Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session KI2: MFE Regime OptimizationInvited
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Chair: Steven Scott, Princeton Plasma Physics Laboratory Room: Chatham Ballroom C |
Tuesday, November 17, 2015 3:00PM - 3:30PM |
KI2.00001: On the continuous confinement transition between ``baseline'' and ``hybrid'' plasmas in JET with an ITER-Like Be/W Wall Invited Speaker: Joelle Mailloux Experiments with the JET ITER-like wall have shown that confinement is often lower than the IPB98(y,2) scaling in low beta plasmas, typical of the ``baseline'' domain foreseen for ITER Q$=$10 operation, while high beta ``hybrid'' plasmas have achieved improved confinement compared with the scaling. JET ``baseline'' and ``hybrid'' plasmas differ in terms of beta, q95 and initial q-profile, as well as operational aspects such as gas injection rate. These results motivated an investigation to determine if there is a bifurcation or a smooth transition between the two domains and to identify the key plasma parameters explaining the departure from the scaling. Plasmas with initial q-profiles typical of the ``baseline'' and ``hybrid'' domains were compared at same beta and q95. Additionally, the heating power was varied to produce plasmas with a range of beta, keeping the same initial q-profiles. The results show confinement to be insensitive to the initial q-profile, but to increase with respect to the IPB98(y,2) scaling as power increases, such that H98 increases from 0.85 to 1.2 as normalized beta rises from 1.4 to 2.5. The detailed scan in heating power shows that the power degradation of confinement is weaker than that of the IPB98(y,2) scaling, with a smooth, continuous transition between the two domains. The weak power dependence of confinement is thought to be due to the interplay between many factors affecting core transport (ExB flow shear; fast ion pressure; electromagnetic effects; collisionality) and H-mode pedestal height (which is consistent with peeling-ballooning stability modelling in shots with low gas rate injection rates). Operational factors also play a role: e.g. high gas injection is needed to avoid high W concentration, however, this reduces the energy confinement. These results show the importance of including all key core and edge physics processes to predict the confinement behavior in future devices. [Preview Abstract] |
Tuesday, November 17, 2015 3:30PM - 4:00PM |
KI2.00002: High Internal Inductance for High $\beta_N$ Steady-State Tokamak Operation Invited Speaker: J.R. Ferron An attractive scenario for steady-state tokamak operation at relatively high values of the internal inductance, $l_i$ $>1$, has been demonstrated at DIII-D. The more peaked current density profile leads to reduced core energy transport and higher ideal stability limits that could eliminate the need for n$\ge$ 1 active stabilization coils at $\beta_N\approx$ 4, or enable $\beta_N\approx$ 5 with wall stabilization. The scenario's potential is shown by discharges at $l_i$ $\approx$ 1.3 with high bootstrap current fraction $\mbox{f$_{BS}$ $\approx$ 0.8}$, high plasma pressure $\beta_N\approx$ 5 and excellent confinement H$_{98(y,2)}$ $\approx$ 1.8. This very high $\beta_N$ discharge with q$_{95}$ =7.5 has noninductive current fraction f$_{NI}$ $>1$ and too much bootstrap current in the H-mode pedestal, so $l_i$ decreases with time. To achieve a stationary current profile, the key is to maximize $\beta_N$ and f$_{BS}$ while maintaining $l_i$ high enough for stability through choice of q$_{95}$ or by reduced pedestal current. DIII-D modeling shows that with q$_{95}$ reduced to lower f$_{BS}$ to $\approx$ 0.5, a self-consistent equilibrium has $l_i$ $\approx$ 1.07 and $\beta_N\approx$ 4 (below the n=1 no-wall limit) with q$_{95} \approx$ 6. The remainder of the current can be externally-driven near the axis where the efficiency is high. Discharge tests with similar $l_i$ in the ITER shape at q$_{95}$=4.8 have reached f$_{NI}$=0.7, f$_{BS}$=0.4 at $\beta_N\approx$ 3.5 with performance appropriate for the ITER Q=5 mission, H$_{89} \beta_N$/q$_{95}^2$ $\approx$ 0.3. The $l_i$ was shown to increase further above 1, to enable higher self-consistent f$_{BS}$ and $\beta_N$, by reducing pedestal pressure and bootstrap current density through application of n = 3 resonant magnetic fields. With similar fields for ELM mitigation, and neutral beam and electron cyclotron current drive sources for near-axis current drive, the high $l_i$ scenario is a potential option for ITER. The increased core confinement can help mitigate the effect of reduced pedestal pressure. [Preview Abstract] |
Tuesday, November 17, 2015 4:00PM - 4:30PM |
KI2.00003: Access conditions, energy and particle confinement of the I-mode regime on Alcator C-Mod Invited Speaker: Amanda Hubbard Experiments on C-Mod have shown an extended operating range for I-mode at higher magnetic fields, offering options for high-performance, ELM-suppressed operation in future devices. Stationary regimes without significant ELMs are a requirement for ITER and other large burning devices. The I-mode regime offers one potential solution. It features a strong Te and Ti pedestal, up to 1 keV, without a density pedestal. I-mode has been demonstrated on the C-Mod, ASDEX Upgrade and DIII-D tokamaks, over increasingly wide parameter ranges [1]. On C-Mod, global energy confinement is comparable to H-mode, with H98 between 0.7 and 1.2. Scaling of $\tau _{\mathrm{E}}$ with P$_{\mathrm{heat}}^{-0.3}$ is more favorable than H-mode. This lack of saturation and the natural stability to ELMs can now be understood in terms of pedestal stability, with pressure and current gradients well away from stability limits. Impurity confinement $\tau_{\mathrm{imp}}$ is similar in level and scaling to that in L-mode, 15-30 ms for both Ca and Mo, vs 0.1-1 s in H-mode. Key questions for extrapolation to other devices are the conditions for L-I transitions and for avoiding transitions to H-mode. An important new result is that the L-I threshold is independent of field, while the upper range of power for I-mode increases with B$_{\mathrm{T}}$ leading to a wider operating space; at 5 T and above, many discharges remain in stationary I-mode with the full heating power of 5 MW. Scaling thresholds with size suggests that I-mode should be obtainable on ITER. Some I-modes have been observed up to 8 T. Another key question for any regime is compatibility with boundary solutions. In usual operation with Bxgrad drift away from the X-point, heat flux is predominantly to the inner divertor leg. Impurity seeding is used to reduce the flux, taking advantage of low $\tau _{\mathrm{imp}}$. I-modes have now been extended to near-balanced double null. \\[4pt] [1] A.E. Hubbard et al, IAEA FEC 2014, EX/P6-22. [Preview Abstract] |
Tuesday, November 17, 2015 4:30PM - 5:00PM |
KI2.00004: Progress Toward Steady State Tokamak Operation Exploiting the high bootstrap current fraction regime Invited Speaker: Q. Ren Recent DIII-D experiments have advanced the normalized fusion performance of the high bootstrap current fraction tokamak regime toward reactor-relevant steady state operation. The experiments, conducted by a joint team of researchers from the DIII-D and EAST tokamaks, developed a fully noninductive scenario that could be extended on EAST to a demonstration of long pulse steady-state tokamak operation. Fully noninductive plasmas with extremely high values of the poloidal beta, $\beta_p \geq 4$, have been sustained at $\beta_T \geq 2\%$ for long durations with excellent energy confinement quality (H$_{98y,2}$ $\geq1.5$) and internal transport barriers (ITBs) generated at large minor radius ($\geq0.6$) in all channels ($T_e$, $T_i$, $n_e$, $V_{Tf}$). Large bootstrap fraction ($f_{BS}\sim$80$\%$) has been obtained with high $\beta_p$. ITBs have been shown to be compatible with steady state operation. Because of the unusually large ITB radius, normalized pressure is not limited to low $\beta_N$ values by internal ITB-driven modes. $\beta_N$ up to $\sim$4.3 has been obtained by optimizing the plasma-wall distance. The scenario is robust against several variations, including replacing some on-axis with off-axis neutral beam injection (NBI), adding electron cyclotron (EC) heating, and reducing the NBI torque by a factor of 2. This latter observation is particularly promising for extension of the scenario to EAST, where maximum power is obtained with balanced NBI injection, and to a reactor, expected to have low rotation. However, modeling of this regime has provided new challenges to state-of-the-art modeling capabilities: quasilinear models can dramatically underpredict the electron transport, and the Sauter bootstrap current can be insufficient. The analysis shows first-principle NEO is in good agreement with experiments for the bootstrap current calculation and ETG modes with a larger saturated amplitude or EM modes may provide the missing electron transport. [Preview Abstract] |
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