Recent physical results and diagnostic development of Heavy Ion Beam Probing in TJ-II L.G. Eliseev 1 , L.I. Krupnik 2 , A.V. Melnikov 1 , A. Alonso 3 , A.A. Chmyga 2 , C. Hidalgo 3 , I.A. Krasilnikov 1 , A.D. Komarov 2 , A.S. Kozachek 2 , S.M. Khrebtov 2 , V.A. Mavrin 1 , J.L. de Pablos 3 , M.A. Pedrosa 3 , S.V. Perfilov 1 , Yu. I. Tashchev 2 , A. I. Zhezhera 2 1 Institute of Tokamak Physics, NRC Kurchatov Institute, 123182, Moscow, Russia 2 Institute of Plasma Physics, NSC KIPT, 61108, Kharkov, Ukraine 3 Asociación EURATOM-CIEMAT, 28040, Madrid, Spain The direct measurements of an electric potential and its fluctuations in a core plasma are of a primary importance for the understanding the confinement improvement mechanisms and the role of electric field E r in toroidal plasma confinement. Heavy Ion Beam Probe diagnostics, which is a unique and powerful tool to study directly the plasma potential  and turbulence characteristic in the core plasma and in the edge as well, is used in TJ-II (four- period flexible heliac, B 0 = 1 T, <R> = 1.5 m, <a> = 0.22 m). 150 keV Cs + ions are used to probe ECR and NBI heated plasmas (P ECRH  0.6MW, P NBI  1MW) from the edge to the core [1]. Poloidally resolved potential and density measurements provide short range correlations, poloidal electric field E pol = ( 1 - 2 )/x, x~1 cm. The turbulent particle flux:  r =  EpolxBtor = < n e ~ v r ~ > [2] and poloidal plasma turbulence rotation [3] were retrieved for the broadband turbulence and for the quasicoherent modes as well [4]. In the experiments with ECRH power modulation plasma potential was found to follow the electron temperature evolution [5]. It was found recently that the amplitute of the potential modulation  is a function of plasma density. The mean potential profile evolution in the discharge with density increase is shown in Fig. 1. Plasma was created and heated by one gyrotron with P ECRH = 0.3 MW. The power of the second gyrotron was modulated from 0 to P ECRH = 0.1 MW. The potential profile evolves with density raise from the positive hill to the negative well following the conventional trend [2]. Such trend was found first in TM-4 tokamak [6], later on it was generalized for T-10 tokamak and the helical systems CHS, TJ-II and LHD [7]. Fig.1 shows that  is pronounced over the whole plasma radius for all densities. It also shows that  decays with density. Potential profile evolution with local modulations is presented in Fig 1. This behavior may be explained by the electron temperature evolution. The similar behavior was found in the T-10 tokamak [8]. 39 th EPS Conference & 16 th Int. Congress on Plasma Physics P2.050