Soft x-ray and bolometric tomography
The measurement of the emitted electromagnetic radiation gives a variety of important physical information, for example on plasma equilibrium and stability and its associated MHD dynamics, on the energy balance and confinement properties, on the impurity content and radiation losses and on plasma-wall interaction. Tomography is a widely used technique for recostructin with good spatial accuracy and high time resolution the radiation emissivity distribution. With tomography the plasma is viewed along a large number of collimated chords using arrays of detectors. From the measurement of the mean emissivity performed by each detector along its line of sight the emissivity spatial distribution is obtained through a tomographic reconstruction technique which was firstly used in medical sciences.
The seven cameras view the plasma through seven individual portholes open in the same poloidal cross-section of the vacuum chamber. They are inserted to their operational positions by means of seven almost identical remotely controlled bellows drive systems directly connected to the high vacuum environment of the RFX discharge chamber (base pressure<10-8 mbar). The systems are anchored to the RFX machine supporting structure through specially designed beams.
The seven cameras are equipped partly by Si SXR detectors (3 cameras) and partly by low noise metal film miniaturized bolometers (3 cameras). Two versions of the seventh camera and of its insertion system, located on the equatorial plane, will be realized. They can easily be interchanged in order to have available additional SXR or bolometric detectors. Totally 98 lines of sight for SXR tomography and 48 for bolometric tomography are available, the two imaging techniques being performed simultaneously.
Since the RFX first wall can be baked up to 350°C during plasma operation the detector heads are cooled. The coolant flows inside a coiled pipe realized inside the cameras. The head temperature is continuously monitored and a feedback controlled unit allows the operation of detectors at steady temperature as low as 10°C even with the machine first wall baked at 350°C. Since the fluid conditioning unit has the capability to operate both as a refrigerator and as a heater, if necessary the detector heads can be baked up to 90ºC.
In the version presently used on RFX the electronics for the bolometric signal conditioning has a bandwidth limited to 4 kHz, whereas in the new version for the tomography the bandwidth will be 10 kHz. The detector in itself could guarantee even better time resolution provided an appropriate data analysis technique is used as described in ref. . In the SXR tomography the bandwidth of the detection system is mostly limited by the capacitance of the cable which drive the signal from the detctors up to the first amplification stage. Depending on the overall amplification the bandwidth ranges from approximately 20 to 150 kHz.
Considering the large amount of experimental data and the need of performing on-line tomographic reconstruction VME standard has been chosen for data acquisition to allow local on-board elaboration of the signals.