Retarding field energy analyzers for ion temperature measurements in the boundary plasmas of the tokamak ISTTOK and TJ-II stellarator

The retarding field energy analyzer (RFEA) remains the more reliable diagnostic to measure the ion temperature in the boundary plasmas of magnetic fusion devices. A compact, simple design RFEA have been developed for investigations on the tokamak ISTTOK and TJ-II stellarator. More recently a five-channel RFEA has been successfully tested allowing the simultaneous measurement of the ion temperature profile. The conditions of the RFEA operation in poor alignment along magnetic field are considered.Аналізатор енергії з затримуючим потенціалом (АЕЗП) залишається найбільш надійним діагностичним пристроєм для виміру температури іонів поблизу границі плазми термоядерних установок з магнітним утриманням. Компактний простий пристрій АЕЗП розроблено для досліджень на токамаці ISTTOK і стеллараторі TJ-II. Порівняно недавно п’ятиканальный АЕЗП був успішно випробуваний і дозволяє здійснювати синхронні виміри профілю температури іонів. Визначено умови роботи АЕЗП при поганій орієнтації його уздовж магнітного поля.Анализатор энергии с задерживающим потенциалом (АЭЗП) остается наиболее надежным диагностическим устройством для измерения температуры ионов вблизи границы плазмы термоядерных установок с магнитным удержанием. Компактное простое устройство АЭЗП разработано для исследований на токамаке ISTTOK и стеллараторе TJ-II. Сравнительно недавно пятиканальный АЭЗП был успешно испытан и позволяет осуществлять синхронные измерения профиля температуры ионов. Определены условия работы АЭЗП при плохой ориентации его вдоль магнитного поля

TOF method in plasma potential measurements by HIBD

The heavy ion beam diagnostic (HIBD) developed for the tokamak ISTTOK (R = 0.46 m, a = 0.085 m, B = 0.5 T, I = 6-9 kA) is based on a multiple cell array detector (MCAD), which collects simultaneously a “fan” of secondary ions originated along a primary beam trajectory in collisions with the plasma electrons and separated by the magnetic field of the tokamak. Utilization of the traditional electrostatic energy spectrographs for the plasma potential measurements in experiments with MCAD is very complicated. This paper presents the current results of adaptation and mastering of the alternative time-of-flight (TOF) technique. Three schemes of the measurements are considered: i) “integral” scheme of the average plasma potential measurements by a pulsed primary beam, ii) “quasi-local” scheme of the measurements of plasma potential drop between neighbouring sample volumes, and iii) “local” scheme of plasma potential profile measurements. The electronics used in TOF energy analyzer (TOFEA) consist of charge sensitive and fast shaping amplifiers, constant fraction discriminator and time-toamplitude converter with resolution ∆t/t = 10⁻⁴. The TOFEA resolution ∆t/t = 3×10⁻⁴ has been achieved in mastering experiments with a pulsed (250 ns) primary beam carried out to the primary detector in magnetic field of the tokamak. With plasma the resolution is reduced 2.5 times due to decreasing of signal-to-noise ratio caused by plasma loading of MCAD. The changes of the average plasma potential during discharges with minor disruptions have been obtained by TOF energy analysis. The results of this experiment allow to conclude the reliability of TOF technique in plasma potential measurements by HIBD with MCAD. On the base of the obtained data and experience a four-channel TOFEA for the plasma potential profile measurements has been elaborated

Perturbative transport experiments on TJ-II Flexible Heliac

Transport properties of TJ-II are explored performing perturbative experiments and taking advantage of TJ-II flexibility. Rotational transform can be varied in a wide range, which allows one to introduce low order rationals and to study their effect on transport. On the other hand, confinement properties can be studied at very different rotational transform values and for different values of magnetic shear: Experiments on influence of the magnetic shear on confinement are reported. In these cases a Ohmic current has been induced in TJ-II plasma to modify magnetic shear and to evaluate itsd effect on confinement, showing that negative shear improves the confinement. Heat transport is also reduced by locating a low order rational near the power deposition profile. Plasma potential profiles have been recently measured in some configurations up to the plasma core with the Heavy Ion Beam Probe (HIBP) diagnostic and the electric field values measured in low-density plasmas are consistent with neoclassical calculations near the plasma core. Plasma edge turbulent transport has been studied in configurations that are marginally stable due to decreased magnetic well. Results show a dynamical coupling between gradients and turbulent transport. Finally, cold pulse propagation has been studied showing ballistic non diffusive propagation

3D effects on transport and plasma control in the TJ-II stellarator

The effects of 3D geometry are explored in TJ-II from two relevant points of view: neoclassical transport and modification of stability and dispersion relation of waves. Particle fuelling and impurity transport are studied considering the 3D transport properties, paying attention to both neoclassical transport and other possible mechanisms. The effects of the 3D magnetic topology on stability, confinement and Alfvén Eigenmodes properties are also explored, showing the possibility of controlling Alfvén modes by modifying the configuration; the onset of modes similar to geodesic acoustic modes are driven by fast electrons or fast ions; and the weak effect of magnetic well on confinement. Finally, we show innovative power exhaust scenarios using liquid metals