Time-of-flight refractometry for robust line integral electron density measurements and control in ITER

Abstract

A new method of electron density measurements in magnetic confinement plasmas—time-of-flight refractometry (TFR)—is proposed for application in the International Thermonuclear Experimental Reactor (ITER) in order to get robust line integral electron density information. The method provides unambiguous density measurements directly from the measurements of time delay of microwave pulses propagating through the plasma. Generally, both O-and-X probing modes are available for use in the measurements. At ITER conditions (BT∼5 T), X-mode probing in the equatorial plane seems to be attractive for application due to transparency of ITER plasma for electromagnetic emission between lower and upper cutoffs at approximately 60–105 GHz frequency band. When these relatively low frequencies are used for measurements, both electron cyclotron resonance absorption and first mirror problems should be avoided. Also, relatively simple implementation of the access to plasma and comparatively low sensitivity to vibrations and misalignment are expected. General approach to TFR is given in the article. The choice of probing frequencies of TFR for ITER has been conducted. Estimations of expected time delays for different density profiles have been performed. Parameters of TFR prototype used on the T-11M tokamak and results obtained are discussed. A conclusion of sufficiency of recently attained accuracy of the measurements for TFR application on large tokamaks has been done. Some ways of improving the measurement accuracy have been proposed and tested. Advantages and problems of TFR application are discussed.