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TJ-II:Electron Cyclotron Emission: Difference between revisions
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Electron | [[File:TJ-II_ECE_Antenna.png|315px|thumb|right|Diagram showing the position of the ECE antenna-mirror system at TJ-II.]] | ||
Electron temperature profiles are measured at [[TJ-II]] | |||
< | by means of a 16 channel heterodyne radiometer, | ||
<ref>[ | covering the frequency range 50–60 GHz, corresponding to the second harmonic of [[:Wikipedia:Electron cyclotron resonance|electron cyclotron]] emission (ECE) in X-mode polarization at a magnetic field of 0.95 T on the plasma axis. | ||
The sampling rate is 100 kHz. | The measurements are performed from the low field side (LFS) in the horizontal midplane (between [[TJ-II:Sectors|sectors]] C4 and C5, φ = 315°), but most of the channels receive radiation from the high field side (HFS). In normal operation conditions, the frequency depends on a known way on the magnitude of the magnetic field B. Tuning properly the receiver system, each frequency corresponds to a different value of the major radius ''R'' (according to ''B(R)''). | ||
The signals | The system is operated close to the strong [[TJ-II:Electron Cyclotron Resonant Heating|ECR heating source]] (f<sub>ECRH</sub> = 53.2 GHz). | ||
To protect the radiometer against stray radiation from the gyrotron, the radiometer band is split into two parts. | |||
The second harmonic emission above and below 53.2 GHz are measured separately by means of microwave couplers in the signal path. | |||
== Calibration == | |||
The system is calibrated absolutely by comparing room temperature with liquid nitrogen temperature. | |||
<ref name="Luna">E. de la Luna, J. Sánchez, V. Tribaldos, and T. Estrada, ''Multichannel electron cyclotron emission radiometry in TJ-II stellarator'', [[doi:10.1063/1.1315636|Rev. Sci. Instrum. '''72''', 379 (2001)]]</ref> | |||
<ref>E. de la Luna et al, ''Electron cyclotron emission measurements on TJ-II stellarator plasmas'', [[doi:10.1016/S0920-3796(00)00492-0|Fusion Engineering and Design '''53''', Issues 1-4 (2001) 147-151]]</ref> | |||
The optical system and the transmission line of the ECE diagnostic were designed to allow the calibration to be performed outside the vacuum vessel keeping the arrangement of the diagnostic. | |||
To calibrate, the transmission line is opened close to the diagnostic port (C5-bottom). Then the port flange, which holds the optical system and the wave-guide up to that point, is extracted from the torus and assembled with the same alignment outside the vacuum vessel. | |||
A check for the radiometer stability is performed periodically by using a stable noise source at the input of the radiometer. Such a noise source is also used to test the linearity of the system and to calibrate any possible change that may occur in the electronics of the diagnostic. | |||
== Data analysis == | |||
The emission can be simulated by the [[TRECE]] ray tracing code. | |||
<ref name="Tribal">V. Tribaldos and B. P. van Milligen, ''Electron cyclotron emission calculations for TJ-II stellarator'', [[doi:10.1088/0029-5515/36/3/I02|Nucl. Fusion '''36''', 283 (1996)]]</ref> | |||
The local radiation temperature is assumed to be a function only of the local electron temperature at the resonant layer; however, if the plasma is not Maxwellian or if the plasma is optically thin, the measured radiation temperature is no longer equal to the electron temperature. | |||
The effect of polarization rotation can be neglected once the correct polarization for the pure X mode on-axis is chosen. Even at high density (worst condition) the radiation coming from the plasma bulk experiences a rotation below 5°.<ref name="Tribal" /> | |||
The spatial resolution is about 1 cm.<ref name="Luna" /> | |||
The typical sampling rate is 100 kHz. | |||
The raw signals in the [[TJ-II:Shot_database|TJ-II database]] are called 'ECE1' ... 'ECE16', and the processed (calibrated) signals 'TECE1_' ... 'TECE16_' (units: keV). | |||
== References == | == References == | ||
<references /> | <references /> | ||