@misc{oai:ir.soken.ac.jp:00002160,
author = {周, 航宇 and ツォウ, ハンギュウ and ZHOU, Hangyu},
month = {2016-02-17, 2016-02-17},
note = {Impurity control is significantly important to reduce substantial power loss and to achieve high performance plasmas in controlled nuclear fusion research. Effective ion charge, Zeff, is a key parameter to characterize the impurity contents in fusion plasmas andthe radial distribution gives clear information on the impurity transport. The Zeff value hasbeen observed using bremsstrahlung continuum in high-temperature plasmas. A Zeffdiagnostic system based on a visible spectrometer has been newly designed and constructedinstead of an old interference filter system in Large Helical Device (LHD) to eliminate lineemissions and to obtain Zeff in low-density plasmas. An astigmatism-corrected Czerny-Turner-type visible spectrometer coupled with a charge coupled device (CCD) has beeninstalled with a 44-channel optical fiber array to detect the visible bremsstrahlungcontinuum profile in LHD. The diagnostic system has been absolutely calibrated with astandard tungsten lamp and an integrated sphere to analyze the visible bremsstrahlungcontinuum at 53Onm quantitatively.
A full vertical bremsstrahlung profile (-0.6m≤Z≤0.6m) has been successfullyobserved for horizontally elongated plasma cross section in both discharges using H2 gas-puffing at normal density range (ne≤1014cm-3)and solid H2 multi-pellet injection atextremely high-density range (ne≥1014cm-3). The line emissions are entirely eliminatedfrom the signal due to the high spectral resolution with which the bremsstrahlungcontinuum can be clearly separated from spectral lines, whereas nonuniformbremsstrahlung emission originating in the ergodic layer is indentified in the radial profilefor the normal discharges with outwardly shifted magnetic axis configurations (Rax>3.60m).At present, the lower half of the vertical bremsstrahlung profile (-0.6m≤Z≤0m) isapplicable to the Zeffprofile analysis for the gas-puff discharges in inwardly shiftedmagnetic axis configurations (Rax≤3.6Om). On the contrast the vertical bremsstrahlungprofile measured from the pellet discharges is fully available for the Zeff profile analysis inall the magnetic axis configurations because the plasma edge boundary shrank in the high-density operation and the nonuniform bremsstrahlung emission disappeared.
Local bremsstrahlung emissivity profile has been derived from the verticalbremsstrahlung profile as a function of normalized minor radius, p, through Abel inversiontaking finite β-effect into account. Errors in the calculation due to the magnetic surfacedistortion have been examined with uncertainty of the edge plasma boundary. The resultindicates that determination of the normalized minor radius for each observation chordgives the largest influence on the error estimation rather than the chord lengthdetermination. Thus, the Zeff profile has been calculated from the radial bremsstrahlungemissivity profile by considering the temperature and density profiles in a wide densityrange of 3.0≤ne≤30×l013cm-3 and for a variety of density profiles. The Zeff valuesmeasured in the present system are checked by neutral-beam-heated discharges with H2 andC pellet injections, in which the Zeff values should be close to 1 and 6, respectively. Theresult shows a good agreement between the measurement and the estimation. The line-averaged Zeff value decreases from 3.8 to 1.4 as the density varies from 3 to 9×1013cm-3inthe gas-puff discharges with Rax:3.6m, while it keeps a constant value close to 1.0 fordensity range of 9 to 30×1013cm-3 in the pellet discharges with Rax3.85m. The Zeff profileshave been extensively analyzed in the LHD discharges. As a result, it is found that the Zeffprofiles becomes generally flat in helical plasmas even if the density profile is changed topeaked, flat or hollow one. It indicates that the impurity partial pressure is radially constantto the electron or ion pressure.
Total bremsstrahlung radiation has been studied from the visible bremsstrahlungprofile by integrating over the whole energy region and plasma volume. The totalbremsstrahlung radiation is correlated with several plasma parameters for the gas-puff andpellet discharges. It is found that the total bremsstrahlung radiation quickly increases inproportion to the square of the density for the pellet discharges, while it is roughly constantagainst the density in the gas-puff discharges. The total radiation loss is entirely dominatedby the total bremsstrahlung radiation in the high-density pellet discharges. The ratio of thetotal bremsstrahlung radiation to the total input power only ranges in 3-10% in the gas-puffdischarge. However, the ratio starts to increase with the density in the pellet discharges andreaches 40% at ne=30×1013cm-3. Flat Zeff profiles are also found not only in the gas-puffdischarges but also in the pellet discharges. It strongly suggests that the impurityaccumulation is not occurred in the pellet discharges of LHD.
The Zeff profile diagnostics based on the visible bremsstrahlung in LHD has oftenbecome difficult because of the presence of the nonuniform visible bremsstrahlungemissions originating in the edge ergodic layer. For solving this problem, an alternativeapproach to the Zeff profile diagnostics using a space-resolved flat-field extreme ultraviolet(EUV) spectrometer has been newly adopted to detect the bremsstrahlung continuum in theEUV wavelength range of 7.0-7.5nm. The EUV bremsstrahlung intensity profiles havebeen measured and checked for all the magnetic configurations in LHD. It is found that thenonuniform bremsstrahlung emission can be entirely eliminated by use of the EUVbremsstrahlung. As a result, the Zeff profile has been successfully measured for most ofdischarges regardless of magnetic field structures of the ergodic layer which can be usuallycontrolled by the magnetic axis shift in the range of 3.5≤Rax≤4.1m. The Zeff profilesmeasured in the EUV range are compared with those measured in the visible range forinwardly shifted contigurations (Rax≤3.60m).The result is in good agreement between thetwo Zeff profiles. It verifies that the use of the EUV bremsstrahlung continuum is a novelway for the Zeff diagnostics in toroidal plasmas with such nonuniform bremsstrahlungernissions at the plasima edge. Typical analysis on the EUV bremsstrahlung also shows afairly flat Zeff profile with error bars of ±14% in the outwardly shifted configuration ofRax=3.75m at which the Zeff profile diagnostics is not available from the visiblebremsstrahlung measurement., application/pdf, 総研大甲第1375号},
title = {A study of spatial structures of bremsstrahlung continuum and Zeff based on visible spectroscopy in LHD},
year = {}
}