It often said that JET is similar to ITER. In fact, it is more correct to say that ITER is similar to JET. JET, with its long history of operation and many scientific achievements, has provided the basic template for the ITER design. As a result, JET’s mission is clear: support ITER in all possible aspects!

In the last two years of operations, the JET team has made substantial achievements in improving system performance, testing new technologies for ITER and resolving remaining physics issues. In addition new hardware and new and upgraded diagnostics have been successfully commissioned. The machine is now in a period of enhancement ‘shutdown’ which will last for a year, during which new ITERlike plasma facing components will be installed (see JETInsight June 2009) and the neutral beam power increased by 50 per cent. This shutdown is an excellent opportunity to look back and summarise the achievements of the last two years. The following few examples highlight the impact JET experiments have had on the design of ITER components and on the operating scenarios.

We currently have approximately 350 scientists from Europe and 100 scientists from other international collaborations participating in the JET programme. They will be involved in the analysis of the data taken over the last two years, a period in which we have carried out some very successful experiments.

Francesco Romanelli, EFDA Associate Leader for JET

ITER will operate in three different phases: the first, a non-activated phase will use hydrogen or helium plasmas to test the machine performance while keeping it accessible for repairs. Then there will follow two phases with increasing fusion power, one to test shielding provisions and another to reach full fusion power. Thanks to JET’s remarkable flexibility, its experimental programme has been able to imitate the ITER non-activated phase and compare its performance to normal deuterium operation. This will increase the ITER team’s confidence in being able to predict the ultimate fusion performance right from the first phase of operation.

JET influences ITER’s hardware

From 2007 onwards, JET has performed experiments focusing on toroidal field ripple effects. In every tokamak the field generated by the toroidal field coils is imperfect. The coils produce a small scale fluctuation in the magnetic field a “ripple”. Some ions and electrons get trapped in the lower magnetic field areas, resulting in energy and particle losses so it is important to reduce the ripple as much as possible. The more coils on the machine, the smaller the ripple between them. In other words JET, with a total of 32 toroidal field coils, has a very small ripple level while ITER, with only 18 coils, might face a substantial loss of energy from the plasma. To consolidate the ITER design, JET carried out a series of experiments taking advantage of its ability to operate with various combinations of currents in odd and even numbered toroidal field coils. The results led to a revision of the ITER design of the ferritic inserts in the wall between the toroidal field coils. These should reduce the amount of ripple to a tolerable level allowing ITER the high fusion gain (Q≥10) it is designed to achieve. There is also a need to test possible designs for the Ion Cyclotron Resonance Heating antenna (ICRH, see JETInsight October 2008) for ITER. Its baseline scenario is the High Confinement Mode, which is, in most cases, accompanied by Edge Localised Modes (ELMs). These instabilities cause abrupt changes in the edge density of the plasma that results in large amounts of the Radio Frequency power being reflected back to the antennas. The Radio Frequency generators then have to power down briefly to avoid overloading. As a result the amount of power coupled into the plasma is significantly reduced. Experiments in the last two years with the JET ITER-Like Antenna have showed that the concept chosen for ITER is indeed capable of powering through ELMs and that the power coupled to the plasma should meet the requirements of ITER.

Dealing with disturbances

ELMs are also a concern because of the heat loads and the erosion they cause on plasma facing components. Another JET contribution to ITER is to develop and test techniques to moderate the ELMs. One possibility is to add a voltage pulse to the vertical stability coils, in effect giving the plasma a vertical kick. As a result the plasma moves down a few centimetres and shrinks before it recovers its original position and size. Each kick triggers an ELM and high kick frequencies result in many small ELMs instead of fewer large ones. Fortunately, this technique does not reduce energy confinement to a significant degree. Another concept for reducing the size of ELMs uses a set of coils to produce so-called Resonant Magnetic Perturbations (RMP) at the plasma edge. Yet another is the injection of a train of small pellets, each triggering an ELM. Six pellet injectors are planned in ITER for fuelling fusion reactions and for mitigating the ELMs. ELM control with kicks, RMPs and pellets has been extensively studied in 2009 and will be again in the 2011 experimental campaign. Over the years, ITER has benefited from the experience gained at JET. Paul-Henri Rebut, leader of the JET design team and JET Director until he became Director of ITER-EDA, said in 2006, on receiving the Hannes Alfvén Prize:”Without JET, ITER would not exist today.” That is certainly true; however the honour comes with important responsibilities.

Petra Nieckchen

Left: Moderating Edge Localised Modes (ELMs):An additional voltage pulse to the vertical stability coils momentarily move the plasma down a few centimetres and shrinks it, triggering an ELM each time; middle: Picture taken with visible light camera shows Toroidal Field ripple induced losses (indicated by the arrow) on the poloidal limiter for a plasma with one per cent ripple. The experimental results are consistent with the calculations done beforehand; right: The ITER-Like Antenna fully mounted in JET. Hidden behind the rods, but clearly visible, are the copper ‘straps’ that radiate the power into the plasma.


JET support for ITER in 2008-2009:

  • Advanced scenario experiments in JET bolster expectations for ITER
  • Full ITER discharge simulations prompt design improvements for ITER coils
  • Experiments with increased toroidal field ripple show necessity of having low ripple in ITER
  • Threshold power for high confinement regime (H-mode) in helium plasmas, as planned for initial phase of ITER operation, is not higher than in deuterium plasmas
  • Ion cyclotron heating used to control sawteeth and avoid neo-classical tearing modes, which deteriorate confinement
  • ITER-like ion cyclotron antenna tested and confirmed to be tolerant to ELMs
  • Severity of Edge Localised Modes (ELMs) mitigated by several different techniques, controlled vertical plasma displacements (‘kicks’), resonant magnetic perturbations (RMP), trains of small pellets and even gas puffs
  • Severity of plasma disruptions mitigated by powerful puffs of gases such as neon and argon
  • Documentation of deuterium retention in carbon and material migration in the vessel for future comparison with ITER-Like Wall