# Analysis of the New Thomson Scattering Diagnostic System on WEST [PERSON]
## 1 Introduction  
The French tokamak WEST, located in Cadarache, South of France, is an upgrade of the Tore Supra tokamak, which achieved first plasma in 1988 [1]. After leading plasma control for long pulse operations for 25 years, the initial toroidal carbon limiter of this tokamak has been turned into a tungsten divertor to support ITER design and operation with this specific plasma facing material.  
WEST is a medium-sized tokamak (R = 2.5 m, a = 0.5 m, Bt = 3.7 T). Its two main objectives are to study the behavior of ITER representative tungsten plasma facing components in a divertor configuration, as well as to control plasma for long pulses (\(\sim\)1000 s) in a metallic environment.  
In order to measure plasma density and temperature profiles in WEST, IRFM is designing a new Thomson Scattering diagnostic (Figure 1) with a radial resolution of few millimeters inside the pedestal [2, 3, 4].  
The design of the diagnostic is based on two key points: the interaction between the laser beams and the plasma and the collection of the scattered light by the endoscope.  
Up to three lasers of 2 J at 30 Hz will be installed under the tokamak. The focused laser beams will come into the vacuum vessel from below and through a Brewster window. The scattered light, which is the result of the interaction between the laser beams and the plasma, will be transported by optical fibers to polychromators located below the tokamak.  
After vertically crossing the tokamak, the laser beams will be stopped by a laser dump, and the main characteristics of the beams will be analyzed by a control box.  
An endoscope inside the vacuum vessel, using actively cooled optical components, would allow for high spatial resolution inside the pedestal. The rear part of the endoscope will be attached on a frame, fixed in turn to the building to make the endoscope and the tokamak mechanically independent of each other. To do so, at the level of the vacuum flange, a stainless steel bellows will be used to ensure vacuum tightness while keeping the endoscope free from the vacuum vessel movements (Figure 2).  
The optical system, composed of an actively cooled mirror, is located at the front part of the endoscope (placed inside the vacuum vessel), its bracket and a lens system allowing for focalization in the optical fibers (Figure 3). To ensure viability of the measurements, the tilt of the mirror needs to be calculated.  
Temperature is also another important parameter to be taken into account, since this diagnostic operates in the near infrared light. To avoid stray radiation, the temperature of all components must be kept lower than 200 \({}^{\circ}\)C.  
In this study, mechanical, thermal, hydraulic and electromagnetic aspects are considered to evaluate the efficiency of the endoscope. Global and local calculations have been carried out in order to understand the behavior of critical components and to validate the whole design before launching a call for tenders for manufacturing.  
Figure 1: (**a**) Laser path. (**b**) Sectional view of the WEST tokamak, including the Thomson scattering diagnostic.  
Figure 2: Representation of the connection between the diagnostic and the vacuum vessel.