The Development Of Plasma Electron Density Diagnostic System On J-TEXT

The J-TEXT tokamak (Joint Texas Experimental Tokamak,formerly named TEXT-Utokamak in University of Texas at Austin)facility has been constructed at HuazhongUniversity of Science and Technology since the end of 2004.This machine has a majorradius R of 105 cm and a minor radius of 27 cm.The J-TEXT tokamak can reach thetypical plasma current I_p=350 kA at a centerline magnetic field where B_t=3 T,with theline averaged electron density n_e=3×10¹⁹m^-3.After the J-TEXT reconstruction was finished,achieving the first plasma has become topone task.Thus,we need to monitor the process of hydrogen gas being ionized and theplasma being generated.Therefore,a H_αemission detection system was first planed todevelop on J-TEXT facility,meanwhile,in order to monitor the visible emission during thewhole plasma discharge process,a CCD plasma visible light imaging system wasdeveloped on J-TEXT.After the achievement of first plasma,the system of diagnosticsrelated to basic parameters of plasma need to be built to get the basic characteristics ofplasma.Moreover,the electron density is an essential parameter in tokamak plasma physicsresearch,and millimeter-wave or far infrared laser interferometer was widely used tomeasure the electron density.Comparing to the far-infrared laser interferometer,theMillimeter-wave interferometer is simple and low cost,so microwave interferometer wasplaned to build in the early stage of the J-TEXT operation.After that,a multi-channelfar-infrared laser interferometer will be installed for the measurement of electron densityprofiles.This thesis was written on the basis of these four diagnostic systems.All the diagnostic systems described in this thesis are related to electromagnetic wavemeasurement,which can be classified into two categories.One is the measurement of theelectromagnetic emission and it is passive diagnostic,the H_αline emission detection systemand CCD imaging system belong to this category,the other is active measurement.In thismethod,electromagnetic wave is served as a probe to interact with the plasma.And fromthe change of the electromagnetic wave,some information of the plasma can be concluded.The microwave or the far infrared laser interferometer works in this manner. A H_αline-emission detection system is developed on J-TEXT,which is used todetermine the H_αemission level during the gas breakdown and hereafter to control thestartup of plasma current,i.e.the H_αemission signal was used to determine whether andwhen to fire the Ohmic heating capacitor banks,which are applied to drive the plasmacurrent ramp-up.The experimental results confirm that the H_αemission criterion isacceptable for controlling the plasma current start-up in the J-TEXT tokamak.Furthermore,the system was configured as a multi-channel system to guarantee the supply of the H_αsignal and to study the poloidal asymmetry of H_αemission.A plasma visible light imaging system has been installed on the J-TEXT tokamak.The tangential viewing scope of the system can cover the whole cross section.Theresolution of the color digital CCD camera is 659×494,with maximum frame rate of 110f/s.The system provides two capturing options:black and white or color.The system canmonitor the whole plasma discharge process of the visible emission in real time.A 2 mm microwave interferometer is designed to measure plasma electron density forthe J-TEXT tokamak.The system is configured as a frequency-modulated interferometerand views the plasma vertically through a set of large diagnostic ports.The probing waveand the reference wave recombine in a three-ending mixer.The resulting IF amplified andthen filtered by a band-pass filter.Finally,the sawtooth wave signal and the processed oneare transferred to the phase comparator.The output is proportional to the line-averageddensity.A multi-channel far-infrared (FIR) HCN laser interferometer was developed tomeasure plasma electron density profile on the J-TEXT.Phase modulation was achieved bya rotating grating,with a modulation frequency of 10 KHz,corresponding to the systemwith temporal resolution 0.1 ms.The beat signal was detected by TGS detector.Phase shiftbrought by the plasma was measured by the phase comparator with phase detectionsensitivity was 0.06 fringe.The phase shift signals were collected by the data acquisitionsystem and processed immediately by a program,and thus we can get the electron densityinformation in real time.