Results 1 - 9 of 9
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[en] The effects of edge-localized modes (ELMs) on divertor particle and heat fluxes were investigated for the first time in the Experimental Advanced Superconducting Tokamak (EAST). The experiments were carried out with both double null and lower single null divertor configurations, and comparisons were made between the H-mode plasmas with lower hybrid current drive (LHCD) and those with combined ion cyclotron resonance heating (ICRH). The particle and heat flux profiles between and during ELMs were obtained from Langmuir triple-probe arrays embedded in the divertor target plates. And isolated ELMs were chosen for analysis in order to reduce the uncertainty resulting from the influence of fast electrons on Langmuir triple-probe evaluation during ELMs. The power deposition obtained from Langmuir triple probes was consistent with that from the divertor infra-red camera during an ELM-free period. It was demonstrated that ELM-induced radial transport predominantly originated from the low-field side region, in good agreement with the ballooning-like transport model and experimental results of other tokamaks. ELMs significantly enhanced the divertor particle and heat fluxes, without significantly broadening the SOL width and plasma-wetted area on the divertor target in both LHCD and LHCD + ICRH H-modes, thus posing a great challenge for the next-step high-power, long-pulse operation in EAST. Increasing the divertor-wetted area was also observed to reduce the peak heat flux and particle recycling at the divertor target, hence facilitating long-pulse H-mode operation. The particle and heat flux profiles during ELMs appeared to exhibit multiple peak structures, and were analysed in terms of the behaviour of ELM filaments and the flux tubes induced by modified magnetic topology during ELMs. (paper)
[en] The basic in–out divertor asymmetry with respect to ion B × ∇B direction has been examined in EAST by changing the divertor configuration from upper single null (USN) to lower single null (LSN) during the same discharge without reversing BT. It is remarkable that the in–out asymmetry is reversed when moving from USN to LSN. However, modeling with SOLPS, taking into account classical drifts, shows little difference. The divertor magnetic configuration also affects the access to H-modes, favoring DN or near-DN divertor configurations on EAST. ELMs further enhance the in–out divertor asymmetry, leading to greater particle and heat deposition on the outer target with a broader footprint, presumably arising from enhanced ELM transport in the outboard region
[en] The modelling studies of SOL-divertor plasmas in EAST tokamak with high heating and driving power have been carried out for the design of W/Cu divertor. The edge plasma code package SOLPS5.0(B2.5-Eirene) and Monte Carlo impurity transport code DIVIMP are used for the modelling. The plasma density at the core-SOL interface Nedge or the separatrix density nsep is varied extensively in order to survey the operation of the divertor, the profiles of the plasma parameters and heat fluxes at the target plates of divertor have been obtained. For nsep = 0.937 x 1019 1/m3, the heating and driving power Pin = 14 MW on the device produces the peak heat flux at the outer target plate Qt = 15.3 MW/m2 with the SN configuration and the traditional divertor (no gas or radiation target). The radiative divertor modelling with Ne puffing and the transport modelling of W impurity from the target plates of divertor are also performed.
[en] The first systematic assessment of divertor performance was made in the EAST superconducting tokamak for both single null (SN) and double-null (DN) divertor configurations under Ohmic and L-mode plasma conditions. Particle and heat fluxes are higher at outboard divertor targets than those at inboard divertor targets for both SN and DN. However, the DN divertor configuration exhibits a much stronger in-out asymmetry favoring outer divertors. DN operation also leads to an up-down asymmetry with higher particle fluxes to the divertors with their ∇B drift toward the X-point. Reversing toroidal field direction shows a strong influence on the divertor asymmetries. The plasma exhibits a large amplitude broadband turbulence in the outer divertors for both SN and DN, which is significantly reduced in the inner divertors, especially for DN. This coincides with a broad divertor SOL profile on the outboard side. Initial comparisons are also made with SOLPS-B2/EIRENE modeling.
[en] The Experimental Advanced Superconducting Tokamak (EAST) has demonstrated, for the first time, long-pulse divertor plasmas over 400 s, entirely driven by lower hybrid current drive (LHCD), and further extended high-confinement plasmas, i.e. H-modes, over 30 s with predominantly LHCD and advanced lithium wall conditioning. Many new and exciting physics results have been obtained in the quest for long-pulse operations. The key findings are as follows: (1) access to H-modes in EAST favours the divertor configuration with the ion ∇B drift directed away from the dominant X-point; (2) divertor asymmetry during edge-localized modes (ELMs) also appears to be dependent on the toroidal field direction, with preferential particle flow opposite to the ion ∇B drift; (3) LHCD induces a striated heat flux (SHF), enhancing heat deposition away from the strike point, and the degree of SHF can be modified by supersonic molecule beam injection; (4) the long-pulse H-modes in EAST exhibit a confinement quality between type-I and type-III ELMy H-modes, with H98(y,2) ∼ 0.9, similar to type-II ELMy H-modes. (paper)
[en] Full text: The ELM-resolved particle and power loads on divertor targets were studied in a wide range of discharge conditions in the EAST superconducting tokamak, mainly using divertor triple Langmuir probe (DTLP) arrays embedded in the target plates, along with infrared (IR) camera and other key divertor diagnostics. The target particle and power loads for type-I, type-III and mossy ELMy H-mode plasmas are investigated, respectively. The experiments were performed with lower hybrid current drive (LHCD) alone or combined with ion cyclotron resonance heating (ICRH), with Lithium wall coatings. For a typical type-I ELM in EAST, the divertor heat load is about 10% the plasma stored energy, while it is about 1 - 2% for a typical type-III ELM in EAST. The mossy ELMs induce even less change to the plasma stored energy, i.e., ∼ 0.1%. The peak heat fluxes on the divertor targets for mossy, type-III and type-I ELMs are characteristically ∼ 0.1 MW/m2, ∼ 2 MW/m2 and ∼ 10 MW/m2, respectively. The mossy ELMy H-mode in EAST may provide a potential scenario for ITER. In the present ongoing campaign of EAST, mossy ELMy H-mode duration has been lengthened to 1 s with LHCD+ICRH, which is only limited by ICRH pulse length and may be readily extended to longer values. In an ELM-free period, the power deposition obtained from DTLPs was compared to that from IR camera, which is not ELM-resolved at present. And the results of the two cross diagnostics are comparable. Statistically, the frequencies for mossy, type-III, and type-I ELMs are 1 - 2 kHz, 200 - 800 Hz, and < 100 Hz. During both type-I and type-III ELM phases, the SOL width and plasma- wetted area on divertor targets broaden insignificantly, compared to that between ELMs. This insignificant broadening is independent of the auxiliary heating technique. The mid-plane SOL width for type-III ELMs in EAST is ∼ 8 - 15 mm, being different to that during type-I ELMs. The divertor in-out asymmetry during both type-III and type-I ELMs has also been studied. For type-III ELMs, divertor plasmas favor the outboard target in LSN, which exhibits an even stronger in-out divertor asymmetry in DN. Also in the ongoing campaign the fine structure of ELMs, i. e., ELM filaments were observed by DTLPs. The typical interval time between adjacent ELM filaments for type-III ones in EAST is ∼ 150 - 250 μs. (author)
[en] A series of experiments has recently been carried out in EAST under different plasma conditions to investigate the basic divertor performance, divertor and SOL screening efficiency and radiative divertor effect. Detached divertor plasmas have been achieved by density ramp-up. It is found that CIII emission from the low-field side (LFS) exhibits a strong dependence on poloidal locations and plasma operation regimes from methane (CH4) puffing experiments. In addition, the radiative divertor experiments by injection of mixed Ar (5.7% Ar in D2) into the outer divertor chamber reduce the peak heat flux by 50% at the outer target plate, which also reduce the divertor plasma temperature. The in-out heat flux distribution asymmetry is improved.
[en] The neon puffing for inducement of a Radiative Improved (RI) mode has been investigated on HT-7 tokamak. The results show that the central electron temperature is increased 1.4 times and electron density is strongly peaked with ne0/nGR about 0.7. It has an improved energy confinement (17–25 ms) and improved particle confinement (7.5–23 ms) at the same time. The confinement factor H (H = τe/τeITER93ELMy-free) of 0.96 is obtained. There is a large fraction of radiation power losses. In our cases it is up to 80% of total input power. This kind of plasma has a high Z-effective, but there is not detrimental concentration of the seeded impurity at the core plasma, which induce a reduction in the growth rate of Ion Temperature Gradient (ITG) driven modes. Fluctuation experimental measurement obtained by CO2 scattering with kθρs of 1.4 also shows a reduction of turbulence in the core plasma after neon seeding