K. Inoue

N. Iwasawa

T. Kato

T. Takahashi

Y. Kondoh

National Institute for Fusion Science (NIFS-Repository)

§ 42. Single Particle Orbit Calculation ofNeutral Beam Injected Fast Ion in FRCsTakahashi, T., Inoue, K., Kato, T., Iwasawa, N.,Kondoh, Y. (Gunma Univ.)are injected at an angle of 19 degrees to the geometric axisin the same manner as the FIX device, the fast ions travellarge axial extent. Although this experimental setupsuppresses the wall orbit loss significantly, however, anadditional technique to enhance the effect of heating insidethe separatrix is necessary, such as a high fluxing of FRe.0.6 ,..--,...--....----......---.---r----,1) T. Asai et al., Phys. Plasmas 7, (2000) 2294.N[w/m)j4.0x 10"3.5 x lOll3.0xlOb2.5x 1062.0xlOIlJ.5 x 101>LOx 10"5.0 x lO5().l)xJ(}[)1.0z/zmir•• •0.5 •• ••,-.... •• •-.§. fa, •••2'0.4 •I .,•• •• ••• ••0.3 ,. .... It'-'0.2 "..···i··16:::t r·0.10.00.01.0100,....-, )(80 )( • 2 -1.........1 • f- A 4)(• 660A )()( 8+ 1040 +•At )(20 A )(A t)(0 J. A • i t0 2 4 6 8 10Fig. 2. The correlation of the magnetic moment between atthe first mirror reflection and at the third reflection.Fig. 3. The deposition power profile on the r-z plane (z > 0 ).The white solid line is the separatrix.o 0.1 0.2 0.3 0.4 0.5 0.6III / (q 211/J)/m)Fig. 1. The fractions of the confined fast ions to the ionized neutralbeam injected particles (10 keY) versus the number of theirmirroring motion.The Neutral Beam (NB) injection experiment has beencarried out at the FIX device in Osaka University. I)Improvement of confinement and electron heating areobserved so far. The objectives of this study are to discusshow the NB injected fast ion confinement property dependson the mirror ratio, what mechanisms cause the end andorbit loss on the wall, and where and how much the fast iondeposit its energy. Interaction between NB particles andplasma is taken into account by the Monte-Carlo method. Inprescribed FRC equilibria with a strong magnetic mirror,single particle orbits calculation starts where NB particlesare ionized. Calculation parameters are relevant to the NBexperiment on the FIX device. Figure 1 shows thedependences of the confined fast ion fractions to the totalionized NB particles a on the number of fast ions'mirroring motion. These fractions are calculated for variousmirror ratios, 2, 4, 6, 8, and 10; these are defined by the ratioof the fl ux function at the wall and mirror point to the one atthe wall and midplane1p(r =rw'z =Zmir)/1p(r =rw'z =0).For the case of the mirror ratio of 2, almost all of the fastions are lost from the mirror point at the first arrival at themirror point. A longer confinement and a gradual loss areobserved for an equilibrium with higher magnetic mirror. Iffast ions move regularly, then mirror reflected ions areconfined for infinitely long time; this conflicts with thecalculated results. Thus the fast ions are in non-adiabaticmotion that destroys the mirror confinement. In order toclarify the non-adiabaticity, the correlation of the magneticmoment between at the first and the third mirror reflection ispresented in Fig. 2. As all plotted points are scatteredrandomly, it is found that the correlation disappearscompletely. Taking into account the slowing down collisionbetween fast ions and plasma electrons, we also calculatethe local energy deposition by fast ions. It appears that thedominant deposition occurs outside the separatrix andbetween the X-point and the mirror end. The reasons of thisare as follows:1. Fast ions travel for a long duration around the mirrorreflection point.2. There exists a significant plasma density in the edgelayer of this equilibrium.3. Although fast ions move wider radial extent on themidplane, they are concentrated in the mirror region.For an effective plasma heating, the dominant depositionshould be located inside the separatrix. Since NB particles363

Single Particle Orbit Calculation of Neutral Beam Injected Fast Ion in FRCs

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