A. "Sanpei

C." Suzuki

H. Himura

M. Sugihara

S. Masamune

T. Akiyama

National Institute for Fusion Science (NIFS-Repository)

Accuracy enhancement of simple systemof millimeter-wave interferometer for lowdensity plasmasSanpei, A., Sugihara, M., Himura, H., Masamune,S., (Kyoto Inst. Tech.),Akiyama, T., Suzuki, C.Measurement of electron density proﬁle of the edgeand divertor plasma is very important issue for study offusion plasma physics. In 2008, we had developed a prot-type homodyne interferometer system to measure theelectron density distribution in the diverter region. Thehomodyne interferometer system is simpler than stan-dard heterodyne systems and can be developed at a rea-sonably low cost. One of the important issues of thisstudy is the development of multi channel measurementsystem, which will be applied for low density regime suchas diverter plasma of LHD. Developed a prot-type inter-ferometer system was applied for a low aspect ratio (A)reversed ﬁeld pinch (RFP) plasma, then obtained a pre-liminary experimental result. However, an analysis fromthe obtained experimental result is less-accurate becausefringe jump is more likely to occur with much number offringes. In order to accurize our system, in this study,we have adopted a cross detector which simultaneouslydetects reference and transmitted waves from two ports.The use of cross detector has lead to improved accuracyin the phase shift measurement. All data are obtainedfrom the low-A RFP device, REversed ﬁeld pinch of LowAspect eXperiment (RELAX)1, 2), which has low elec-tron density ∼ 1018−19.The schematic of developed system is shown inFig.1. Microwave frequency for the probing beam hasbeen carefully selected. Beam frequency must be muchhigher than the cut oﬀ frequency of the plasmas atthe center since refraction by a plasma density gradi-ent may become a serious problem at low frequencies.In this study, we choose 60 GHz microwave oscillator3).The microwave oscillator is IMPATT diode (HUGES,47174H-1280), which has maximum power 25.4 dBm,and a cross detector which simultaneously detects ref-erence and transmitted waves from two ports. In orderto protect the oscillator from reﬂected wave, an isolatoris inserted on circuit. A directional coupler (HUGES,45324H-1210) splits the microwave to probe and refer-ence signals. This system is simple and the use of crossdetector has lead to improved accuracy in the phase shiftmeasurement.Here, we show a experimental result. The exper-iment is carried out the typical discharge of the low-ARFP device RELAX. Figure 2 shows the time evolutionof (a) the plasma current Ip and (b) the line averagedelectron density ne measured using developed interfer-ometer in typical RELAX plasmas. The density evolvesand reaches a maximum value in the current rising phase.Fig. 1: Schematic of the developed interferometer sys-tem.After that, the density decays to a steady state value inthe current ﬂat-top phase. The rapid decay of the den-sity is called density pump out, which results from imbal-ance of suﬃcient ionization and insuﬃcient conﬁnementcaused by insuﬃcient formation of the RFP conﬁgura-tion. It is typical density behavior in RFPs, and we haveobserved the similar density behavior in RELAX.Fig. 2: Time evolution of (a) the plasma current Ip and(b) the line averaged electron density ne measured usingdeveloped interferometer in typical RELAX plasmas.The newly developed 60 GHz homodyne interferom-eter, characterized by simplicity and low cost, has beenshown to be applicable to RELAX particularly in the lowdensity regime.1) S. Masamune et al., Trans. of Fusion Sci. Technol. 51(No. 2T) (2007) 197.2) S. Masamune et al., J. Phys. Soc. Jpn. 76 (No.12)(2007) 123501.3) I. H. Hutchinson, Principle of Plasma Diagnostics(2002).189§40. c  E nt  S  S  f Milli t  I t f t r f  Low Density PlasmasSanpei, A., Sugihara, M., Himura, H., Masamune, S. (Kyoto Inst. Tech.),  Akiyama, T., Suzuki, C.

Accuracy Enhancement of Simple System of Millimeter-wave Interferometer for Low Density Plasmas

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