IPP’s ELISE test rig gains world record
Heating facility for the ITER fusion reactor / high-energy particle beam of record quality
The objective aimed at by the ITER (Latin for “the way”) international test reactor is ambitious: In order to heat the plasma to many million degrees Celsius, two high-energy particle beams with heating powers of 16.5 megawatts each are to be pumped into the 800-cubic-metre plasma volume. The cross-section of these particle beams will be of about door size, and will thus greatly exceed the beams used hitherto, which have done with roughly plate-size cross-sections and much lower powers.
The ITER test device, now being built as an international venture at Cadarache in France, is to demonstrate that an energy-yielding fusion fire is feasible. Like the sun, a future fusion power plant is to derive energy from fusion of atomic nuclei. The fuel, a hydrogen plasma, has to be confined in a magnetic field cage without contacting the walls and then be heated to ignition temperatures exceeding 100 million degrees. ITER is to generate 500 megawatts of fusion power, this being ten times as much as the plasma heating input.
Development work on the ELISE test rig
The so-called "neutral particle heating" will take on about half of this plasma heating: Fast hydrogen atoms injected through the magnetic field cage into the plasma transfer their energy to the plasma particles by means of collisions. Present heating systems, e.g. that in IPP’s ASDEX Upgrade device at Garching, thus bring the plasma to a multiple of the sun’s temperature at the push of a button. The ITER large-scale device, however, imposes enhanced requirements: For example, the particle beams have to be much thicker and the individual particles be much faster than hitherto so that they can penetrate the voluminous plasma deeply enough. Instead of the electrically positively charged ions used hitherto to produce the particle beam it is therefore necessary to use negatively charged ions, which are extremely fragile. The high-frequency ion source developed at IPP for this purpose was adopted as prototype in the ITER design. The contract for adapting the source to ITER requirements also went to IPP at the end of 2012.
With the ELISE (Extraction from a Large Ion Source Experiment) test rig, a source was being investigated in the past two years that was already half as big as the subsequent ITER source. It produces a particle beam with a cross-sectional area of about a square metre. The increased size called for revision of the previous technical solutions for the heating method. ELISE has thus advanced step by step to new orders of magnitude. Recently, the ion source achieved operation pulses lasting one hour, during which a stable and homogeneous ion beam of nine amperes lasting 20 seconds could be produced every three minutes. The gas pressure in the source and the quantity of the electrons retained conformed to ITER specifications. In short, a world record.
Background: the technical details
To enable hydrogen atoms to be accelerated, they have to be made tangible to electric forces as charged particles – as positively or negatively charged ions. This is done in the ion source: High-frequency waves injected into hydrogen gas ionise and disintegrate part of the hydrogen molecules. The plasma created, a mixture of neutral particles, negative electrons and largely positively charged ions, flows to a first lattice-shaped electrode. Through the several hundred apertures of this lattice an equally large number of individual ion beams are extracted from the plasma. On being accelerated through another two lattices, the finger-thick individual beams finally merge as a wide single beam, whose cross-section in ELISE is about a square metre.
If the surfaces of the ion source are coated with appropriate material, e.g. caesium, the hydrogen atoms there can then take up electrons. This provides the negatively charged hydrogen ions needed for ITER. To get rid of the unwanted, simultaneously extracted electrons from the plasma, their flight to the first lattice is obstructed by a transversal magnetic field in the plasma. Small permanent magnets incorporated in the second lattice then guide the electrons out of the beam for good. The much heavier ions, on the other hand, keep flying almost unhampered. It is not only this magnetic inner life that makes the ELISE lattices technical masterpieces; there is also an elaborate water-cooling system that, despite the high wall load during the heating pulse, keeps every individual aperture in place within hundredths of a millimetre in relation to its partner in the following aperture.
To make all this function properly, there are numerous individual parameters that have to be precisely tuned to one another, e.g. the high-frequency power, the caesium concentration, the wall temperature, the lattice voltages and the magnetic field for deflecting the electrons. Only then does one get the desired result, viz. a stable and homogeneous beam of fast, negatively charged hydrogen atoms. To enable the fast ions later in ITER to traverse the magnetic field unhampered into the plasma, they first have to be neutralised again. Finally, as fast hydrogen atoms they are injected into the plasma and surrender their energy to the plasma particles.
Where does it go from here?
Meanwhile the ion source was opened again for the first time since commissioning: Once the source has been cleaned, operation will be resumed at full power to attain the full target values. The system in its original size will then be investigated by ENEA’s Padua research institute in collaboration with IPP. As preparation, the Italian team will train here for the next two years and at the same time development at ELISE will be ongoing.