Synchrotron emission

Runaway electrons can be confined long enough in present day tokamak discharges to gain energies of the order of several tens of MeVs. At these energies they emit synchrotron radiation in the (near) infrared wavelength range, which can easily be detected by thermographic cameras. On TEXTOR a unique diagnostic exploiting this synchrotron radiation has been developed. It consists of an infrared camera, which views the plasma tangentially towards the direction of electron approach. This camera, an Inframetrics 760 BB, (upgraded to 12 bit), includes a liquid nitrogen cooled HgCdTe detector, focusing lens and two scanning mirrors, one horizontal and one vertical. With this camera a 2D-TV picture is generated according to the NTSC standard. One line is scanned in 65 ms, a complete picture consists of 256 lines and is scanned in 16.7 ms. One complete frame contains, therefore information on space and time simultaneously. The detector is sensitive in the wavelength range of range 3-12 micron, but since CaF2 optics is used, this is effectively from 3-8 micron.

The diagnostic provides a direct image of the runaway beam inside the plasma. From the spectral features the runaway energy can be obtained, the intensity of the radiation is a measure of the number of runaway electrons, and the synchrotron spot carries information on their perpendicular momentum and spatial distribution. Figure 1 gives an example of such synchrotron measurement at TEXTOR in the wavelength range 3-8 micron where the emission reaches its maximum for 20-30 MeV electrons.

Figure 1: Tangential view into a low density TEXTOR-94 discharge with a thermographic camera looking into the direction of electron approach. In frame A recorded at t=0.5 s no synchrotron radiation is observable and only the wall structure can be recognized. In frame B, recorded at t=1.5s, the synchrotron radiation starts to develop and in frame C the full extent of the spot is visible from which the size of the runaway beam can be determined.

With this diagnostic several interesting results have been reported:

• In steady state discharge conditions an exponential increase in the synchrotron radiation signal is observed. This was the first evidence of the so-called secondary generation mechanism. This is the process in which existing runaway electrons kick thermal electrons into the runaway regime by close Coulomb collisions. This mechanism is predicted theoretically and is thought to dominate the runaway production during disruptions in large tokamaks. Disruption generated runaway electrons in TEXTOR have also been identified with this synchrotron radiation diagnostic.
   
•  In discharges with MHD activity observations have been made of a fast loss of runaway electrons in the stochastic region outside magnetic islands, whereas within the islands the runaway electrons are almost perfectly confined. These snake-like structures of the runaway beam have been observed at q=1 and q=2.
   
• Relativistic runaway electrons can be used to probe the scale size of magnetic turbulence in the core of the plasma. These experiments are done in low-density discharges, heated additionally by neutral beam injection. A decay of the runaway population after switch-on of the beam indicated the loss of these electrons due to increased magnetic turbulence. However, a noticeable delay (up to 1 s) in the response of the synchrotron signals showed that the loss of runaway electrons occurred at energies well below the energy range of the diagnostic (20-30 MeV). The critical energy for loss of runaway electrons depends on heating power, and is related to the scale size of the magnetic turbulence. From these observations it was derived that the radial correlation length of magnetic turbulence in the core of the plasma is 0.2 cm in Ohmic discharges, increasing to several cm with 0.6 MW heating.