Fusion energy project uses spectrometers

Fusion energy is different from fission in that it uses Deuterium-2 and Tritirium-3 to produce Helium-4, leaving virtually no nuclear waste, but a large amount of energy as a result. Nuclear fission uses uranium-235 and plutonium-239 to get a chain reaction that can be difficult to terminate. To achieve a nuclear fusion, a substantial energy barrier of electrostatic forces must be overcome. At large distances two nuclei repel one another because of the repulsive electrostatic force between their positively charged protons. If two nuclei can be brought close enough together, however, the electrostatic repulsion can be overcome by the attractive nuclear force, which is stronger at close distances.

Using deuterium-tritium fuel, the energy barrier is about 0.01 MeV. In comparison, the energy needed to remove an electron from hydrogen is 13.6 eV, about 750 times less energy. The result of the fusion is an unstable 5He nucleus, which immediately ejects a neutron with 14.1 MeV. The recoil energy of the remaining 4He nucleus is 3.5MeV, so the total energy liberated is 17.6 MeV. This is many times more than what was needed to overcome the energy barrier.

The testing done in the United Kingdom will lead to ITER (International Thermonuclear Experimental Reactor), an even bigger torus currently built in Cadarache, France. There, a full-scale electricity-producing power plant will start working in 2019. JET does the much-needed experiments on how to successfully set-up a fusion reactor. The next step will be DEMO (DEMOnstration Power Plant), which is to be constructed starting in 2024 and operational from 2033. Pending construction of ITER, JET remains the only large fusion reactor with facilities dedicated to handling the radioactivity released from D-T fusion. During a full experimental campaign in 1997 JET achieved a peak fusion power of 16MW, with equates to a measured gain Q of approximately 0.7. Q is the ratio of fusion power produced to input heating power. To break-even, a Q value greater than 1 is required. ITER is expected to reach a Q value of 10, which relates to 500 megawatts of output power for 50 megawatts of input power. DEMO should quadruple these amounts.

Avantes’ spectrometers measure the state of the torus’ walls 24 hours a day, 7 days a week. They will be running for several years to come. One of the main challenges for fusion reactors is the compatibility between a reactor-grade plasma and the materials facing the plasma. JET did use carbon until October 2009. In a vast refurbishment exercise the carbon tiles were removed against tungsten and beryllium tiles as foreseen in ITER. Since May 2011 JET has been operating with a full metal wall, which makes JET basically a small ITER. This makes JET even more valuable for the fusion community and ITER in particular.

Tungsten is very resistant to high temperatures (melting at 3422 degrees Celsius), but it is a heavy element (proton number 74) that can pollute plasmas considerably: it gets highly ionised in extreme plasma temperatures, which causes immense energy losses due to plasma radiation, and dilutes the D-T fuel. Beryllium is a light element with a proton number just 4. However it melts at just 1284 degree Celsius. The combination of beryllium and tungsten had never been tested in a tokamak.

The JET experimental programme will focus on optimising operating scenarios compatible with the ITER-like Wall. The level of retained tritium and its dependence on plasma parameters are determined by using Avantes spectrometers. Plasma performance is tested to show that the level of tungsten reaching the core is acceptably low. The lifetime of the wall will be studied with ITER-relevant power loading provided by increased heating.

Sebastijan Brezinsek is Deputy Leader of the JET task force of European scientists, which explore the ITER-Like Wall, and is responsible for all spectrometers used. He says: 'Even in the case of a major malfunction, no damage is done to either the torus, nor the environment. However, we want the walls to be used for as long as possible, therefore we are testing different materials. The Avantes spectrometers help us understand the effect of the plasma on the wall.'

Beryllium is used in the main wall, whereas tungsten, with its high melting point, is the choice for the exhaust component known as the divertor. It has to withstand high heat flux. It makes a 50 per cent increase in heating power possible. Brezinsek adds: 'With these even higher temperatures, it is vital that we constantly monitor the state of the walls and the degradation of the materials used in it. Avantes’ spectrometers have been connected to our central software solution. We used Avantes’ DLL-files, that gave us the possibility to extract all needed data.'

Avantes delivered 53 individually triggered systems, housed in 19-inch rack mountable cases. They are located approximately 100 metres away from the torus itself, to prevent any radiation from interfering with the measurements. Special systems have been developed, with slits effectively 0.6mm high and 25 to 100 micrometers wide. Focal length is 75mm and Sony ILX 511 detectors have been used. Gratings vary between 830 and 2400 lines per mm. Plasma is a fast changing charged gas, therefore fast data acquisition and a high repetition rate are essential. They measure from 380 to 960nm, from visible to near infrared. They are divided in different groups: some are used for a general overview, while most focus on a specific area.

In the core of the torus is the vacuum vessel where the fusion plasma is confined by means of strong magnetic fields and plasma currents (up to 4 tesla and 5 mega amperes), since no solid material can withstand the extremely high temperature of the plasma. In the current configuration the major and minor radii of the plasma torus are 3 metres and 0.9 metres respectively, and the total plasma volume is 200 cubic meters. A divertor at the bottom of the vacuum vessel allows escaping heat and gas to be exhausted in a controlled way. The plasma is heated through neutral-beam injection, which involves the introduction of high-energy atoms into the heated plasma. The atoms are ionised as they pass through the plasma and are trapped by the magnetic field. The high-energy ions then transfer part of their energy to the plasma particles in repeated collisions, increasing the plasma temperature.

Avantes delivered custom-made spectrometers carrying special features such as low noise detectors, which haven't been implemented before. Furthermore, the embedded software was specifically written for EFDA JET: timing and synchronisation were altered and independent triggering introduced to reduce dead time lost to a minimum. Klaas Otten, technical sales manager at Avantes, says: 'We are used to building specifically designed spectrometers. Therefore we have our own design team and construction is also done in-house. For EFDA JET, we were in close contact with the on-site technical team to fulfill their every need.'