Ion cyclotron heating
How to pump 20 MW of power into 1 gram of plasma
13 Jan 2020
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Pat Brans
To power the ion cyclotron system, the ITER Organization and its partners are designing not only new antennas, which will be housed in the tokamak vessel, but also the robust transmission lines that will connect the antennas to radio frequency power sources two buildings away.
By generating waves at a frequency that matches the oscillation of specific ion species in the plasma, the ion cyclotron resonance heating (ICRH) system transfers energy to the plasma. ICRH is responsible for delivering 20 MW of input heating power through two 50-tonne launchers.
One of three plasma heating systems on ITER, ion cyclotron resonance heating will deliver 20 MW to the plasma in the form of electromagnetic power at frequencies between 40 and 55 MHz, depending on the scenario. System parameters such as frequency, power or spectrum will be controlled in real time by the central control system, CODAC.
At roughly 3.5 metres long, with a cross section of 2 x 2 metres and a weight between 45 and 50 tonnes, the antennas will be huge objects occupying two of the vacuum vessel's equatorial ports. Each antenna will have eight input transmission lines feeding into an array of "current straps," which face the plasma.
"We'll be firing a very large amount of power at a very small amount of matter," says Bertrand Beaumont, leader of the Ion Cyclotron Lower Hybrid Section. "20 MW is something like the power needed to heat 2,000 houses during a European winter. As you might imagine, this much power delivered to about 1 gram of matter will cause the plasma to reach a very high temperature, very quickly."
From the local power grid to the vacuum vessel
Getting that amount of power to the antennas is no small feat. Starting from the local power grid, using transformers and rectifying units, 22 KV of AC power will be transformed to 30 KV of DC power to feed the power amplifiers. The power from eight amplifiers, reaching up to 3 MW each, will then be transferred to the machine via rigid coaxial lines.
The coaxial lines run from the Radio Frequency Building, where the transmitters and high voltage power supplies will be housed, through the Assembly Building to the Tokamak Building. An impedance matching system will sit at the point just before the lines penetrate the Tokamak Building, adapting the load of the antennas to the output load of the power amplifiers.
"The power amplifiers are based on the broadcasting technology that was used for the shortwave communications of very powerful radio systems, such as Voice of America," says Beaumont. "What we're doing is a continuation of that standard technology. What's different is that our level of power is much higher. So we have to combine power from a number of amplifiers to get the unit power we need in each of the transmission lines."
The ICRH launchers (dark blue) are shown in their equatorial ports. In total, approximately 1.5 km of transmission lines across three buildings will be required to connect the wave-generating ion cyclotron sources to the antenna feeds. Two other external heating systems—electron cyclotron (green, at left) and neutral beam injection (beam lines at right) will combine with the ion cyclotron system to bring the ITER plasma to 150 million degrees Celsius.
"In one transmission line we want to transmit 3 MW. To do this, we use rigid coaxial lines, which are around 300 mm in diameter. The lines use straight runs, bends, and are also fitted with switches and measurement points. The inner conductors are supported by ceramic, and the insulating dielectric gas running inside is controlled and pressurized."
Taking proven technology to new levels
The ITER ion cyclotron is not required for First Plasma, but at the very earliest for the first pre-fusion power operation campaign (beginning December 2028). "It is still years away," says Beaumont. "Even though we're working with proven technology, we're taking that technology to new limits. It's going to take a lot of time to pull it all together with our partners."
ITER Organization is already very busy working to the targets—and so are the three contributing Domestic Agencies: Europe, India and the United States. While the ITER Organization is developing the comprehensive design of the antenna, responsibility for procurement lies with the European Domestic Agency. The preliminary design of the antenna is expected to be completed by the end of 2020, and the final design by the end of 2022.
Europe expects to complete the Radio Frequency Building by the middle of 2020. The ITER Organization is developing the interfaces with the Tokamak Building, ensuring both the confinement and fire zone boundaries at the points where the coaxial cables enter.
The Indian Domestic Agency is providing the radio frequency transmitters and roughly half the high voltage power supplies, customizing existing technology. The ITER Organization will procure the other half of the high voltage power supplies.
The US Domestic Agency is developing the transmission line and matching system, using simulation tools and high-power tests in a dedicated lab. The team there expects to start production soon, to ensure delivery of the transmission lines in time for integration with the transmitter and high voltage power supplies during the assembly phase leading up to First Plasma.
"We're drawing on a big base of knowledge from JET and other experimental reactors that used ion cyclotron systems," says Beaumont. "Using scaling laws, we have taken what has been proven to work and extrapolated to our target power levels and duration. Our ultimate target is to provide power for a pulse lasting 1 hour, with a power amplification ratio (Q) equal to 5. We're designing the whole system to reach that target."