# Detecting Plasma Detachment in the Wendelstein 7-X Stellarator Using Machine Learning
## 2 Methods  
### Experimental Setup  
The Wendelstein 7-X (W7-X), based in Greifswald, Germany, is the largest stellarator type fusion device in the world, aiming at demonstrating the reactor relevance of optimized stellarators. The main objectives are achieving plasma discharges as long as 30 min and heating power of 10 MW. The first plasma was produced in 2015; the road to steady-state operation consists of various Operating Phases (OP) to gradually increase the machine performance, carry out experiments and test new ideas. For the first experimental runs (OP1.1) [13] it had a simple setup of plasma facing components, using 5 inertially cooled discrete graphite inboard limiters. In OP1.2 [14] a more expanded set of PFCs, including a test divertor, which already had the exact same shape as the water cooled divertor presently being installed, were installed enabling a significant increase in plasma density and heating power. Thanks to the new set of PFCs, feedback controlled divertor gas fuelling capability and a surface conditioning [15], the circumstances were satisfactory to reach complete power detachment in the device [11].  
In order to facilitate smooth operation, W7-X is equipped with a wide variety of diagnostics. Video diagnostics, both infrared and visible, play a key role in the visualization of several different plasma phenomena and in the protection against dangerous plasma-wall interactions. The visible video diagnostics is seen in Figure 1, it is based on ten cameras in toroidally viewing \"oblique equatorial\" (AEQ) ports, covering 95% of the interior. Due to the thick cryostat vessel of the device, each camera is attached to the end of 2-meter-long, flexible tubes to be able to reach the inner end of the observational ports [5]. The so-called Event Detection Intelligent Cameras (EDICAMs) serve as the workhorse for the visible video diagnostics. The EDICAMs use a special 1.3 megapixel CMOS sensor with non-destructive read capability, enabling fast monitoring of smaller, predefined Regions of Interest (ROIs) in parallel to a normal, slow framing (100 Hz), full-frame overview [6]. The camera can perform simple operations on the ROI data in real-time, which can adapt and change the readout process and generate various output signals. The camera hardware has two main parts: the camera head, containing only the most necessary electronics including the sensor module, and the FPGA-based image processing and control unit (IPCU), mounted in a control PC as a PCIe extension card; these two components are connected via a 10 gigabit/s fiber link running through the previously mentioned flexible tube.  
Since the cameras are installed in a tangential observation geometry, they see the projected cross-sections of the 3-dimensional plasma. Under normal operational conditions the plasma emits visible radiation only at its edge, thus a slim radiation belt can be observed in the images. The shape of this radiation belt is not trivial: W7-X is based on a helias configuration, i.e., the plasma lacks axial symmetry; in other words, the poloidal cross-section of the plasma changes continuously when moving along the toroidal direction. Therefore, the observed radiation belt is the superposition of poloidal cross-sections at different toroidal locations within the camera's view range. Even so, the changes in the size of the plasma--or more precisely, the location of visible light emission--can be observed surprisingly well by looking at the radiation belt.