T. Pereira

Edge plasma pressure measurements using a mechanical force sensor on the tokamak ISTTOK

In the present paper we report on a novel mechanical probe, which is able to measure the plasma pressure directly. The probe consists of two pendulums whose heads are exposed to the tokamak edge plasma, while the deflection is measured very sensitively outside the plasma by means of semi-conductor strain gauges. The plasma pressure was successfully measured in the ISTTOK edge plasma, its value being in good agreement with that derived from the electrical probe data (pp = 1–10 Pa). Furthermore, we discuss the possibility of determining the ion temperature Ti = pp/n − Te by combining the pressure measurement with those of n and Te from the electrical probes. Although the derived ion temperatures—besides that in the region close to the limiter—were reasonable, its uncertainty is still very large.

On the Nature of Stochastic Excited Oscillations: Subdwarf B Star PG 1605+072 as a Case Study

It is well known that the near discontinuities of the background state thermodynamic quantities, like those encountered in the upper layers of some stars, reflect acoustic-gravity waves and thereby partially contain them within the stellar interior. These reflections help to establish resonant cavities in which oscillations of discrete characteristic frequencies propagate. These oscillations, when excited, produce variations on the light curve. We present an investigation of the observed light curves of variable stars using a theoretical time series calculated on the assumption that the observed light curve is the superposition of sinusoidal waves of a given frequency, each one representing a stellar pulsation. By analyzing different simulated light curves, we show that the nature of the excitation of pulsating stars can be gathered from the time dependence of the observed modes amplitudes, in spite of the limited information contained in some observational data. This analysis is based on the temporal variation of the oscillation amplitudes during several consecutive nights. It is expected that if an oscillation is excited and damped by a mechanism in stochastic equilibrium, the probability distribution of its amplitude, A, will be well defined. An interesting property is that the ratio of the standard deviation σA over the mean value μA is of the order of 0.52. Furthermore, we show from observational light curves that there is no stochastic mechanism exciting the oscillations in the subdwarf B star PG 1605+072. This new independent observational result solidifies the case that in subdwarf B stars, pulsations are originated by the excitation of intrinsically overstable modes.

Real-time tomography system at ISTTOK

A real-time plasma position control system is mandatory to achieve long duration (up to 250 ms), Alternating Current (AC) discharges on the ISTTOK tokamak. Such a system has been used for some time supported only on magnetic field diagnostic data. However, this system is clearly challenged when the plasma current is low, rendering it inoperative during the plasma current reversal. A tomography diagnostic with 3 pinhole cameras and 8 silicon photodiode channels per camera was installed and customized to supply alternative plasma position to be used for plasma position control. As no filtering is applied most of the radiation detected is in the visible/near-UV range. The data acquisition and control system is based on a 2MSPS, 32 channel acquisition ATCA module and the data processing is performed on a GPU that is connected to another ATCA module via the PCI-Express port for fast data access. Control commands are relayed to the plasma positioning PF power supplies via optical serial ports. In this work, an overview of some of the tomographic reconstruction algorithms most commonly used for tokamak plasmas is done and an assessment is made on the best candidate for the proposed real-time implementation. The tomography acquisition and plasma control systems operating at ISTTOK are also described. This system aims at achieving the following goals: (i) execute a tomographic reconstruction; (ii) determine the average emissivity position from it; (iii) calculate the shift from the axis and (iv) supply the vertical field power supply with the desired current value, all in less than 100 μs. The horizontal magnetic field power supply unit, essential for vertical plasma positioning, is foreseen to be integrated in the system and will have no impact in the processing time.

A generic communication protocol for remote laboratories: An implementation on e-lab

The remote laboratories at IST (Instituto Superior Técnico), e-lab, serve as a valuable tool for education and training based on remote control technologies. Due to the high number and increase of remotely operated experiments a generic protocol was developed to perform the communication between the software driver and the respective experimental setup in an easier and more unified way. The training in these fields of students and personnel can take advantage of such infrastructure with the purpose of deploying new experiments in a faster way. More than 10 experiments using the generic protocol are available on-line in a 24×7 way.

Distributed e-lab setup based on the Raspberry Pi: The hydrostatic experiment case study

e-lab is a remote laboratory which provides the control of real physics experiments over the internet. The robustness of the laboratory framework has allowed the implementation of new experiments outside the main physical laboratory, for instance in high schools and science centres. To reduce costs and the number of maintenance interventions, a new and simple hardware platform to access and control the remote experiment, including video encoding and streaming was developed over the Raspberry Pi computer. Such hardware platform and network implementations are presented in this paper as a case study of a new hydrostatic e-lab experiment. The solution has been bench-marked against previous hardware configurations and proved to be adequate for the given purpose.

Tomographic Visualization for Plasma Position Control in ISTTOK

Plasma positioning on the ISTTOK tokamak has usually been done, resorting to magnetic pickup coils. However, during alternating current (ac)-type discharges with plasma current reversal, this method has been found to be inadequate. The recently installed tomography diagnostic can be an alternative for determining the plasma position. The presented image shows tomographic reconstructions of the ISTTOK plasma during ac discharges, showing the position of maximum and average emissivity positions.

Amplitude variations in the sdBV star PG 1605+072: Another beating time scale?

PG 1605+072 has an unique and complex oscillation spectrum amongst the pulsating members of the EC 14026 stars. It has the longest periods and the richest, most puzzling frequency spectrum. We present a quantitative analysis of the photometric time-series obtained at 1-m telescope of the South African Astronomical Observatory. Thirteen oscillation parameters, frequencies, amplitudes and initial phases were determined from a 45 h time-series. Our work confirm previous observational results. The observed frequencies are within a difference smaller than 2.7% of the theoretical values, and less than 0.1% of other previous studies. We also infer the existence of variation of a periodicity of 4–5 days on the amplitude of the observed modes, similar to the yearly time-scale variation found by previous studies. Furthermore, we found a new frequency of 2133

Multi-scale study of the isotope effect in ISTTOK

\mathrm{\mu Hz}

Integration of EPICS subsystem control on FireSignal

which has not been previously reported, its origin being yet unclear.

Design and instrumentation of a magnetic field micro-probe mapper: An e-lab aparatus to map a coil's magnetic field

The isotope effect, namely the isotope dependence of plasma confinement, is still one of the principal scientific conundrums facing the magnetic fusion community. We have investigated the impact of isotope mass on multi-scale mechanisms, including the characterization of radial correlation lengths () and long-range correlations (LRC) of plasma fluctuations using multi-array Langmuir probe system, in hydrogen (H) and deuterium (D) plasmas in the ISTTOK tokamak. We found that when changing plasma composition from the H dominated to D dominated, the LRC amplitude increased markedly (10–30%) and the increased slightly (~10%). The particle confinement also improved by about 50%. The changes of LRC and are congruent with previous findings in the TEXTOR tokamak (Xu et al 2013 Phys. Rev. Lett. 110 265005). In addition, using biorthogonal decomposition, both geodesic acoustic modes and very low frequency (<5 kHz) coherent modes were found to be contributing to LRC.