Thomson scattering diagnostic on the ETE tokamak: status and progress

Abstract

In order to measure the plasma temperature and density in the spherical tokamak ETE, a one-channel Thomson scattering system was implemented. During the upgrade of capacitor banks and optimization since the beginning of operation, the plasma pulse duration has increased from 1.5 ms up to 12 ms with plasma currents varying from 10 kA to 60 kA. During this phase, the electron temperature was increased from 20 eV to 160 eV with densities as high as 3.5×1019 m −3 . Presently, the Thomson scattering diagnostic is being upgraded based on the time-delay technique, that consists in using fibers of different lengths to transmit the scattered light signals to the same polychromator. This system will allow measurements of electron temperature and density profiles with ten spatial points per laser shot and per polychromator. This work describes in details the Thomson scattering system, presents a selection of results obtained by this system since the initial phase of operation, and shows details of the proposed upgrade of the Thomson scattering system. The ETE Spherical TokamakThe ETE (Experimento Tokamak Esférico) is a medium-size spherical torus with an aspect ratio of 1.5, major radius of 0.3 m, toroidal field of 0.4 T and plasma current of 0.22 MA for the first stage of operation [1]. The objectives of the project are focused on plasma edge investigation, development of diagnostics and training on tokamak operation. The vacuum vessel was manufactured with Inconel 625 with an external diameter and height of 1.2 m and an internal tube with a diameter of 0.18 m, which houses the ohmic solenoid and the inner legs of the toroidal coils. Good access for diagnostics is provided by 58 Conflat ports (12xCF14", 4xCF250 and 42xCF40). The vacuum system comprises three pumps: one turbo drag pump (1500 l/s) backed by an oilfree diaphragm (4 m 3 /h) and an auxiliary mechanical pump (30 m 3 /h). A base pressure of 7.8×10 −8 Torr was achieved after conditioning the vessel walls with baking (T < 150 o C) and DC glow discharge cleaning (GDC) with helium gas.There are three capacitor banks that storage energy for the toroidal, ohmic heating and equilibrium coils. The energy of the capacitor banks is being increased by adding more capacitors and increasing the maximum voltage rating. Presently, the acquisition system is based on CAMAC and oscilloscopes that, in a near future, will be replaced by VME technology [2]. Table 1 summarizes the status of the proposed diagnostics for ETE. For this initial phase a set of electromagnetic and optical diagnostics are already installed. For edge plasma investigation a 10 keV Fast Neutral Lithium Beam (FNLB) is under development with glassy β-eucryptite source that will furnish beam currents up to 1 mA/cm 2 [3]. To measure the electron temperature and density profiles a one-channel Thomson scattering (TS) system was implemented. Thomson Scattering Diagnostic in ETEThe TS is based on a 10 J Q-switched ruby laser (λ = 694.3 nm, pulse duration: 30 ns) that probes the plasma horizontally at the mid-plane. Th...

First results on the fast neutral lithium beam diagnostics probing the edge plasma of the ETE tokamak

“…The difference on the intensity of both signals (with and without the application of the beam) is proportional to the electron density of the plasma under the following conditions: plasma temperature T e < 100eV and low attenuation of the Li beam. These conditions have been observed at the outer window position (R = 48cm) and for the present status of development of ETE [17] (the energy delivered by the capacitor banks represents approximately 1 4 of the total energy expected for the initial stage of operation of ETE). In this radial position, the temperature measured by Thomson Scattering diagnostics remained always below 30eV [18].…”

Stray light analysis for the Thomson scattering diagnostic of the ETE Tokamak

“…Each fiber has different lengths (time delay line) to relay the scattered light signals to the same detector. 2,3 The scattered light by the plasma is detected by a polychromator that presents four channels for temperature measurements. The polychromator channels have the following spectral responses: 695 nm < channel 1 < 700 nm, 700 nm < channel 2 < 714 nm, 714 nm < channel 3 < 754 nm, and 754 nm < channel 4 < 880 nm.…”

“…This work proposes a detailed stray light analysis of the diagnostic installed at the ETE (Experimento Tokamak Esférico). 2,3 The diagnostic was simulated in ZEMAX software where the scattering profiles used for the mechanical surfaces involved (steel, inconel, aluminum, and graphite) were measured in laboratory in order to have near realistic results. With this simulation it was possible to identify and quantify the main points that contribute to the stray signals and changes in the system were implemented to reduce the stray light.…”