Heavy ion beam probing (HIBP) is a unique diagnostics to study the core plasma potential and turbulence. Advanced HIBPs operate in the T-10 tokamak and TJ-II flexible heliac with fine focused (<1 cm) and intense (100 µA) beams. They provide measurements in the wide density interval n e = (0.3-5) × 10 19 m −3 , in a wide range of Ohmic and electron cyclotron resonance heated (ECRH) discharges with various currents at T-10, and in the wide range of magnetic configurations with ECR and neutral beam injection (NBI) heating at TJ-II. Time evolution of the radial profiles and/or local values of plasma parameters from high field side (HFS) to low field side (LFS), −1 < ρ < 1, is observed in TJ-II by 125 keV Cs + ions in a single shot, while LFS (+0.2 < ρ < 1) is observed in T-10 by 300 keV Tl + ions. Multi-slit energy analyzers provide simultaneously data on the plasma potential ϕ (by the beam extra energy), plasma density n e (by the beam current), poloidal magnetic field B pol (by the beam toroidal shift), poloidal electric filed E pol that allows one to derive the electrostatic turbulent particle flux Γ E×B . The cross-phase of density oscillations produces the phase velocity of their poloidal propagation or rotation; also it gives the poloidal mode number. Dual HIBP, consisting of two identical HIBPs located ¼ torus apart provide the long-range correlations of core plasma parameters. Low-noise high-gain electronics allows us to study broadband turbulence and quasi-coherent modes like geodesic acoustic modes and Alfvén eigenmodes.

Alfvén eigenmodes (AEs) were studied in low magnetic shear flexible heliac TJ-II (B 0  =  0.95 T, R 0  =  1.5 m, 〈 a 〉   =  0.22 m) neutral beam injection (NBI) heated plasmas (P NBI  ⩽  1.1 MW, E NBI  =  32 keV) using the heavy ion beam probe (HIBP). L-mode hydrogen plasmas heated with co-, counter- and balanced-NBI and electron cyclotron resonance heating (ECRH) were investigated in various magnetic configurations with rotational transform ι(a)/2π  =  1/q ~ 1.5–1.6. The HIBP diagnostic is capable of simultaneously measuring the oscillations of the plasma electric potential, density and poloidal magnetic field. In earlier studies chirping modes have been observed with 250 kHz  <  f AE  <  380 kHz in combined ECR and NBI heated plasmas at low density n ¯ e   =  (0.3–1.5)  ×  1019 m−3. In this paper we report the observation of chirping modes obtained with NBI only in plasmas with densities similar to those of earlier studies and obtained after lithium evaporation in the vacuum vessel. The absence of ECRH in the discharges studied here shows that ECRH is not a necessary ingredient to obtain chirping modes in TJ-II but rather a tool for obtaining low-density discharges. Using the HIBP we deduce that the location of the AE chirping mode is  −0.8  <  ρ  <  0.8. Chirping modes have a specific spatial structure: electric potential perturbations have a ballooning character, while the density and B pol perturbations are nearly symmetric for both ECRH  +  NBI and NBI-only plasmas. On TJ-II, the dominant effect on the nonlinear evolution of the AE from the chirping state to the steady-frequency state is the magnetic configuration, determined by the vacuum ι and plasma current I pl.

Previous investigations of small-scale density fluctuation by means of correlation reflectometry in T-10 tokamak revealed the existence of several density fluctuation types and strong radial and poloidal variation of their amplitudes and correlation properties. This paper is focused on the new measurements of the 3D spatial distributions of the amplitudes, the radial correlation lengths and the long range correlations along the field lines for the different turbulence types. The properties of the density fluctuations were systematically studied with the improved reflectometers, data analyzing and acquisition hardware. The density fluctuations were measured by heterodyne correlation reflectometry using ordinary mode. New T-10 antenna set have horn antennae arrays at four places distributed toroidally and poloidally over tokamak torus. The experiments confirmed previously found strong poloidal amplitude asymmetry of the broad band and the quasi-coherent oscillations and the uniform poloidal distribution of stochastic low frequency fluctuations. The presence of those turbulence types was also proved by the measurements of perturbation properties using heavy ion beam probe diagnostic. The radial correlation measurements were performed at four poloidal angles to understand the poloidal dependence of the radial correlation length for the different fluctuation types. The significant decrease of the radial correlation lengths towards the high magnetic field side was observed for quasi-coherent and stochastic low frequency turbulence types. The long range correlations along the field lines were measured by the reflectometers in two cross-section separated by 1/4 of the torus. The reflectometers have the same frequency thus provide reflection from the same magnetic surface. Reflection radii are chosen by the frequency variation of the launched wave from shot to shot in a series of reproducible discharges. The measurements were carried out at the low and the high magnetic field side with two currents and simultaneous reverse of the direction of the toroidal magnetic field and the plasma current. Resonance radii were also calculated using 3D tracing of the magnetic field line and demonstrate good agreement with experiments. These results allow to propose the new approach for the current profile measurements in tokamaks.