Performance calculations yield a substantial operating window for Q > 10 inductive operation for ITER. The figures below show values of fusion power (P_fus) and energy multiplication (Q) as a function of the auxiliary heating power (P_aux) for a range of plasma currents. The operating points have H_H = 1 and n/n_GW = 0.85. There is a strong increase in Q and P_fus with the plasma current and obviously a strong increase in Q with reduced auxiliary heating power. This emphasizes that the operation space is multidimensional and that plasma parameters can be adjusted to optimize the fusion performance according to whether high Q or high fusion power (e.g. to maximize the neutron wall loading) is required.

A more complete view of the range of plasma parameters at which Q=10 operation is possible can be seen below. Q=10 is maintained inside the shaded region.

The results illustrate

• the flexibility of the design,
• its capacity for responding to factors which may degrade confinement while maintaining its goal of extended burn Q=10 operation,
• its ability to explore higher Q operation.

The design is able not only to study the standard operating regime, but is flexible enough to achieve enhanced performance within the cost constraint. To extend the achievable range of Q values (and to counteract any unforeseen degradation of confinement), the possibility of operating the device with plasma currents up to ~17.4 MA (q₉₅ ~ 2.6) is being planned for, albeit at reduced pulse length (~100s). The capability of operation at fusion powers up to 40% higher than the reference value (though under the assumption of no increase in total neutron fluence) is included in the design to enhance the possibility of ignited operation and to accommodate the possibility that higher b values than assumed are achieved. The range of possibilities for different degrees of conservatism in the amount of helium impurity accumulating in the plasma can also be seen in the following diagram.

Steady-state and hybrid operation

The operation space for steady state operation, in terms of fusion power versus confinement enhancement factor, and showing the transition from hybrid to true steady-state operation, is illustrated below for I_p = 12 MA and P_CD= 100 MW. Contours of constant n/n_GW and b_N are indicated, as is the threshold for Q = 5 operation. For a given value of fusion power (and hence Q), as the confinement enhancement factor, H_H, increases (simultaneously decreasing plasma density and increasing b_N), the plasma loop voltage falls towards zero. For example, operation with V_loop = 0.02 V and I_p = 12 MA, which corresponds to a flat-top length of 2500 s, is expected at H_H = 1, Q = 5, n_e/n_GW = 0.7, and b_N = 2.5. True steady-state operation at Q = 5 can be achieved with H_H = 1.2 and b_N = 2.8. This analysis indicates that a long pulse mode of operation is accessible in ITER.

These results can be presented another way, as shown below. 

The range of performance follows the predictions and limits (e.g. in plasma density) of the bulk of today’s experiments, in which an "H-mode" transport barrier is established just inside the separatrix, and the internal radial temperature and density profiles within the plasma are constrained.  Under certain conditions of magnetic shear, the power entering the plasma core is sufficiently high that a second transport barrier occurs inside the first, limiting even more the heat conduction across the plasma.  If such a barrier can be controlled, it would lead to improved confinement.  Furthermore, the “bootstrap current”, a part of the plasma current driven by the plasma itself, would be enhanced by such an internal barrier, reducing the need for external non-inductive drive systems. In this case the curves in the figure would move significantly to the right, increasing the Q of hybrid and steady state operation. Investigations of these conditions will be carried further on ITER, in its non-inductive operation scenarios.

Further information on ITER plasma operation can be found in the ITER Plant Description.