Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

estimates indicate that it will be difficult to achieve h-mode in the non-dt phases in iteR. in addition,
development of appropriate ion cyclotron resonance heating scenarios for the early phases
of the iteR research plan is needed to ensure that these phases will be as relevant as possible to
subsequent d-t operation.
techniques for plasma breakdown and current ramp-up and ramp-down consistent with Iter operating
constraints
Vessel Cleaning at Full Field: a relatively prosaic but very important issue for iteR is developing
the means to clean the vessel walls of loosely bound elements such as carbon, beryllium, and
water vapor prior to tokamak operation. during the breakdown and startup phase of fusion devices,
lightly bound elements are released from the wall, which may cause the discharge to collapse
radiatively before reaching temperatures high enough to permit the induced electric field to
drive significant current. This problem is well known in existing tokamaks, and various methods
of in-situ cleaning of the chamber walls have been developed. however, most of these methods
are not applicable to iteR because they have been developed for pulsed machines in which the
toroidal field is typically not present during the cleaning process. in iteR, superconducting magnets
produce the 5.3 tesla toroidal field, and it is highly desirable to maintain the field constant at
its design value. Thus, the research issue here is to develop an effective means to clean the iteR
vessel walls while maintaining a constant 5.3 tesla toroidal field.
Ramp-up and Ramp-down: Present iteR scenarios call for the plasma current to be ramped up
in 65-100 seconds and a diverted plasma to be formed relatively early during the ramp-up. heating
power is to be applied at or soon after the plasma becomes diverted to keep the internal inductance
low and avoid vertical instability. The transition to h-mode is to be avoided until late in
the ramp-up. a key question is then how to manage this ramp-up scenario to minimize the transformer
consumption (i.e., resistive volt-seconds), so that the maximum volt-seconds will be available
at full plasma current. to answer this question with more extensive modeling, one needs to
know the plasma transport properties — e.g., energy, particle, and impurity diffusion coefficients
— during this transient ramp-up phase. The problem is complicated by the fact that the total current
radial profile is evolving on the ramp-up time scale, and this will feed back on the transport,
which in turn affects the current profile. The effectiveness of the heating and current drive sources
for reducing the transformer consumption during ramp-up needs verification with respect to
the associated transport and resistive mhd limitations.
The current ramp-down in iteR is complicated by the requirements to maintain mhd stability
(i.e., achieving a soft landing) and avoid additional flux consumption while exiting from burn.
here again, knowledge of the particle and energy transport coefficients is necessary for modeling
this phase of the discharge. additional issues include whether and when the plasma makes a
transition back to l-mode, how to keep the plasma coupled to the mid-plane antennas for heating
and to the divertor strike points for particle and power handling, and how to keep the plasma diverted.
While “normal” ramp-down is described here, these issues also arise for “emergency” shutdown
if an incipient disruption cannot be mitigated and the discharge must be terminated. Thus
a variety of initial conditions for the ramp-down phase must be considered.
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