyed towards the region of maximum shrinking of the plasma column where they accumulate. The interpretation is related to the topology of MHD m = 0 and m = 1 modes: the reconstruction of the magnetic topology shows that the highly radiating region corresponds to the presence of peripheral m = 0 magnetic islands well detached from the wall. The emerging indication is that in RFPs a reduction of the m = 0 activity could be a way to overcome the density limit.