|UC Davis PDG: ECE Imaging Diagnostics|
Electrons in an external magnetic field will gyrate around magnetic field lines. This gyromotion gives rise to electron cyclotron emission (ECE) at harmonics of the electron cyclotron frequency, which is proportional to the local magnetic field. If the electrons are sufficiently hot and sufficiently dense, then the plasma is considered optically thick. Under these conditions, the electron cyclotron emission (ECE) is directly proportional to the electron temperature and independent of all other plasma parameters.
In tokamak plasmas, the toroidal magnetic field decreases monotonically with radius. As the electron cyclotron frequency is proportional to the total magnetic field, the emissions at a given frequency are emitted from a very specific layer of the plasma corresponding to a given magnetic field. Measuring the ECE power as a function of frequency allows the electron temperature to be computed as a function of plasma radius. Conventional ECE radiometry employs a single wideband receiver to resolve multifrequency ECE into a spatially varying temperature profile, and has been a standard diagnostic for magnetic fusion plasmas for over thirty years. Such systems, however, are generally limited to one dimensional horizontal measurements along the major radius and often feature a relatively poor poloidal spatial resolution. Spatially imaging the emission onto a vertically-aligned array of detectors expands the capabilities of ECE radiometry to two dimensions and is referred to as ECE imaging.
A new generation of ECE Imaging diagnostics is now under development, initially for the TEXTOR tokamak. The approach taken is to collect broad bandwidth radiation on each element of the imaging antenna array, which is subsequently separated by frequency band. This, in essence, attaches a broadband multifrequency heterodyne radiometer electronics to each element of the one dimensional ECEI array. Using a 16 element array, and an 8 band receiver attached to each array element, time-resolved 16×8 images of electron temperature profiles and fluctuations of the TEXTOR plasma are acquired with vertical and horizontal channel spacings of 11 mm and 8 mm, and spot sizes of 13 mm and 9 mm, respectively. Similar systems have been proposed for a variety of different toroidal plasmas, including a number of non-tokamak plasma devices such as the Large Helical Device (LHD).
The unique features of the ECE Imaging diagnostics derive from the use of wideband, low cost Schottky diode mixer arrays coupled with innovative, low cost electronics. Follow the links below to learn more about both the technology employed in ECE Imaging.
Imaging array design and fabrication
2-D ECE Imaging Electronics
Quasi-Optical Notch Filters
UC Davis has fabricated and installed multichannel ECE Imaging systems on a number of fusion plasma tokamaks across the world. Follow the links below for a description of the systems involved, and to sample data collected with these systems.
ECE Imaging on the TEXTOR tokamak in Germany
ECE Imaging on the KSTAR tokamak in Korea
ECE Imaging on the RTP tokamak in the Netherlands (Reference Only)
ECE Imaging on the TEXT-U tokamak in the U.S.A. (Reference Only)