Science in the spotlight

How to reach 200 million degrees

Plasma heating methodsWhen operating a fusion experiment JET may be the hottest place in the universe, but can examining the workings of some everyday items here on earth help understand the challenges and complexities of heating the plasma to extreme temperatures?

“In the late 2020s ITER will reach fusion burn, pouring out energy like a little star,” Professor Steve Cowley, Director of CCFE, predicts.

The key goal of fusion research is the creation of a ‘burning plasma’ – a plasma which is producing energy whilst the input energy is reduced significantly or switched off altogether. Reaching this state is an essential step towards fusion power generation.

To achieve this feat several conditions are necessary. The plasma needs to be heated and sustained at extreme temperatures; anything less than hundreds of millions of degrees centigrade is not enough. And whilst maintaining this, the hot gas needs to be confined and controlled in such a way as to keep its density and ensure minimal heat loss.

How on JET, the world’s largest fusion device, do scientists and engineers achieve and maintain such high temperatures? How can tens of millions of Watts of power, equivalent to around 10,000 electric fan heaters, heat a mere hundredth of a gram of fuel? Like the numerical fan heater analogy, simplified explanations may lie in comparisons with the workings of several everyday things.

A light bulb, a cappuccino machine, a microwave oven, even the opening break of the balls in a game of pool can all provide an insight into how fusion fuel (deuterium and tritium), with the equivalent weight of a postage stamp, is heated to temperatures over ten times hotter than the centre of the sun.

Three main methods are used to heat the plasma in a fusion machine: Ohmic heating, neutral beam injection and radio frequency waves. They are complemented by the magnetic field which prevents heat loss and provides effective insulation.

Getting started

As the plasma created in the fusion process is an electrical conductor, initial heating can be achieved by passing a current through it. The coil in the central column of the JET vessel acts as a primary coil for what is a huge transformer. The secondary winding of the transformer is provided by the plasma itself, through which a large current can be induced. This resulting plasma current produces heat, just as a wire heats up when an electric current flows through it.

Bring on the light bulb; an old-style one with a tungsten filament and a useful object to explain the process. CCFE Chief Technologist Tom Todd details the analogy: “We can heat the plasma up to about twenty million degrees by passing an electrical current through it, like the filament being heated in a tungsten light bulb to roughly two thousand degrees. However unlike in tungsten, which rises in electrical resistance as the temperature increases, the plasma resistance actually reduces rapidly as its temperature rises, so that for a given plasma current we cannot achieve very much higher temperatures by this means alone, and have to add some other forms of heating.” The extra heating required to reach 150 million degrees comes in the form of two external methods – neutral beam injection and radio frequency waves.

Breaking the pack

Neutral Beam Injection (NBI) involves firing very fast particles into the plasma. The action of introducing the fast particles can be seen as similar to the opening break in a game of pool. When this happens, the fast motion of one billiard ball transfers energy to the other balls. Once inside the JET vessel, the newly introduced high energy particles crash into the plasma particles, give up some of their energy and in doing so make it considerably hotter.

An alternative analogy for this process is heating the milk for a cappuccino. To obtain the froth, milk is heated by firing steam into it. At this point, the steam is high-energy water and its fast particles give up some of their energy to heat the milk, which is also mostly water. After heating, the milk and the small quantity of injected steam equilibrate to the same temperature – albeit much hotter than before.

Plasma heating on JETSo how does NBI work? Neutral beam energy is produced in two distinct phases. Firstly, a beam of energetic ions is produced by ionising a gas and applying an accelerating potential to a set of water–cooled copper extraction grids which both accelerate and focus the ion beam. The energetic ion beam cannot be immediately injected into the Ohmically pre-heated plasma as it would not be able to penetrate the confining magnetic fields. Thus, a second phase neutralises the energetic ions by passing them through a neutral gas where sufficient numbers of them can charge-exchange to form energetic neutral atoms that can be injected into the plasma.

The JET NBI system is capable of using hydrogen, deuterium, tritium or helium as the source gas for creating neutral beams, which gives tremendous flexibility to the scientists designing experiments. To penetrate sufficiently deeply into the plasma, the injected particles have to be travelling at tremendous speed, so acceleration potentials of up to 140,000 Volts are used. Once on the move the atoms can travel at more than 3,000 km per second; well over six million mph or about 1% of the speed of light.

“The NBI system is the most powerful heating system on JET, with the potential to deliver up to 35MW of heating power for up to 20 seconds,” explains CCFE Neutral Beam Physicist David Keeling. “With the application of this level of power, the plasma can heat from several tens of millions of degrees to over 300 million ºC in a couple of seconds. Because of the power, reliability, and the added advantage of diagnostic information, NBI is used in just about every experiment on JET.”

Tuning into the plasma

A third method of heating the plasma is Radio Frequency (RF) heating, which complements the Ohmic heating and Neutral Beam Injection. To discuss, this we turn to the ubiquitous kitchen device the microwave oven, and a description often used by CCFE’s Technology Programme Manager Michael Porton when giving guided tours of the JET machine.

‘Your microwave at home excites water molecules in your food to transfer their increased energy to the bulk and raise its temperature. And by launching radio-frequency waves into the plasma we can excite the particles to ultimately raise the overall plasma temperature. By launching waves of differing frequency we can target differing parts of the plasma, for example on JET we launch waves ~50MHz in order to excite the ions within the plasma. By contrast if we launch waves ~170GHz we could excite the electrons in the plasma, as is used on other tokamaks around the world.

“Finally, by small changes in the frequency around these values we can also change the depth of the ions or electrons we excite. Much like playing a trombone, if you extend or shorten the length of the instrument you change the frequency of the sound produces. If we change the length of the transmission line we can change the frequency of the waves launched into the plasma thus penetrating to differing depths within the plasma.”

On JET, the heating system Michael is describing as analogous to playing a trombone is known as Ion Cyclotron Resonance Heating (ICRH). It launches waves from a number of antennae in the vessel walls, combining to supply up to 20 MW of power to the plasma. As the plasma particles move they spiral along the magnetic field lines at a particular frequency (number of circuits they make per second), which is called a cyclotron frequency.

To achieve the most efficient heating of the plasma, the wave frequency needs to resonate or match with the cyclotron frequency of the ions or electrons being heated. Put another way, the frequency with which the waves oscillate needs to match the rotation frequency of the ions or electrons.

Radio wave heating is undertaken with an ICRH systemwhere waves in the frequency range 23 – 57 MHz (FM frequencies are around 100MHz) are launched from a number of antennae in the vessel walls to resonate and heat the ions in the plasma. As the ions have different resonance frequencies, depending on the magnetic field strength at their location – ICRH heating can be applied selectively to specific areas of the plasma – enabling the use of radio wave heating to importantly minimise plasma instabilities in some cases.

JET is a role model of how the plasma will be heated in ITER. The major heating systems described (Ohmic heating, neutral beam injection and radio wave heating) will form the backbone of plasma heating on ITER – and will be essential to demonstrate burning plasmas with high fusion yield.