Stellarator Plasma Physics
Dr. Maciej Krychowiak
Max Planck Institute for Plasma Physics, Greifswald, GermanyA
Background
Dr. Maciej Krychowiak is a physicist in the Max Planck Institute for Plasma Physics and works with Wendelstein 7-X (W7-X), the world’s largest superconducting stellarator. Dr. Krychowiak is using spectroscopy to study the edge layer of plasma in W7-X, and evaluate the possibility of a future fusion power plant.
Dr. Krychowiak explains more about his work with W7-X, “The goal in fusion research is to confine a fusion plasma in a magnetic cage within a vacuum chamber, and to heat it to a high temperature, around 100 million degrees. At these temperatures heavy hydrogen isotopes, deuterium and tritium, collide at high speeds and fuse, producing helium nuclei and a lot of energy that can be harnessed. This is similar to what occurs in the centres of stars.”
“There are two major approaches to confine the plasma within a magnetic field, the stellarator and the tokamak. Stellarators are more complex to build than tokamaks as the magnetic field coils are not flat, but are in a coiled 3D shape. There are currently some smaller stellarators with diameters up to 5 m, but W7-X is currently the only large stellarator with an 11 m diameter.”
“It is vital to understand the complex behaviours of the plasma in W7-X at different coil currents and with this at different magnetic field configurations, to get the plasma in the centre as hot and dense as possible. We use spectroscopy to observe the edge of the plasma at the divertor, where the plasma impacts with the wall of the stellarator. We have theoretical models, and we need spectroscopic measurements to validate them and maintain a hot plasma in the centre.”
Figure 1: Diagram describing the geometry of the spectroscopic observation of edge plasma at the divertor of Wendelstein 7-X stellarator. There are lines of sight (LOS) from above (port AEF) and from the side (port AEI) used for tomographic reconstruction of e.g. CII and CIII radiation. The 2D radiation image shows an example simulation result of plasma emission zone close to the horizontal divertor.
Figure 2: Tomographic reconstruction of measured CII and CIII radiation in the so-called detached plasma condition in which the edge plasma radiates excessive energy at a certain distance to the divertor (black lines) reducing the heat fluxes to the wall components. The red contour lines depict the structure of the magnetic field in the plasma edge. The yellow polygon encloses the region of crossing lines-of-sight in which the tomograms of the plasma emission are created.
Challenge
The edge layer of plasma in a stellarator is a critical region to study, but conditions in this zone fluctuate rapidly and unpredictably, requiring precise diagnostic tools to capture the full range of phenomena occurring within. Dr. Krychowiak outlines the challenges he faces, “The plasma is complex, and modelling the plasma is very difficult. We have to prove through experiments whether the models are correct or not, so high optical resolution spectroscopic measurements are very important.”
“When hot plasma hits the walls, it releases impurity atoms which can contaminate the plasma centre. This effect can be reduced by deliberate increase of edge plasma radiation to reduce the plasma temperate in the edge and by this to minimise the plasma heat influx to the wall. We need sensitive spectrometers to study the plasma radiation in this edge layer at the wall… we have in total 324 fibre optics behind the walls, and we are trying to image the plasma from there using visible and UV light. We have many of these sightlines into the edge plasma but they only overlap in a small area and they don’t align perfectly due to geometric limitations.”
“We need to split the light into wavelengths onto a camera chip in a horizontal line, and arrange the fibres at the slit of the spectrometer. For this we need spectrometers with good optical quality, and need 7 or 8 such systems.”
Spectroscopy and camera equipment needs to operate in this high-energy environment, where the speed of reactions and light emissions from the plasma are difficult to capture with conventional instruments. Dr. Krychowiak requires multiple high-performance spectrometer and camera systems.
The optical imaging of the IsoPlanes and ProEMs is very good, and we have great spectral resolution. The grating options enable us also to capture overview spectra which is very useful and gives us high flexibility.
Dr. Maciej Krychowiak
Solution
To overcome the challenges of this complex, cutting-edge application, Dr. Krychowiak’s team have integrated over 10 IsoPlane 160/320/320A spectrometers and ProEM 1024 EMCCD cameras into their diagnostic suite, “We have quite a lot, there are four IP320 and upgraded IP320A spectrometers with cameras, and through a new tender in 2021 we ordered four more cameras and four spectrometers, as well as two more spectrometers for cameras we already had.”
The Isoplane-320/320A spectrometers are the best imaging spectrometers in their class, featuring superior astigmatism-free optical design and capturing high resolution data from each of the 324 fibres simultaneously. Their low optical aberrations and precise imaging capabilities allowed Dr. Krychowiak to measure the plasma with exceptional accuracy. “With the IsoPlanes we can detect among others hydrogen, carbon, nitrogen, neon, argon and their different ionization states, the good spectral resolution is very important. We also use holographic gratings and can separate lines that are often very close to each other, allowing us to map 54 lines of sight on a single camera chip, which is very useful. Everything on one device, all remotely controlled.”
Paired with these spectrometers, Dr. Krychowiak utilizes Pro-EM HS 1024 EMCCD cameras. Due to their high quantum efficiency and low effective read noise, ProEM is an ideal detector to capture the faint emission lines emitted by the plasma. “We use the ProEM 1024 x 1024 EMCCD cameras with a special coating that allows for the highest possible quantum efficiency. These cameras are fast but more important is the high sensitivity and the low noise readout, because our spectral lines are weak. We also start from 360 nm and go up to 915 nm, so we can detect over a wide spectral range.”
This combination of advanced spectrometers and cameras provides Dr. Krychowiak’s team with a comprehensive solution for observing and analyzing the plasma’s behavior in real time, helping them gain valuable insights into how to optimize the performance of Wendelstein 7-X and contributing to the ongoing development of fusion energy.
Reference
Krychowiak, M., König, R., Henke, F., Barbui, T., Flom, E., Kwak, S., et al. (2021). Gaussian Process Tomography of carbon radiation in the transition to detached plasmas in the Wendelstein 7-X stellarator. In G. Giruzzi, C. Arnas, D. Borba, A. Gopla, S. Lebedev, & M. Mantsinen (Eds.), 47th EPS Conference on Plasma Physics. Geneva: European Physical Society.
