Theory of Interaction of Memory Patterns in Layered Associative Networks

A synfire chain is a network that can generate repeated spike patterns with millisecond precision. Although synfire chains with only one activity propagation mode have been intensively analyzed with several neuron models, those with several stable propagation modes have not been thoroughly investigated. By using the leaky integrate-and-fire neuron model, we constructed a layered associative network embedded with memory patterns. We analyzed the network dynamics with the Fokker-Planck equation. First, we addressed the stability of one memory pattern as a propagating spike volley. We showed that memory patterns propagate as pulse packets. Second, we investigated the activity when we activated two different memory patterns. Simultaneous activation of two memory patterns with the same strength led the propagating pattern to a mixed state. In contrast, when the activations had different strengths, the pulse packet converged to a two-peak state. Finally, we studied the effect of the preceding pulse packet on the following pulse packet. The following pulse packet was modified from its original activated memory pattern, and it converged to a two-peak state, mixed state or non-spike state depending on the time interval

Improvement in Cryogenic Stability of the Model Coil of the LHD Helical Coil by Lowering the Temperature

Helical coils of the Large Helical Device are pool-cooled superconducting magnets, in which propagation of a normal-zone has been observed several times at about 86% of the nominal current of 13.0 kA. It is planned to improve the cryogenic stability by lowering the inlet temperature. In order to estimate the effect, the cryogenic stability of a model coil of the helical coil was examined in saturated and subcooled helium. Liquid helium is supplied from the bottom of the model coil, and it is exhausted through the winding to the current-leads tank. The inlet helium is subcooled by a pre-cooler. A normal zone was initiated by a heater on the conductor at the bottom of the coil. In saturated helium of 4.4 K and 0.12 MPa, the minimum current to propagate over the next turn varies from 10.7 to 11.2 kA in the four cases that are without or with additional thermal shields, and before or after being subcooled. The difference is considered to be caused by the change of quality of saturated helium inside the winding or by the change of the wetted condition of the conductor surface. The minimum currents are higher at the lower temperatures in subcooled helium. It is raised up to 11.7 kA at 3.5 K of the temperature inside the winding. The propagation velocity at each minimum current is almost same. Namely, the propagation velocities at the same current are slower at the lower temperature in subcooled heliu

Sparse and Dense Encoding in Layered Associative Network of Spiking Neurons

A synfire chain is a simple neural network model which can propagate stable synchronous spikes called a pulse packet and widely researched. However how synfire chains coexist in one network remains to be elucidated. We have studied the activity of a layered associative network of Leaky Integrate-and-Fire neurons in which connection we embed memory patterns by the Hebbian Learning. We analyzed their activity by the Fokker-Planck method. In our previous report, when a half of neurons belongs to each memory pattern (memory pattern rate $F=0.5$), the temporal profiles of the network activity is split into temporally clustered groups called sublattices under certain input conditions. In this study, we show that when the network is sparsely connected ($F<0.5$), synchronous firings of the memory pattern are promoted. On the contrary, the densely connected network ($F>0.5$) inhibit synchronous firings. The sparseness and denseness also effect the basin of attraction and the storage capacity of the embedded memory patterns. We show that the sparsely(densely) connected networks enlarge(shrink) the basion of attraction and increase(decrease) the storage capacity

Excitation properties and cryogenic stability of helical coils for the LHD

The helical coils for the Large Helical Device are the world\u27s largest pool-cooled superconducting coils in operation. These were expected to be cryostable up to 13.0 kA at 4.4 K on a basis of the measured recovery currents in all the short samples. However, a normal-zone was induced at higher than 11 kA repeatedly. It propagated to the finite length and recovered within several seconds except at 11.45 kA. Because of slow current diffusion into a pure aluminum stabilizer, a normal zone can propagate dynamically below the cold-end recovery current. The excitation tests have been carried out, and average 11.65 kA has been achieved by grading the current in the three blocks of the helical coil. The disturbance during excitations and the cryogenic stability of the coil are presente

Results of the first excitation of helical coils of the Large Helical Device

The helical coils of the Large Helical Device are large scale pool-cooled superconducting coils. A conductor made of NbTi-Cu compacted strands and a pure Al stabilizer was developed to attain high cryostability. The design current of the conductor is 13.0 kA at 4.4 K, which produces a toroidal magnetic field of 3 T at a major radius of 3.9 m. The first excitation test up to 6.5 kA was conducted successfully is the beginning of the first cooling period. The higher excitations were tried in the second cooling period. The first propagation of a normal zone was observed when reaching 11.2 kA, and it recovered within 5 s. In the next step, a wide propagation occurred at 11.4 kA, and the quench detection system worked. The coils were designed to satisfy `cold-end\u27 stability by using the recovery current measured in short samples, but a normal zone propagates at lower than the recovery current in the composite conductor stabilized by very low resistive meta

First-order Raman spectra of double perovskites AB′1/2B'{1/2}B''{1/2}O3

First principles computations of Raman intensities were performed for perovskite-family compound CaAl$_{1/2}$Nb$_{1/2}$O$_3$ (CAN). This compound features 1:1 (NaCl-type) ordering of Al and Nb superimposed onto the $b^-b^-c+$ octahedral tilting. Raman tensor for CAN was computed using the package for first-principles computations ABINIT (URL \underline {http://www.abinit.org}). Computations performed for both untilted cubic ($Fm\bar{3}m$) and tilted monoclinic ($P2_1/n$) CAN structures showed that the strongest Raman lines are associated with the ordering of Al and Nb. The computed spectrum agreed qualitatively with the experimental data measured on powder (CAN is available in polycrystalline form only). The effect of cation disorder on the Raman intensities was considered using phenomenological theory of light scattering in the vicinity of a phase transition. We suggest that, for certain modes, the corresponding Raman intensities depend primarily on the average long range order while, for other modes, the intensities are determined by fluctuations of the order parameter.Comment: 4 figures, submitte

Results of stability test in subcooled helium for the R&D coil of the LHD helical coil

Helical coils of the Large Helical Device are pool-cooled superconducting magnets. The operating current is restricted below about 90% of the design current because a normal-zone has propagated dynamically at several times at almost the same current. In order to estimate the effect of lowering temperatures on the cryogenic stability, an R&D coil was made of the same conductor. The cryogenic stability of the R&D coil was examined in saturated and subcooled helium. A normal-zone was initiated by a heater inserted between the conductor and the layer to layer spacer. The propagation was detected by voltage taps. In saturated helium of 4.4 K and 0.12 MPa, the minimum current to begin propagation is 10.7 to 10.8 kA. It becomes higher at the lower temperature, and it exceeds 11.7 kA in subcooled helium of 3.5 K as a temperature inside the R&D coil

Results of the Excitation Test of the LHD Helical Coils Cooled by Subcooled Helium

Large helical device, the largest superconducting stellarator, has been operated for the research of fusion plasma since 1998. The toroidal field of almost 3 T is produced by a pair of pool-cooled helical coils, in the innermost layers of which a normal-zone had been induced several times at the bottom of the coil at higher currents than 11.0 kA. Since the field is not the highest there, the local cooling conditions are probably deteriorated by bubbles gathered by buoyancy. In order to improve the cryogenic stability by subcooling, an additional cooler with two-stage cold compressors was installed at the inlet of the coil in 2006. The inlet and outlet temperatures of the coils were successfully lowered to 3.2 K and 3.8 K, respectively, with a mass flow of 50 g/s. In spite of a half charging rate to reduce AC losses, a normal-zone was induced near the top of the coil at 11.45 kA. It propagated to one side and stopped near the inner equator, where the field is the highest. In comparison with the stability tests with a model coil, the local temperatures of the innermost layers near the top is considered to have been raised up to almost the saturated temperature of 4.4 K by charging. The excitation method was revised to waiting cool-down at 11.0 kA, and the excitations up to 11.5 kA have been attained