A present status of the diagnostics in EAST is presented. As a very important step of the EAST project, tens of diagnostics were employed for operation and protection of the machine, plasma control and physics analysis to accommodate requirement for the study on plasma performance in EAST with upgraded radio frequency （RF） injection power up to 4 MW （both LHCD and ICRF） in the forthcoming years. Recently, new diagnostics are set up to provide several profiles of key plasma parameters for the further evaluation and better understanding of the plasma performance and physics in EAST, including a 25-channel Thomson scattering system for density and electron temperature, a metal bolometry for radiated power, an X-ray crystal spectroscopy for both ion and electron temperatures and plasma rotation velocity, etc. It is expected that these diagnostics would play an important role in data analysis and interpretation combined with integrated modeling.
Operation of HT-7 tokamak in a multicycle alternating square wave plasma current regime is reported. A set of AC operation experiments, including LHW heating to enhance plasma ionization during the current transition and current sustainment, is described. The behaviour of runaway electrons is analysed by four HXR detectors tangentially viewing the plasma in the equatorial plane, within energy ranges 0.3-1.2 MeV and 0.3-7 MeV, separately. High energy runaway electrons （~MeV） axe found to circulate predominantly in the opposite direction to the plasma current, while the number of low energy runaway electrons （~tens to hundreds of keV） circulating along the plasma current is comparable to that in the direction opposite to the plasma current. AC operation with lower hybrid current drive （LHCD） is observed to have an additional benefit of suppressing the runaway electrons if the drop of the loop voltage is large enough.
This paper reports that an experimental investigation of fast pitch angle scattering （FPAS） of runaway electrons in the EAST tokamak has been performed. From the newly developed infrared detector （HgCdTe） diagnostic system, the infrared synchrotron radiation emitted by relativistic electrons can be obtained as a function of time. The FPAS is analysed by means of the infrared detector diagnostic system and the other correlative diagnostic systems （including electron-cyclotron emission, hard x-ray, neutrons）. It is found that the intensity of infrared synchrotron radiation and the electron-cyclotron emission signal increase rapidly at the time of FPAS because of the fast increase of pitch angle and the perpendicular velocity of the energetic runaway electrons. The Parail and Pogutse instability is a possible mechanism for the FPAS.
