Installation of ITER-like Ion Cyclotron Resonance Heating at JET, October 2007
A central activity of the 2007 JET Shut Down is the installation of a new Ion Cyclotron Resonance Heating system (ICRH) referred to as the "ITER-like ICRH" system. To a very large extent the in-vessel part of this activity has now been completed.
ICRH is an established plasma heating technology using electromagnetic waves in a frequency range comparable to the cyclotron frequency of ions (30 – 55MHz in JET's magnetic fields) to increase the energy of the plasma ions. The ITER-like ICRH antenna is a further development of the existing ICRH systems towards more ITER-relevant conditions.
The baseline scenario for ITER is the High Confinement Mode, which is in most cases accompanied by "Edge Localised Modes" (ELMs). ELMs are disruptive events at the plasma edge, which are fast and strong variations in the equivalent loading impedance that the plasma presents to the antenna. For conventional ICRH antennas this impedance change at the antenna reflects the RF energy back along the feed transmission lines to the generators which then must be tripped very briefly to avoid damage and this obviously leads to a performance reduction of the system. The ITER-like antenna is one of several new ICRH systems constructed in an ELM- or load-resilient way, which means that although the plasma may vary strongly, this does not translate into large impedance variations, thus allowing the generators to couple RF power to the plasma despite the ELMs and thus retaining the ELM-free system performance.
As the walls of a future fusion reactor have to be covered to a high fraction by the blanket and only a small area is available for antennas etc, it is important that any heating system has a high power density. The ITER-like antenna will be capable of delivering 8MW/m2 to an ELMy plasma at a relatively high spacing between antenna and plasma, which is another challenging condition for the real ITER ICRH system.
The exploitation and optimisation of the JET ITER-like ICRH antenna will be a very important step towards mastering the challenges of ITER.
In-vessel view of the JET ITER-like antenna (left) during its installation
9th October, 2007 |
