Progress in Mirror-Based Fusion Neutron Source Development

The Budker Institute of Nuclear Physics in worldwide collaboration has developed a project of a 14 MeV neutron source for fusion material studies and other applications. The projected neutron source of the plasma type is based on the gas dynamic trap (GDT), which is a special magnetic mirror system for plasma confinement. Essential progress in plasma parameters has been achieved in recent experiments at the GDT facility in the Budker Institute, which is a hydrogen (deuterium) prototype of the source. Stable confinement of hot-ion plasmas with the relative pressure exceeding 0.5 was demonstrated. The electron temperature was increased up to 0.9 keV in the regime with additional electron cyclotron resonance heating (ECRH) of a moderate power. These parameters are the record for axisymmetric open mirror traps. These achievements elevate the projects of a GDT-based neutron source on a higher level of competitive ability and make it possible to construct a source with parameters suitable for materials testing today. The paper presents the progress in experimental studies and numerical simulations of the mirror-based fusion neutron source and its possible applications including a fusion material test facility and a fusion-fission hybrid system.

Threefold Increase of the Bulk Electron Temperature of Plasma Discharges in a Magnetic Mirror Device

This Letter describes plasma discharges with a high temperature of bulk electrons in the axially symmetric high-mirror-ratio (R=35) open magnetic system gas dynamic trap (GDT) in the Budker Institute (Novosibirsk). According to Thomson scattering measurements, the on-axis electron temperature averaged over a number of sequential shots is 660±50  eV with the plasma density being 0.7×10^{19}  m^{-3}; in few shots, electron temperature exceeds 900 eV. This corresponds to at least a threefold increase with respect to previous experiments both at GDT and at other comparable machines, thus, demonstrating the highest quasistationary (about 1 ms) electron temperature achieved in open traps. The breakthrough is made possible by application of a new 0.7  MW/54.5  GHz electron cyclotron resonance heating system in addition to standard 5 MW heating by neutral beams, and application of a radial electric field to mitigate the flute instability.

Spatial pattern formation in external noise: theory and simulation

Spatial pattern formation in fluctuating media is researched analytically from the point of view of the order parameters concept. A reaction-diffusion system with external noise is considered as a model of such media. Stochastic equations for unstable mode amplitudes (order parameters), the dispersion equation for averaged amplitudes of unstable modes, and the Fokker-Planck equation for the order parameters are obtained. The theory developed makes it possible to analyze different noise-induced effects including the variation of boundaries of ordering and disordering phase transitions depending on the parameters of external noise.

Experimental evidence of high-beta plasma confinement in an axially symmetric gas dynamic trap

In the axially symmetric magnetic mirror device gas dynamic trap (GDT), on-axis transverse beta (ratio of the transverse plasma pressure to magnetic field pressure) exceeding 0.4 in the fast ion turning points has been first achieved. The plasma has been heated by injection of neutral beams, which at the same time produced anisotropic fast ions. Neither enhanced losses of the plasma nor anomalies in the fast ion scattering and slowing down were observed. This observation confirms predicted magnetohydrodynamic stability of plasma in the axially symmetric mirror devices with average min-B, like the GDT is. The measured beta value is rather close to that expected in different versions of the GDT based 14 MeV neutron source for fusion materials testing.

Differences in amplitude-voltage relations between minimal and composite mossy fibre responses of rat CA3 hippocampal neurons support the existence of intrasynaptic ephaptic feedback in large synapses

Computer simulations and electrophysiological experiments have been performed to test the hypothesis on the existence of an ephaptic interaction in purely chemical synapses. According to this hypothesis, the excitatory postsynaptic current would depolarize the presynaptic release site and further increase transmitter release, thus creating an intrasynaptic positive feedback. For synapses with the ephaptic feedback, computer simulations predicted non-linear amplitude-voltage relations and voltage dependence of paired-pulse facilitation. The deviation from linearity depended on the strength of the feedback determined by the value of the synaptic cleft resistance. The simulations showed that, in the presence of the intrasynaptic feedback, recruitment of imperfectly clamped synapses and synapses with linear amplitude-voltage relations tended to reduce the non-linearity and voltage dependence of paired-pulse facilitation. Therefore, the simulations predicted that the intrasynaptic feedback would particularly affect small excitatory postsynaptic currents induced by activation of electrotonically close synapses with long synaptic clefts. In electrophysiological experiments performed on hippocampal slices, the whole-cell configuration of the patch-clamp technique was used to record excitatory postsynaptic currents evoked in CA3 pyramidal cells by activation of large mossy fibre synapses. In accordance with the simulation results, minimal excitatory postsynaptic currents exhibited "supralinear" amplitude-voltage relations at hyperpolarized membrane potentials, decreases in the failure rate and voltage-dependent paired-pulse facilitation. Composite excitatory postsynaptic currents evoked by activation of a large amount of presynaptic fibres typically bear linear amplitude-voltage relationships and voltage-independent paired-pulse facilitation. These data are consistent with the hypothesis on a strong ephaptic feedback in large mossy fibre synapses. The feedback would provide a mechanism whereby signals from large synapses would be amplified. The ephaptic feedback would be more effective on synapses activated in isolation or together with electrotonically remote inputs. During synchronous activation of a large number of neighbouring inputs, suppression of the positive intrasynaptic feedback would prevent abnormal boosting of potent signals.

Environmental factors which may have led to the appearance of colour vision

It is hypothesized that colour vision and opponent processing of colour signals in the visual system evolved as a means of overcoming the extremely unfavourable lighting conditions in the natural environment of early vertebrates. The significant flicker of illumination inherent in the shallow-water environment complicated the visual process in the achromatic case, in particular preventing early detection of enemies. The presence of two spectral classes of photoreceptors and opponent interaction of their signals at a subsequent retinal level allowed elimination of the flicker from the retinal image. This new visual function provided certain advantages concerning reaction times and favoured survival. This assumption explains why the building blocks for colour vision arose so early, i.e. just after the active predatory lifestyle was mastered. The principal functions of colour vision inherent in extant animals required a more complex neural machinery for colour processing and evolved later as the result of a change in visual function favouring colour vision.

A Method for Measuring Apparent Illumination

Knowledge of perceived illumination is very important for almost all experimental investigations in colour science. We consider here a method for measuring apparent illumination based on the perceptual phenomenon of fluorescence. The underlying hypothesis is that the minimal illumination level at which a patch begins to look fluorescent exactly corresponds to the respective chromatic component of the apparent illumination in the scene. We report results of three experimental studies: (1) measurements of apparent illumination for a flat mondrian presented on a screen and mondrians made of paper presented in a specially designed box with controlled illumination observed monocularly; (2) measurements of apparent illumination in a ‘rich’ 3-D scene; (3) measurements to test the linearity of the subjective illumination space.

The results confirm the basic hypothesis of the relation between fluorescence phenomena and perceived illumination and demonstrate that the proposed method gives reliable and accurate values for each of its chromatic components (errors were in the range 4% – 10%), eg for paper mondrians there was a strong correlation between the level and colour of illumination and the fluorescence. In the case of mondrians presented on a screen in darkness the measured apparent illumination was found unstable, however, as would be expected given that the context of such a scene is insufficient for a unique solution of the photometric problem. In the second experiment we found that the measurement results remain constant when the context of a ‘rich’ 3-D scene is changed and that they follow the changes in illumination conditions. In the third experiment we found that the linear relation f = f1+ f2 holds for the measured apparent illumination ( f) produced by combining of two other apparent illuminations ( f1, f2).

Visual Associative Memory Simulates the McCollough Effect

The McCollough effect (ME) refers to the phenomenon that, after a few minutes' exposure to gratings differing in both orientation and colour, subjects perceive similarly oriented achromatic gratings as if they were tinted with complementary hues. The traditional explanation of the ME as an adaptation of detectors selective for colour and orientation suffers from a number of inconsistencies: (i) the ME lasts much longer than ordinary adaptation, the decay of the effect being completely arrested during a night's sleep, or by occluding the eye for a long time; (ii) the strength of the ME is practically independent of the intensity of the adapting light; and (iii) a set of related pattern-contingent aftereffects discovered later would require, for explanation on similar lines, new detectors specific to other patterns. These properties can be explained, however, in the framework of associative memory and novelty filters.

A computational model has been developed which consists of (i) an input layer of two (left and right eyes) square matrices with two analog receptors (red and green) in each pixel; (ii) an isomorphic associative neural layer, each neuron being synaptically connected with all receptors of both eyes; and (iii) an output layer (novelty filter). Modification of synaptic efficacies conforms to the Hebb learning rule. After a few presentations of coloured gratings the model displays the ME, which is slowly destroyed by subsequent presentations of random pictures. With a sufficiently large receptor matrix the effect lasts a thousand times longer than the period of learning. Continuous darkness does not change the strength of the effect. Like the real ME, the model does not display interocular transfer, but with other adapting patterns it shows the disparity-contingent colour aftereffect (thus confirming the connections with both eyes). The model can account for different pattern-contingent colour aftereffects without assuming any predetermined specific detectors. Such detectors are constructed in the course of adaptation to specific stimuli (gratings).

On the Nature of the Gated Colour Opponency in the On-Units of the Frog Retina: Electrophysiological Study and Model

Ganglion cells of the ON-type in the frog retina produce colour-dependent responses differing in temporal patterns (short bursts to excitation of red-sensitive cones as opposed to prolonged discharges if blue-sensitive ‘green rods’ are excited). Their gated colour opponency (Kicliter et al, 1981 Brain Research210 103 – 113; Maximov et al, 1985 Vision Research25 1037 – 1049) becomes apparent from the OFF-responses in conditions when the test stimuli are superimposed on a background of another colour. So, when blue glass is introduced in the light beam (decreasing the excitation mainly of red-sensitive cones), an OFF-response is observed, much like the response to the onset of blue light. It has been suggested that opponency in ON-cells is asymmetric, ie that the red signal reaches the blue channel with reversed sign, but not vice versa.

A single-unit-recording study revealed the dependence of ON-cell responses both on the colour of stimuli presented in the centre of the receptive field and on the steady illumination of its surround. Surround illumination was found to favour OFF-responses in ON-units. In some cases even the cessation of blue light elicited an OFF-response with a discharge pattern resembling that of the onset of red light. In these cases an ON-response to yellow glass could also be obtained. These observations prove some degree of symmetry in the opponency of the red and blue channels. It is suggested that feedback from horizontal cells onto photoreceptor terminals is involved in the gated colour opponency. A circuit model that reproduces the observed phenomena is presented.

Colour Constancy in Birds: An Alternative Mechanism?

When recognising a surface colour, the visual system discounts the illumination, apparently by using some reference surfaces (like a spectrophotometer). To recover the illuminant colour it uses signals from different, sometimes remote, parts of the scene viewed either in sequence or in parallel. As a result, humans and animals fail to recognise the colour of a patch that is locally illuminated with a narrow light beam different in colour from the ambient illumination, but show good colour constancy when the beam envelops the surrounding scene.

The results obtained in birds distinguish them from humans and all animals hitherto investigated. The behaviour of hole-dwelling birds was studied in the wild by the method of alternative choice of entrance into experimental nesting-boxes having three entrances marked with coloured stimuli made from papers painted in different shades of blue, grey, or orange (see Maximov and Derim-Oglu, 1996 Perception25 Supplement, 98, and the corresponding WWW site: http://www.digipark.com/science/meta ). The spectral content of the direct sunlight illumination was changed by filters, either locally at the stimuli or over the whole front panel of the experimental box. In such artificial lighting conditions the birds proved to be incapable of using a neighbouring white surface as a sign of the illumination to discount its effect on the colour of objects. These unexpected results can be explained by the ability of animals with colour vision of rather high dimensionality to take recourse to local mechanisms of colour constancy that extract the colour information which is invariant to changes of the illumination, using certain a priori constraints on environmental light spectra. Natural daylight spectra have been shown to satisfy these constraints.

Horizontal cell dynamics: what are the main factors?

The factors potentially determining the dynamics of horizontal cell (HC) responses are: (1) the rate of transmitter release (including its transient component) and removal; (2) the voltage non-linearity of HC non-synaptic membrane combined with its capacitance; and (3) the dynamics of feedback from HCs to photoreceptors. Using, in consecutive order, the models of an isolated HC, a HC with one or two synaptic inputs and a HC of chromatic type, we have analysed the relative importance of three factors in shaping HC responses to the light and electrical current. The most prominent effect on the shape of HC ON responses derives from the voltage-dependency of the non-synaptic membrane. The dynamics of synaptic transmission plays a leading role in shaping the OFF light responses. For depolarizing responses of C-type HCs, the key factor is the electrical feedback from L-type HCs, which provides not only the response of opposite polarity (to red light), but also the typical feedback delay.

Small passerines can discriminate ultraviolet surface colours

Hollow-dwelling passerine birds were tested for UV surface colour discrimination by using the instinct to bring food to nestlings that makes a bird search for the nest under changing conditions. The experiments were carried out on breeding pairs of pied flycatcher (Muscicapa hypoleuca), great tit (Parus major) and tree sparrow (Passer montanus) in the wild by the method of alternative choice of entrance into a double nesting-box with the nestlings in one or the other section. The entrances were marked with sheets of painted papers that had different reflectances in UV. For a human observer, all marks looked achromatic. Birds were trained to discriminate the mark coloured with UV-absorbing paint from a neutral one (with equal reflectance throughout the spectrum). Birds easily learned to search for the UV-absorbing mark, and transferred the acquired habit when tested with new marks that differed only slightly from the initial ones in lightness to eliminate brightness cues.

Chromatic properties of the retinal afferents in the thalamus and the tectum of the frog (Rana temporaria)

In order to clarify physiological mechanisms underlying colour-specific visually guided behaviour, we measured spectral sensitivities of On-fibres projecting to the thalamus and class 2 and 3 fibres passing to tectum opticum. In addition we recorded responses of these fibres to moving coloured papers with known spectral reflectancies. The latter method, here called paper colourimetry, allowed us to change the relative stimulations of the blue-, green- and red-sensitive photoreceptors in any direction desired. Under the photopic conditions used the tectal fibres were driven exclusively by red-sensitive receptors, while the thalamic fibres received strong On-inputs from both red- and blue-sensitive receptors. Due to a partly antagonistic interaction between these inputs the On-fibres acted in a dichromatic way, responding with specific extended low-frequency discharges to all relative increases in blue receptor stimulation, e.g. to a great reduction in red stimulation combined with unchanged blue stimulation. Thus they have functional characteristics which could serve a visual system showing colour constancy.