Group NBI

K. Ikeda

Ikeda,  K.

NBI, Group

NIFS-Repository (National Institute for Fusion Science)

We have reported a spectrally selective imagingsystem for a negative hydrogen ion source in the lastNIFS annual report1). This system have been performedwell to understand distribution of Hα emissions whichis caused by the variation of excited hydrogen popula-tion. It is low electron temperature (∼1 eV) near theplasma grid surface where is the birth area of the hy-drogen negative ions. Main excitation mechanisms forHα emission are dissociative recombination between anelectron and H+2 , and the mutual neutralization betweenH+ and H− in this area. We have also observed theionic plasma condition which maintained by H+ and H−ions. As the percentage of negative ions is increased, Hαemission caused by the mutual neutralization process be-comes dominant. Figure 1 shows the typical waveform ofa hydrogen negative ion density (nH−) and a Hα emis-sion intensity in the extraction region. These signals de-creased by extraction voltage applied between the plasmagrid and the extraction grid. We have deﬁned the re-duction vale of ∆Hα and ∆nH− subtracting the valueacquired before beam extraction from during beam ex-traction as shown in Figure 1. Figure 2(a) shows a plotnH- (1017 m-3 )00.51.01.5-10 -8 -6 -4 -2 0 2Time (s)0123nH- Hα NBI-TS #79625with Cs(1000 min after)H– ExtractionHα x1020  (photons/m2 nm s)ΔHα ΔnH– Fig. 1: Typical example of the waveform of H− densityand Hα intensity in 38kW arc discharge after 1000 minCs conditioning. Both signals decreased by the extrac-tion voltage during 1 sec.of the extraction current (Iex), the absolute value of thereduction H− density (|∆nH− |) and the reduction in theHα intensity (|∆Hα|) against the absolute value of theextraction voltage (|Vex|). The extracted current densityfor Iex is reached 10 mA/cm2 at |Vex|= 8 kV, here the Iexis the drain current (i.e. mixed H− ions and electrons) inthe circuit between the EG and a direct current extrac-tion voltage power supply; it is linearly depended on theVex. The reduction density of |∆nH− | observed at 2 mmfrom the PG surface also depend linearly on the extrac-tion voltage. As the extraction current is 10 mA/cm2,the |∆nH− | is 3.4 × 1016 m−3. The current density Jis the product of the charged particle density n and thedrift velocity v. The drift velocity v = J/n = 1.85× 104m/s when we assume the extraction current is carriedby |∆nH− |. The ﬂow energy of the negative ion is es-timated as 1.8 eV that is nearly value to the electiontemperature in the extraction region of this source. Wealso found the linear increase on the reduction value for|∆Hα| measured at z = 4 mm, where is as close as pos-sible to the position of H− measurement. The lineardependence is appeared between |∆Hα| and |∆nH− | asshown in Figure 2(b). This result indicates the reductionin the intensity of Hα is the result from a reduction in theH− ion density by way of mutual neutralization processin ionic plasma with rich H− ions after Cs conditioning.1234560|ΔnH-| (x1016m-3 )  1086420| Vex | (kV)012345I ex (A)IexIex = 10 mA/cm2Parc: 38 kWGas: 0.2 PaBias: 0.2 VH dischargewith Cs(a)(b)02004006008001000| ΔHα | (a.u.)|ΔHα | (z = 4mm)|ΔHα |  vs |ΔnH-| | ΔHα | (a.u.)432 1 0|ΔnH-| (x1016m-3)10008006004002000|ΔnH-|  (z = 2mm)Fig. 2: (a) Extraction drain current Iex and the absolutevalue of the ∆Hα and ∆nH− linearly increase by thestrength of applied extraction voltage. The line for 10mA/cm2 is represented. (b) There is a linear relationshipbetween the |∆Hα| and the |∆nH− | in the optimal Cscondition.1) IKEDA, Katsunori “Observation of Reduction Struc-ture of H− Ions at Extraction Region in a HydrogenNegative Ion Source”, Annual Report of National In-stitute for Fusion Science April 2012 - March 2013,(2013), pp200.216§7. Comparison of a Reduction Value between H– Density and Ha Emissions at Extraction Region in a Negative Ion SourceIkeda, K., NBI Group

Comparison of a Reduction Value between H? Density and Ha Emissions at Extraction Region in a Negative Ion Source

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