Mass spectrometry delivers fusion insights

A quadrupole mass analyser designed for fusion research combines radiation hardness with the ability to distinguish between the light atomic species involved in the nuclear reaction

Nuclear fusion promises to deliver an abundant, safe and sustainable source of energy, but replicating the reactions that power the Sun remains a scientific and engineering challenge. Experimental reactors, such as the ITER facility now being built in France, are designed to heat and confine a plasma containing deuterium and tritium to extreme temperatures, with the aim that the two hydrogen isotopes fuse together to form helium along with a neutron and a huge amount of energy. Crucial to success is understanding what is happening inside the reactor, allowing the plasma composition and properties to be adjusted in real time to initiate and sustain the reaction.

The extreme conditions inside the reactor preclude the use of direct probes, but a combination of external diagnostics can be used to monitor the plasma conditions and the progress of the reaction. To push the capabilities of these techniques, Chris Marcus and colleagues at Oak Ridge National Laboratory (ORNL) in the US have been developing a system that will exploit both mass spectrometry and optical spectroscopy to analyse the complex mixtures of light gases that are released from the tokamak while the reaction is taking place. “Each of these techniques provides complementary information that will help to isolate the hydrogen and helium isotopes we are interested in,” he says.

Complementary diagnostics

As part of this project, Marcus has been working with Hiden Analytical in the UK to develop a mass spectrometer that offers unique capabilities for fusion research.  A key part of the brief was to achieve a higher resolving power for hydrogen and helium isotopes than can be achieved with standard instruments. “Our main objective was to detect small concentrations of helium-4, the main product from the nuclear reaction,” says Marcus. “But commercial instruments are unable to resolve helium-4 from deuterium when it is present at concentrations below about 10%.”

The spectrometer developed by the Hiden team exploits a quadrupole design, which is favoured for its rapid scanning speeds, compact footprint, and the ability to provide real-time analysis of atoms and molecules over a broad mass range. These quadrupole sensors work by ionizing the gas species, and then sending the ions through four cylindrical rods impressed with a composite DC and radio-frequency voltage. The resulting electric field separates the ions according to their mass-to-charge ratio, isolating the species of interest and preventing any other ions from reaching the sensor. The ion current produced at the output can then be related to the relative abundance of each species in the gas mixture.

However, conventional quadrupole analysers can only distinguish between atoms and molecules with a difference in mass of 1 amu. That makes it difficult to isolate helium-4 from deuterium, the species of particular interest for fusion research, since their masses differ by just 0.026 amu. “Although large-rod mass spectrometers are able to separate helium and deuterium, they are not suitable for deploying close to the outlet of a fusion reactor,” says Bob Mellor of Hiden Analytical. “We needed to develop a compact instrument that also delivers the extra resolving power.”

Improving resolution

Fortunately, quadrupole devices offer stable operation in several different zones. Standard instruments exploit a region of stability that can be accessed at lower voltages, but the Hiden team designed a spectrometer that can operate in a different stability zone that can separate helium and deuterium, albeit at significantly higher voltages. “This zone, which we call Zone H, achieves the detection limits that Oak Ridge required, but still enabled the use of a small quadrupole sensor,” says Mellor. The operational mode of the spectrometer can easily be switched between Zone H and the more usual Zone 1, which means that the instrument can also be used for conventional vacuum diagnostics.

Once a prototype instrument had demonstrated the feasibility of this solution, the next challenge for Hiden was to engineer a system that would be immune to the strong magnetic fields and high levels of radiation close to the tokamak. That posed a particular problem for the power and control electronics, since solid-state devices are prone to radiation damage and must be installed behind a radiation shield. As a result, the connecting cable between the power unit and the sensor can be up to 140 m long, which presents problems in connecting the RF voltage that is impressed on the quadrupole sensor rods.

A coaxial cable can be used, but simply connecting the RF voltage over long distances results in significant power loss in the generator. This means that the operating frequency of the sensor must be reduced to keep within the available power budget. “If you go much longer than 10 m, the frequency becomes so low that you don’t have any useful resolving power,” says Mellor.

The solution is to transmit RF power down the long coaxial cable to a set of intermediary electronics, or matching unit, that contains radiation-hard passive components and thermionic valves. “In the matching unit there is a power to-voltage convertor that generates the high-voltage RF to be impressed on the rods,” explains Mellor. “Independent testing at a fusion facility has confirmed that this solution is immune to radiation, magnetic fields, and ground vibrations.”

The resolving power of the mass spectrometer has also been evaluated at ORNL. “We have demonstrated that the quadrupole analyser can detect helium-4 at a concentration of 3% in a mixture containing 97% of deuterium,” says Marcus. “It also provides a fast response time, providing an analysis in less than a second, which is another key requirement for fusion diagnostics.

Experimental validation

In the meantime, Hiden has developed a series of commercial instruments to make the technology available to other fusion developers, as well as for other applications that require similar levels of performance. Both the DLS-2 and DLS-2X offer switchable dual-zone operation, combining high resolving power for light species along with high-performance residual gas analysis for vacuum diagnostics. They are available in two possible configurations: one with a Zone H that achieves an ultrahigh mass resolution of 0.0065 amu for species with masses of up to 10 amu, and the other providing a separation of 0.02 amu over a mass range extending to 22.5 amu. While the resolving power and sensitivity are the same for both instruments, the DLS-2X also features remote electronic control for use in harsh environments.

Meanwhile, the HAL 101X includes all the features that were developed for the gas analyser system being developed and tested at ORNL. That includes dual-zone operation and radiation-hard electronics, as well as an additional analysis mode in Zone 1 – called Threshold Ionization Mass Spectrometry (TIMS) – that can help to distinguish between species with similar mass-to-charge ratios when they are present in sufficiently high concentrations.

The Hiden team is now working with Marcus to investigate the capabilities of the HAL 101X to distinguish between other light gases that are important for fusion research. “We have established that we can isolate neon, which can be used as a catalyst for the fusion reaction,” says Mellor. “The usual difficulty with neon is that its mass-to-charge ratio is overlapped by argon ions, but Zone H can be used to separate neon-20 from doubly charged argon.” More challenging will be to resolve tritium from hydrogen deuteride, which are separated by just 0.006 amu. “It may be possible by using the TIMS method in Zone 1, or by further increasing the applied DC and RF voltages to achieve a higher mass resolution in Zone H,” says Marcus.

While those investigations continue, Marcus is continuing to evaluate the HAL 101X on a diagnostic test stand at ORNL. “The next steps for the mass analyser will be to advance it into the production phase for the full diagnostic suite,” says Marcus. “That will yield a complete system that has been validated for operational use.”