# 5.6 Ion Cyclotron Heating

Ion cyclotron waves will be used to heat the plasma. Based on
calculations of plasma transport, 20 MW of heating power will be
required.

A design that can deliver this power has been obtained. It consists of
four two-strap antennas mounted in main horizontal ports, as shown in
**Fig. 5.6-1.**

The amount of power that can be delivered to the plasma depends on a
number of things that are not yet well-quantified, such as:

> ![](media/image1.wmf){width="5.333464566929134in"
> height="4.138976377952756in"}• Distance between the first wall and the
> plasma separatrix at the outer midplane -- a smaller distance allows
> higher power per antenna.
>
> • Maximum voltages that can be sustained in the antenna and
> transmission line -- 30 kV is a relatively conservative number; 40 kV
> has been used on some experiments and would provide considerably
> higher power per antenna (P \~ V~max~^2^).
>
> • The density and density profiles (both inside the separatrix and in
> the scrape-off region) of the plasma.

We have concentrated on the antenna configuration and the coupling of
the antenna to the plasma, as these are the most critical parameters for
initial system design. Power sources, along with tuning and matching
equipment and concepts, should follow reasonably conventional (although
state-of-the-art) designs. While optimizing the parts of the system
external to the vacuum vessel is necessary, we do not regard it as
critical.

## Heating scenarios

The rf system is being designed to operate at frequencies from 80 to 120
MHz. **Figure 5.6-2** shows a plot of resonant frequencies of various
ion species as a function of major radius. Based on this plot, operation
at 10 T will use the second-harmonic T resonance at 100 MHz. For initial
non-tritium operation, minority He^3^ can be used for effective heating
at the same frequency. For lower (e.g., 7 T) operation, second-harmonic
D heating (or H minority heating) can be used at f ≈105 MHz.

## Antenna geometry

**Figure 5.6-3** shows a proposed antenna configuration that fits into a
main horizontal port and that can operate at the frequencies chosen
above .

![](media/image2.wmf){width="3.171527777777778in"
height="3.560416666666667in"}![](media/image3.wmf){width="2.716666666666667in"
height="4.0152777777777775in"}

The antenna consists of two current straps, each strap about 15 cm wide
and 117 cm tall. Each strap is grounded to the case at both ends, with a
20-cm long stub at each end to increase the electrical length of the
strap. The straps are grounded at the center point also for increased
mechanical strength. This results in a very strong strap arrangement
that can resist the disruption-induced forces. For reasonable
assumptions about the strap electrical properties, the electrical length
of the straps is near one wavelength in the 80 -- 120 MHz frequency
range, making this configuration feasible.

Each strap is driven by two coax feeds at the locations shown in **Fig.
5.6-3** (by the dashed circles)**.** The two feeds on each strap are
driven out of phase. We assume that there is π phasing between adjacent
straps, since there is no current-drive requirement on the IC system.

The antenna is covered by a Faraday shield consisting of a number of
metal tubes. Active cooling of these tubes will be required during a
shot for pulse lengths of 10 s or greater.

## Power to the plasma

**Figure 5.6-4** shows the maximum voltage in the rf system vs. gap
(distance from the outer separatrix to the antenna surface) , with the
constraint that the IC system must deliver 20 MW of power to the plasma
using four antennas. The line at 35 kV indicates a nominal value of
limiting voltage. A *conservative* design choice would set this value at
30 kV, while a more aggressive design would increase it to 40 kV. These
curves were calculated for the nominal density profiles
(parabolic-to-a-power) with\
\<n\> = 4.5 x 10^20^ m^-3^ and α~ν~ = 0.5 and
0.2.![](media/image4.wmf){width="5.375in" height="3.303472222222222in"}

Based on these results, a four-port system can deliver 20 MW to the
plasma at 100 MHz for up to a 6-cm gap, provided it can operate at 35 V
and\
α~n~ = 0.5. For the steeper edge profile with α~n~ = 0.2, a 4.5 cm gap
would be needed. The present design value for this gap is in the 3 to 4
cm range, so a four-port system should be able to supply the needed
power.