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

although present-day devices have made significant progress in developing individual techniques
and integrated scenarios for achieving plasma conditions consistent with Q = 10 operation in
iteR, research is still required on the specific application of these techniques and scenarios in
iteR. Research issues in this area can be categorized in the following areas:
• Performance capabilities of iteR in non-dt phases.
• techniques for plasma breakdown and current ramp-up/down consistent with operating
constraints on iteR.
• capabilities for heating, fueling, and power exhaust in burning plasma conditions.
• accessibility of enhanced performance regimes in iteR.
For iteR to achieve its Q = 10 goal, there are uncertainties and issues that need to be resolved,
some of which are described in this section. however, the consensus among the world fusion research
community is that none of the issues are insurmountable or show stopping and therefore
iteR has a very good probability of achieving this major objective.
accomplishments
most of the elements required for formation and control of a high-performance iteR plasma have
been and are routinely achieved in the international tokamak research program. For example,
many techniques have been developed to prepare the condition of the discharge vessel, allowing
the formation of a high-purity hydrogenic plasma. techniques for controlling plasma shape,
internal inductance and, to a lesser degree, current profiles during ramp-up, flattop, and rampdown
are routinely employed throughout the worldwide tokamak program. Their success mirrors
the depth of understanding of the mhd behavior of high-temperature plasmas. The scientific and
technological basis for heating plasmas to thermonuclear temperatures and pressures with the
use of neutral beams and radiofrequency wave heating at electron and ion cyclotron frequencies is
well developed. neutral beams have been used to heat plasmas to over 40 kev in tFtR, well over
the ~ 25 kev central temperature required for iteR, while heating at ion cyclotron frequency has
been used in alcator c-mod to increase the plasma pressure to 1.8 atmospheres, essentially equaling
iteR’s operating pressure. techniques for gas fueling and fueling by injection of frozen hydrogenic
pellets have been developed and, together with active pumping, are used to control the
plasma density. an important result is the discovery that fueling from the high-field region of the
vacuum vessel can be used to improve the effective fueling depth and offer some degree of control
over the density profile. excellent progress has also been made in the control of plasma heat
exhaust, one of the most daunting issues to be confronted as plasmas move into the long pulse,
burning regime. a particularly important development was the observation and understanding
of “divertor detachment,” in which a large fraction of heat flux flowing into the divertor region is
dissipated before contacting the divertor target plates. a basis for extended pulse operation has
been established with the discovery in the last few years of the so-called “hybrid mode,” a regime
of improved confinement in which the central safety factor q(0) is clamped near unity. by increasing
q(0) above unity, regimes have been realized in which a large fraction (>50%) of the current
is self-driven (the so-called bootstrap current). These are often augmented by the formation of
“transport barriers,” regions of the plasma that are characterized by relatively steep gradients
and low transport. in combination with current drive from neutral beams and/or radiofrequency
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