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AbstractTomography inversion has been used routinely for processing outputs of plasma radiation diagnostics. Various tomographic algorithms have been developed, with those based on Tikhonov regularization being among the fastest while still providing reliable results. This paper presents a further speed optimization of the minimum Fisher Tikhonov regularization algorithm based on reducing iteration cycles used during the calculation. Ten to twentyfold speed gain is achieved compared to the original implementation. Robustness of the new method is demonstrated using both artificially generated data sets and real data from the soft X-ray diagnostics at the COMPASS tokamak. The advantage gained by the optimization is investigated in particular with respect to the possibility of real-time control of the plasma position; the option of impurity control is also discussed.

This paper presents the first application of the autocorrelation-width technique (Bencze A and Zoletnik S 2005 Phys. Plasmas12052323) to detect and characterize the microturbulence-zonal flow system using fluctuating signals ( , ) measured by arrays of single-tip Langmuir probes in the CASTOR tokamak. Radially localized (?1?cm) random flow structures have been clearly observed with a lifetime of ?1?ms and a long-range poloidal correlation, extending over at least one-fourth of the poloidal circumference.

Small tokamaks have an important role in fusion research. More than 40 small tokamaks are operational. Research on small tokamaks has created a scientific basis for the scaling-up to larger tokamaks. Well-known scientific and engineering schools, which are now determining the main directions of fusion science and technology, have been established through research on small tokamaks. Combined efforts within a network of small and medium size tokamaks will further enhance the contribution of small tokamaks. A new concept of interactive coordinated research using small tokamaks in the mainstream fusion science areas, in testing of new diagnostics, materials and technologies as well as in education, training and broadening of the geography of fusion research in the scope of the IAEA Coordinated Research Project, is presented.

We describe experiments with a biased electrode inserted into the scrape-off layer (SOL) of the CASTOR tokamak. The resulting radial and poloidal electric field and plasma density modification are measured by means of Langmuir probe arrays with high temporal and spatial resolutions. Poloidally and radially localized stationary structures of the electric field (convective cells) are identified and a related significant modification of the particle transport in the SOL is observed.

The poloidal distribution of turbulence in the scrape-off layer (SOL) of the CASTOR tokamak is studied by means of a ring of 32 electric probes covering the whole perimeter of the poloidal cross section. Analysis of floating potential fluctuations in a SOL created in the top part of the machine by shifting the plasma downwards reveals a dominant periodic structure that propagates poloidally in the direction of the Er× Bdrift. Its poloidal mode number is found to be equal to the local safety factor q. Correlation and pulse propagation analyses show that this high mmode is a signature of a single long flute-like structure that is aligned with the magnetic field and snakes around the torus several times before terminating on the limiter.

The statistical properties of the radial propagation of edge localized modes (ELMs) have been investigated in the Scrape-off layer (SOL) region of the JET tokamak. ELM events propagate radially with effective radial velocities in the range of 1000 m s−1. Experimental results show a link between the radial velocity and the size of transport events. This suggests the importance of the competition between both parallel and radial transport to explain particle losses onto the divertor plates. Parallel flows show a transient increase during the appearance of ELMs, providing evidence of a coupling between parallel dynamics and radial transport.

Experiments in the tokamaks TEXTOR, CASTOR, T-10 and ISTTOK, as well as in the reversed field pinch RFX have provided new and complementary evidence on the physics of the universal mechanism of E×B velocity shear stabilization of turbulence, concomitant transport barrier formation and radial conductivity by using various edge biasing techniques. In TEXTOR the causality between transport reduction and induced electric fields in the edge has been for the first time clearly demonstrated. The high electric field gradients have been identified as the cause for the quenching of turbulent cells. A quantitative analysis of the measured transport reduction is in good agreement with theoretical predictions. The scaling of plasma turbulence suppression with velocity shear has been established, revealing the density-potential cross-phase as a key element. Reduction in poloidal electric field, temperature, and density fluctuations across the shear layer lead to a reduction of the anomalous conducted and convected heat fluxes resulting in an energy transport barrier that is measured directly. In CASTOR the biasing electrode is placed at the separatrix in a non-intrusive configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement, as predicted by theory. The impact of sheared E×B flow on edge turbulent structures has been measured directly using a comprehensive set of electrostatic probe arrays as well as emissive probes. Measurements with a full poloidal Langmuir probe array have revealed quasi-coherent electrostatic waves in the SOL with a dominant mode number equal to the edge safety factor. In T-10 edge biasing is clearly improving the global performance of ECR heated discharges. Reflectometry and heavy ion beam probe measurements show the existence of a narrow plasma layer with strong suppression of turbulence. On ISTTOK, the influence of alternating positive and negative electrode and (non-intrusive) limiter biasing has been compared. Electrode biasing is found to be more efficient in modifying the radial electric field E_r and confinement, limiter biasing acting mainly on the SOL. In the RFX reversed field pinch it has been demonstrated that also in RFPs biasing can increase the local E×B velocity shear in the edge region, and hence substantially reduce the local turbulence driven particle flux mainly due to a change in the relative phase between potential and density fluctuations.

The probability density function (PDF) of turbulent transport has been investigated in the plasma edge region of tokamak (JET) and stellarator (TJ-II) fusion devices. PDFs can be re-scaled using a functional form, PDF(Γ_E×B)&equal;L⁻¹g(Γ_E×B/L), where L is directly related with the level of fluctuations in the turbulent flux. This kind of re-scaling holds at different timescales in which the functional form of the PDF changes. The empirical similarity in the PDF of turbulent transport in the edge region in both the JET tokamak and the TJ-II stellarator supports the view that turbulent transport displays universality in fusion plasmas. These results emphasize the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the traditional way to characterize transport based on the computation of effective transport coefficients (i.e. diffusion coefficients) and on average quantities (i.e. average correlation lengths).

A two-dimensional array of electric probes placed on the poloidal plane has been inserted in the edge region of the CASTOR tokamak. The floating potential patterns measured by the probes show the presence of localized structures which are convected poloidally by the E B plasma motion. The lifetime of these structures has been found to be large enough to allow them to complete several full poloidal rotations before dying away. Furthermore, the eddy turnover time of the plasma around the potential blobs is smaller than the lifetime, so that these structures can be called coherent.

A rotating Mach probe is designed and used for measurement of plasma flows on the CASTOR tokamak. Measurements are performed at standard discharge conditions as well as in shots with edge plasma polarization. The one dimensional fluid model is developed to deduce the parallel and perpendicular Mach numbers from experimental data.

With the passage of the Americans with Disabilities Act (ADA) it has become a civil rights violation to deny access to public transportation by people with disabilities. ADA requires transit agencies to provide accessible buses or equivalent services to people with mobility, sensory, or cognitive impairments. Issues concerning people with cognitive impairments, and their access to fixed-route transit services, are examined in this study. The literature concerning access to public transit by people with cognitive disabilities is summarized in this paper, along with exemplary training programs and technologies that have improved transit accessibility for people with cognitive disabilities. On review of the available literature, the authors noted that there was very little information concerning people with cognitive impairments. To fill in this knowledge gap, surveys and interviews were conducted with people with cognitive and other impairments, with people who trained people with sensory and cognitive impairments, and with people who provide special transportation services. A major conclusion of this study is that, for people with cognitive impairments, technological solutions are not the best solutions to increase bus accessibility. Personal interaction is needed to solve each individual situation with the transit user. Training for transit personnel, transit users, and travel trainers is needed to help people with cognitive impairment use the transit system with grace, speed, efficiency, and dignity. Another conclusion of the study is that to be effective all visual signage must be consistent—location, lighting, contrast, and content of visual signs.

With the passage of the Americans with Disabilities Act (ADA) it has become a civil rights violation to deny access to public transportation to people with disabilities. ADA requires transit agencies to provide accessible buses or equivalent services to people with mobility, sensory, or cognitive impairments. Issues concerning people with sensory impairments, and their access to fixed-route transit services, are examined in this study. The literature concerning access to public transit by people with sensory disabilities is summarized in this paper, along with exemplary training programs and technologies that have improved transit accessibility for people with sensory disabilities. A major conclusion of this study is that technological solutions may not increase bus accessibility for people with sensory impairments. One-on-one interaction is needed to solve many individual access problems of the transit users. Training for transit personnel is needed so personnel become aware of, and more sensitive to, the needs of all transit users. Training for the transit user is necessary so that use of the transit system is accomplished with grace, speed, efficiency, and dignity. Training for those who train people with disabilities is necessary so that transit travelers will be informed about all the available services offered by transit agencies. Visual signage must be consistent and highly legible to be effective and includes sign and information location, lighting, contrast, and content.

With the passage of the Americans with Disabilities Act (ADA) it has become a civil rights violation to deny access to public transportation by people with disabilities. ADA requires transit agencies to provide accessible buses or equivalent services to people with mobility, sensory, or cognitive impairments. Issues concerning people with cognitive impairments, and their access to fixed-route transit services, are examined in this study. The literature concerning access to public transit by people with cognitive disabilities is summarized in this paper, along with exemplary training programs and technologies that have improved transit accessibility for people with cognitive disabilities. On review of the available literature, the authors noted that there was very little information concerning people with cognitive impairments. To fill in this knowledge gap, surveys and interviews were conducted with people with cognitive and other impairments, with people who trained people with sensory and cognitive impairments, and with people who provide special transportation services. A major conclusion of this study is that, for people with cognitive impairments, technological solutions are not the best solutions to increase bus accessibility. Personal interaction is needed to solve each individual situation with the transit user. Training for transit personnel, transit users, and travel trainers is needed to help people with cognitive impairment use the transit system with grace, speed, efficiency, and dignity. Another conclusion of the study is that to be effective all visual signage must be consistent—location, lighting, contrast, and content of visual signs.

With the passage of the Americans with Disabilities Act (ADA) it has become a civil rights violation to deny access to public transportation to people with disabilities. ADA requires transit agencies to provide accessible buses or equivalent services to people with mobility, sensory, or cognitive impairments. Issues concerning people with sensory impairments, and their access to fixed-route transit services, are examined in this study. The literature concerning access to public transit by people with sensory disabilities is summarized in this paper, along with exemplary training programs and technologies that have improved transit accessibility for people with sensory disabilities. A major conclusion of this study is that technological solutions may not increase bus accessibility for people with sensory impairments. One-on-one interaction is needed to solve many individual access problems of the transit users. Training for transit personnel is needed so personnel become aware of, and more sensitive to, the needs of all transit users. Training for the transit user is necessary so that use of the transit system is accomplished with grace, speed, efficiency, and dignity. Training for those who train people with disabilities is necessary so that transit travelers will be informed about all the available services offered by transit agencies. Visual signage must be consistent and highly legible to be effective and includes sign and information location, lighting, contrast, and content.

From a given solvable Fokker-Planck equation one can construct a number of other solvable models for diffusion in a stable or bistable potential fields using the Gel'fand-Levitan method of the inverse scattering theory. The simplest way of achieving this is to change the lowest eigenvalue and/or the normalization of the lowest eigenfunction of the ordinary differential equation obtained by separating the time-dependent part. For these cases it is shown that the new probability distribution is expressible in terms of integrals involving the original probability distribution and the Gel'fand-Levitan kernel. The possibility of changing the lowest eigenvalue enables one to find bistable potential fields which would correspond to a well-defined long time relaxation rate for the probability current.

Spectral amplitudes and theoretical seismograms in the vicinity of caustic and critical points are evaluated numerically for acoustic waves propagating in Epstein media. The amplitude decay into the region beyond the caustic is compared with the asymptotic results and good agreement is found at high frequency. At low frequencies the partial reflection enhances the amplitudes at sub-critical angles and the amplitude increases with decreasing thickness of the transition zone. For a thin transition and low frequencies a head wave exists whose amplitude is inversely proportional to frequency.

The system of difference equations with variable coefficients is solved using the J-fraction method. The solution is used to find the phase spectrum of the response (or of reflection coefficient) of a layered medium with known velocities. Also, the inverse problem is solved provided the reflection coefficient is a known function of the wavenumber. At low frequencies the numerical results are in very good agreement with the solution of the differential equation.