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

heating, fueling, and power exhaust in burning plasma conditions
auxiliary Heating: The auxiliary heating and current drive schemes planned for initial operation
of iteR feature 20 mW of electron cyclotron power at 170 Ghz, 20 mW of ion cyclotron power
at 50 mhz and 33 mW of neutral beam injection at 1 mev energy. issues related to electron
cyclotron resonance heating and neutral beam injection are largely technical. although a 1 mW
prototype gyrotron operating at 170 Ghz has been successfully tested for long pulse, the technology
is still in its infancy, and the reliability of 24 such tubes operating simultaneously to deliver
the required 20 mW of electron cyclotron power needs confirmation. although our iteR partners
are adequately addressing gyrotron technology for iteR, the absence of a Us effort on gyrotrons
for iteR raises a concern about the basis for the Us to proceed toward a demo, which in all probability
will require electron cyclotron resonance heating technology. in the case of neutral beam
injection, many years of effort have failed to reach 1 mev beams at the requisite current density.
There are plans to build a full test stand in europe, and an aggressive attack on this issue is underway.
neutral beam injection has been the most reliable and widely used heating and current drive
source in the international tokamak effort and was used to create record-high ion temperatures in
tFtR (see Figure 8). consequently, extrapolations of the tokamak database in critical areas such
as torque generation, rotational control of stability, fast ion physics, and active beam diagnostics
are considered more credible than for other heating and current drive methods.
ion cyclotron resonance heating is also effective in heating tokamak plasmas, as shown in the example
from alcator c-mod (see Figure 9), where it has been used to produce record plasma pressures.
however, unlike electron cyclotron resonance heating and neutral beam injection, where
the technology is still in development but the heating physics is relatively straightforward, ion cyclotron
resonance heating is based on relatively secure technology with some uncertainty in the
physics. The main issue is that ion cyclotron antennas must be in contact with the edge or scrapeoff
layer (sol) plasma so that the antenna impedance can be matched at antenna voltages below
breakdown levels, typically 50 kv. consequently, ion cyclotron resonance heating antennas form
radiofrequency sheaths in the scrape-off layer that can accelerate ions into the plasma facing materials
at energies of a few 100 ev, well in excess of sputtering thresholds. sputtered impurities
T i (keV)
50
40
30
20
10
0
2.6
2.8 3.0 3.2
3.4
Major Radius (m)
4.1
Figure 8. Creation of record high ion temperatures
(> 40 keV) in TFTR by neutral beam heating.
(Figure courtesy of E. Synakowski and R.E. Bell;
for a related article, see D. Mansfield et al., Phys.
Plasmas 3 [1996] 1892.)
4.2
4.3
4.4
Time (sec)
4.5
4.6
4.7
43
Figure 9. RF power at the ion cyclotron frequency in
an Alcator C-Mod plasma discharge increased the
plasma pressure to 1.8 atmospheres, the same pressure
at which ITER will operate. (Figure reproduced
from S. Scott et al., Nucl. Fusion 47 [2007] S598.)