
Citation: | Mingxiang LU, Le HAN, Qi ZHAO, Juncheng QIU, Jianhong ZHOU, Dinghua HU, Nanyu MOU, Xuemei CHEN. Microchannel cooling technique for dissipating high heat flux on W/Cu flat-type mock-up for EAST divertor[J]. Plasma Science and Technology, 2022, 24(9): 095602. DOI: 10.1088/2058-6272/ac684c |
As an important component of tokamaks, the divertor is mainly responsible for extracting heat and helium ash, and the targets of the divertor need to withstand high heat flux of 10 MW m-2 for steady-state operation. In this study, we proposed a new strategy, using microchannel cooling technology to remove high heat load on the targets of the divertor. The results demonstrated that the microchannel-based W/Cu flat-type mock-up successfully withstood the thermal fatigue test of 1000 cycles at 10 MW m-2 with cooling water of 26 l min-1, 30 ℃ (inlet), 0.8 MPa (inlet), 15 s power on and 15 s dwell time; the maximum temperature on the heat-loaded surface (W surface) of the mock-up was 493 ℃, which is much lower than the recrystallization temperature of W (1200 ℃). Moreover, no occurrence of macrocrack and 'hot spot' at the W surface, as well as no detachment of W/Cu tiles were observed during the thermal fatigue testing. These results indicate that microchannel cooling technology is an efficient method for removing the heat load of the divertor at a low flow rate. The present study offers a promising solution to replace the monoblock design for the EAST divertor.
The influence of the isotope mass on plasma confinement properties has been a longstanding issue in magnetically confined plasmas. According to gyro-Bohm scaling [1, 2], the plasma transport diffusivity scales as
The first experimental observation in the TEXTOR tokamak has demonstrated that the amplitude of geodesic acoustic mode (GAM) zonal flows substantially increases during the transition from H to D dominated plasmas [9], which is qualitatively consistent with the GKV simulation [17]. However, up to date, there has been little evidence showing the increase of nonlinear interplay between ambient turbulence and zonal flows in heavier mass isotope plasmas.
In this paper, we present direct experimental evidence to expose the fact that in D majority plasmas, the nonlinear energy transfer plays a dominant role in exciting larger GAM zonal flows by extracting more energy from ambient turbulence, which results in lower turbulent transport and better confinement compared to H majority plasmas. The results provide additional proof for understanding the isotope effects in fusion plasmas.
The experiments were carried out in the Ohmically-heated H and D plasmas at the HL-2A tokamak with a limiter configuration. The major and minor radii are R = 165 cm and a = 40 cm, respectively, line-averaged plasma density
Typical discharge waveforms in H and D dominant plasmas are plotted in figure 1. It can be seen that with the same plasma current, loop voltage and line-averaged density, the plasma stored energy in D majority plasmas is higher than that in H ones, similar to those observed in other devices [4, 5, 7, 8, 10–12, 16]. Figure 1(e) shows time traces of the reciprocating Langmuir probes plunged in the stationary phase of the H and D discharges. The probe measurement in both discharges passed through approximately the same radial locations. It has been confirmed that the plasma horizontal position kept almost unchanged as the probes plunged into the plasma. Figure 2 depicts the edge equilibrium profiles averaged in several similar H (black) and D (red) majority discharges. Here, the ∆r = 0 denotes the LCFS location. As shown in figures 2(a) and (b), the local ne and Te in the edge region (inside the LCFS, ∆r < 0) are both enhanced in D plasmas, suggesting a higher edge pressure gradient, in agreement with higher stored energy achieved in the D plasma (see figure 1(d)). Inside the LCFS, both Vf and Vp are substantially lower in D plasmas than in H ones, leading to a slightly deeper Er well and larger Er shear (
In order to gain an insight into the isotope effects on turbulence levels and associated transport, we compared the RMS values of density, electron temperature and radial velocity fluctuations,
To understand the mechanisms responsible for the reduction of turbulent transport in the isotope deuterium plasmas, we analyzed the spectrum characteristics of edge turbulence and zonal flows in H and D plasmas. In HL-2A, a low frequency coherent mode with long-range toroidal correlations, namely GAM zonal flows, has been routinely observed in floating potential signals [32, 33]. Theories predict that the GAM zonal flow has poloidally symmetric (m/n = 0/0) potential and asymmetric (m/n = 1/0) density perturbations [34, 35]. In the present experiments, it is found that the maximum GAM amplitude presents at ∆r ≈ -3 cm inside the LCFS [36]. Plotted in figure 4 are the frequency spectra of floating potential (
According to theories [34, 35], the GAM frequency
To further compare the strength of the nonlinear coupling in turbulence between H and D plasmas, the squared auto-bicoherence [38], defined as b2(f1, f2) = |B(f1, f2)|2/(〈|X(f1)X(f2)|2〉〈|X(f1 ± f2)|2〉), where B(f1, f2) = 〈X(f1)X(f2)X*(f1 ± f2)〉 and X(f) is the Fourier transform of the fluctuation signal x(t), has been computed. Figures 6(a) and (b) show contour-plots of the squared auto-bicoherence estimated by floating potential fluctuation signals measured at ∆r ≈ - 3 cm in D and H dominant plasmas. It is clear that the bicoherence along the f2 = ±10 kHz and f2 = - f1 ± 10 kHz lines are much more prominent in D plasmas than that in H ones. This comparison illustrates stronger nonlinear coupling occurred between GAM zonal flows and turbulence in D plasmas. The summation of the squared bicoherence is calculated as
In summary, the isotope effects on plasma confinement, edge turbulence and turbulent transport as well as GAM zonal flows have been studied using a two-step Langmuir probe array in H and D majority plasmas in the HL-2A tokamak. Evidence shows that under similar discharge parameters the D plasma has better confinement and lower turbulent transport than the H plasma. Meanwhile, it is observed that the magnitude of GAM zonal flows, the tilting angle of the Reynolds stress tensor, and the turbulence correlation lengths are all larger in the edge region of the D plasma. The results provide direct experimental proof on the importance of the nonlinear energy transfer between turbulence and zonal flows for governing the isotope effects in fusion plasmas.
The authors acknowledge financial support from the National MCF Energy R & D Program (No. 2018YFE0312300), National Natural Science Foundation of China (No. 51706100), the Natural Science Foundation of Jiangsu Province (No. BK20180477) and Fundamental Research Funds for the Central Universities (No. 30918011205).
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