An amplitude modulation reflectometry system is in operation at the stellarator TJ-II. Recently, the first electron density profiles were obtained, showing good agreement with profiles measured by Thomson scattering and lithium beam diagnostics. In order to measure density profiles from the plasma edge, the extraordinary mode of polarization is used. A hyper-abrupt varactor-tuned oscillator used in combination with active multipliers generate two frequency segments: 25-36 and 36-50 etc GHz sharing a unique common wave-guide system (Ka band). The signal is amplitude modulated at 200 MHz and the phase demodulation is done at lower intermediate frequency. The time evolution of the electron density profile was measured under different experimental conditions. In this paper, we present the temporal evolution of the profiles obtained during a transition to an enhanced confinement mode and during cold pulse propagation experiments. We also report the modification in the shape of the density profile measured during a magnetic configuration scan in which a low-order rational surface is moved from the scrape-off layer into the plasma confinement region.

Tokamak plasmas have a tendency to self-organization: the plasma pressure profiles obtained in different operational regimes and even in various tokamaks may be represented by a single typical curve, called the self-consistent pressure profile. About a decade ago local zones with enhanced confinement were discovered in tokamak plasmas. These zones are referred to as internal transport barriers (ITBs) and they can act on the electron and/or ion fluid.Here the pressure gradients can largely exceed the gradients dictated by profile consistency. So the existence of ITBs seems to be in contradiction with the selfconsistent pressure profiles (this is also often referred to as profile resilience or profile stiffness). In this paper we will discuss the interplay between profile consistency and ITBs. A summary of the cumulative information obtained from T-10, RTP and TEXTOR is given, and a coherent explanation of the main features of the observed phenomena is suggested. Both phenomena, the selfconsistent profile and ITB, are connected with the density of rational magnetic surfaces, where the turbulent cells are situated. The distance between these cells determines the level of their interaction, and therefore the level of the turbulent transport. This process regulates the plasma pressure profile. If the distance is wide, the turbulent flux may be diminished and the ITB may be formed. In regions with rarefied surfaces the steeper pressure gradients are possible without instantaneously inducing pressure driven instabilities, which force the profiles back to their self-consistent shapes. Also it can be expected that the ITB region is wider for lower dq/dρ (more rarefied surfaces).

The self-organization of a tokamak plasma is a fundamental turbulent plasma phenomenon, which leads to the formation of a self-consistent pressure profile. This phenomenon has been investigated in the T-10 tokamak in different experiments, excluding profiles with pronounced transport barriers. It will be shown that the normalized pressure profile can be expressed by the equation p N (r) = p(r, t)/p(0, t), over a wide range of plasma densities. It will also be shown that p N (r) is independent of the heating power and the deposition profile of electron cyclotron resonance heating. Experiments show that p N (r) depends only on the value of q at the plasma edge. During rapid current ramp-ups it has been demonstrated that the conservation of p N (r) is established during a time t c < 0.1τ E , with τ E the energy confinement time. It can be concluded that the self-consistent pressure profile p N (r) in tokamaks is linked to the equilibrium of a turbulent plasma.

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A two-dimensional code based on the WKB (Wenzel–Kramers–Brillouin) approximation is used to simulate reflectometry measurements in plasmas with turbulence. In this work we aim to understand, first the role of turbulence in the determination of density profiles with reflectometry and, second, the ability of reflectometry techniques to give reliable information on the characteristics of the turbulence itself. The effects on the profile determination of a rotating turbulence structure with nonperpendicular reflection are analyzed. The influence of the turbulence level, fluctuation wavelengths and antenna beam size on the density profile determination has been studied in static plasma with perpendicular launching. The code has been used to simulate correlation measurements. The results show the correlation of the reflectometry signals for different turbulence parameters. Errors in the correlation length increase when two-dimensional effects become important, though the homodyne signal works better than the phase. The correlation simulations also show the way for new methods to determine the group delay and therefore the density profile.