# 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.pct){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 and Current Drive scenarios![](media/image3.pct){width="2.7159722222222222in" height="3.3069444444444445in"}![](media/image4.pct){width="2.7215277777777778in" height="3.313888888888889in"}

The rf system is designed to operate at frequencies from 75 to 115 MHz.
**Figure 5.6-2** shows a plot of resonant frequencies of various ion
species as a function of major radius for B~0~ = 10 T (the standard
high-field case) and for B~0~ = 6.5 T (for long-pulse AT mode
operation). Heating 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 AT-mode operation,
second-harmonic D heating (or H minority heating) can be used at f ≈ 100
MHz. Fast-wave current drive during AT operation can be done at 75 MHz;
at this frequency the launched power is just below the H/2D resonance
and is predominantly absorbed by the electrons as it travels toward the
center of the plasma.

## Antenna geometry

**Figure 5.6-3** shows the antenna design. It consists of two current
straps, each about 16 cm wide and 125 cm tall. Each strap is grounded to
the case at both ends. The straps are also grounded at their center
points. This results in a very strong arrangement that can resist the
disruption-induced forces.

Electrically, the top and bottom portions of each strap are nearly
independent; each is driven by separate coax feeds and the coupling
between top and bottom segments is small. Each strap is tuned to
resonance at the driving frequency by an adjustable capacitor shown in
**Fig. 5.6-3** that consists of a metal "tongue" on the back of strap
segment and a U-shaped grounded electrode. The electrode is connected to
a moveable cylindrical piston that extends through the back of the
antenna box; electrical connection from the cylinder to the back of the
box is made with finger stock. By moving the electrode, the capacitance
can be adjusted to make the strap segment appear to be λ/2 in length,
capacitively loaded at the center (high-voltage) point.

![](media/image5.pct){width="6.045138888888889in"
height="3.9444444444444446in"}

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, and with π phasing between adjacent straps (worst
case). 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.

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. ![](media/image6.pct){width="5.375in"
height="3.303472222222222in"}

![](media/image7.pct){width="4.459722222222222in"
height="2.7368055555555557in"}