Type-I ELM power loads on the closed outer divertor targets in the HL-2A tokamak

Abstract

The HL-2A tokamak has a very closed divertor geometry, and a new infrared camera has been installed for high resolution studies of edge-localized mode (ELM) heat load onto the outer divertor targets. The characteristics of power deposition patterns on the lower outer divertor target plates during ELMs are systematically analysed with infrared thermography. The ELM energy loss is in the range of 3%–8% of the total plasma stored energy. The peak heat flux on the outer divertor targets during ELMs currently achieved in HL-2A is about 1.5–3.2 MW m−2, the wetted area is about 0.5–0.7 m2, and the corresponding integrated power decay length at the midplane is about 25–40 mm. The rise time of the ELM power deposition is in the range of about 100 μs to 400 μs, and the decay time is typically 1.5 to 4 times longer than the corresponding rise time. Convective transport along open field lines during the ELM rise phase from the midplane towards the divertor targets is implied due to the correlation of parallel transport time in the scrape-off layer (SOL) and ELM power rise time. The peak ELM energy fluence is compared with those predicted by models and with experimental data from JET, ASDEX Upgrade, MAST, and COMPASS. The results, as a whole, show a good agreement.

The effect of impurity seeding into the closed divertor on plasma detachment in the HL-2A tokamak

“…In HL-2A, the distance between baffles and targets at the entrance of the divertor is about 20 mm, which is almost the distance between the baffles and separatrix, d bs , as illustrated in the lower divertor configuration (see figure 1). This is about two to three times the scrape-off heat flux decay length, λ q [29], which is λ q ≈ 5 7 λ n according to the two-point model [27]. Therefore, the HL-2A closed divertor geometry meets the optimum requirements, as mentioned above, and has the potential to make uniquely relevant contributions to this research.…”

“…The HL-2A tokamak (major radius R = 1.65 m, minor radius a = 0.4 m) has a double-null divertor and a divertor geometry with a high degree of closure, where a closely spaced coil triplet with zero net current is used to produce the divertor configuration [29]. Additional ('multipole compensation') coils are added to cancel even the residual far-field of the two (symmetrically top and bottom) divertor triplets over the main plasma region, so that core flux surfaces are close to perfectly circular.…”

“…Divertor heat flux is calculated from the evolution of surface temperature on the target plates, measured by an IR camera [29]. The IR camera is optimized to have its best field of view covering the strike point zones, and to use a reduced array size with a 2 kHz sampling rate.…”

Temporal characteristics of ELMs on the COMPASS divertor

“…The value of τ rise is defined as the time duration for the divertor heat load to increase from 10% to 100% of its maximum and the τ decay represents the characteristic exponential decay time after its maximum as shown by figure 2 in section 3. The recent work from HL-2A tokamak [15] shows that the values of τ rise is nearly equal to the time of ELM parallel propagation (τ || ) defined as τ || = L || /c s , where c s and L || represent the sound speed and the connection length between the upstream and corresponding divertor location, respectively. In [15] the connection length was obtained using a simple formula L || = 2πq 95 R sep (with R sep as outer midplane separatrix position and q 95 as a safety factor at the 95% poloidal flux surface), corresponding to its inter-ELM value.…”

Scaling Laws of Heat Flux Width in the HL-2A Closed Divertor Tokamak

“…[18]. Two infrared cameras with a toroidal separation of 120 ∘ have the spatial resolution of 2 mm, and the highest temporal resolution is 4 kHz, [19] which is used to observe the heat flux width of the edge localized mode (ELM) instability with an accuracy of 14 bits. Their correspondent wavelengths are 8-9.4 µm and 3-5 µm, respectively.…”