Science in the spotlight

The story of a JET pulse

CartoonIn 30 years of operations JET has produced over 80,000 pulses. Although an everyday occurrence during experimental campaigns, how a pulse is prepared and processed deserves its own special story.

So what is meant by the term? A pulse is the name given to the time a plasma is held in the JET vessel. To create fusion in a future power station a plasma, or very hot electrically charged gas, has to be sustained inside the tokamak vessel whilst optimising three specific conditions: temperature, density and confinement time. This is called the fusion triple product and two isotopes of hydrogen, deuterium and tritium, are used to make the reaction occur.

Unlike future stations when the plasma will need to run for several hours or continuously, on JET, each pulse typically lasts around 40 seconds. Although this may seem like a short amount of time, the 80,000 or so plasmas already created in the machine since 1983 have provided crucial information for plasma science by focusing on its behaviour and improving its performance. The pulses have all contributed to an extensive knowledge bank of data (with over 100 TeraBytes of data collected to date) and their analysis plays a key part in the long-term goal of fusion electricity on the grid.

Like all good stories – the story of a pulse has a distinct beginning, middle and end, a strong setting and involves many people in this case engineers, scientists and computer experts who all have fascinating parts to play.

The story begins...

Chronologically, the story begins at 07.00 in the JET Control Room. This is the operational centre of the JET facility where experiments are undertaken in two eight-hour shifts. In this story the first shift of the day is just starting and 15 pulses will be run in its duration.

Before today’s pulses can take place, scientists and engineers from fusion laboratories all over Europe have applied to carry out experiments on JET. Their proposals have been discussed by a Task Force and the direction of experimental campaigns identified. A steering committee has matched submitted proposals with the current scientific requirements of JET, and each experiment has had one lead Scientific Co-ordinator assigned to it.

Back in the Control Room the first pulse of the day, a ‘dry run’, is about to take place. This is the chance early in the morning to check that all the systems needed to operate JET are running smoothly and a profitable day of experiments will follow. None of this, can of course, take place without the staff working in the control room during a given shift and these include the Engineer in Charge, Session Leader, Shift Technician, Scientific Co-ordinator, and Physicist in Charge. All of their roles will become clear as the story unfolds.

Running the pulse

Now to the running of first experimental pulse of the shift. Having checked the status of the machine following the ‘dry run’ pulse, the Session Leader pre-programmes the next pulse. The main responsibilities of the Session Leader are to prepare a realistic experiment plan, which fulfils the wishes of the Scientific Co-ordinator and his/her team as far as is possible within the operational limits of the machine. Based on this plan, the Session Leader prepares the basic types of pulses which need to be run in a session. He/she programmes the details of these pulses – time evolution of things like density, plasma current, magnetic field, plasma shape into the so-called ‘pulse-schedules’, which contain the information required to run a plasma pulse.

Control RoomDuring the shift the Session Leader, starting from the prepared pulses, adapts the pulse schedules in response to the results of previous experiments, and in discussion with the Scientific Co-ordinator. When the Session Leader is happy with the pulse schedule for the next pulse, he/she transmits this schedule to the Engineer in Charge (EIC), who checks that the pulse is safe to run. The Session Leader also communicates with the Physicist in Charge, who is responsible for setting up various diagnostic systems with the Heating System operators.

Before the pulse can begin the power supplies need to be enabled. The power to make a pulse comes partly from electricity directly from the grid and additionally from stored energy in two massive flywheel generators on the Culham site - with roughly 50% coming from each source.

As the flywheels are enabled, in the Control Room, the parameters have been decided and set by the Session Leader and confirmed with the EIC. The EIC then ensures that the operators of the required subsystems, power supplies, computer systems, heating systems and essential diagnostics, are ready and asks the Shift Technician to start the countdown for the pulse. Once this happens, a computer controlled initialisation sequence begins.

Countdown to plasma

After two minutes the sequence is held. At this point the EIC checks that all the systems are functioning correctly. When satisfied the EIC asks the Shift Technician to trigger the pulse - they are the staff who actually ‘push the button’ to make the plasma happen.

The pulse is triggered at zero on the countdown which is marked by a siren noise - an announcement to all in the control room that a plasma will be created on JET. Forty seconds later, the plasma is created and can be seen on a dedicated screen. A number of checks are performed automatically during the pulse and if limits are exceeded the control systems will terminate the plasma by gently ramping the plasma current down.

The Session Leader and the EIC also watch the infra-red camera cameras closely and listen attentively to audio feedback from inside the machine. It is very unlikely, but if anything is judged not to be right, the Session Leader or EIC can push a button, which triggers a gentle rampdown in a similar way to the automatic stops.

In this story, as most commonly happens on JET, the pulse goes ahead as planned. During the first 40 seconds after the end of the countdown, the currents in the large magnetic coils surrounding the vacuum vessel are ramped up to create the required magnetic field inside the vessel. This field has to be just right to allow a plasma to be created inside the vacuum vessel. At 40 seconds a minute amount of gas (deuterium in routine experiments) is injected into the vessel and a strong electric field is induced which ionises the gas, making it into a plasma.

JET plasmaThe plasma which has now been created is a very good conductor of electrical current.The electric field, which was initially used to create the plasma, now generates a strong current in the plasma. This current is ramped up in a controlled way to very high levels, typically 4-5 million amperes. The current in the plasma itself generates a strong magnetic field of its own - this adds to the magnetic field generated by the various magnetic coils to hold on to the very hot plasma, without it cooling down by touching the inside of the vacuum vessel.

Once the plasma is well established, the detailed shape of the plasma is controlled using external poloidal coils. Only the edge of the plasma is visible on the screen in the control room, as only the edge radiates in the visible range of wavelengths.

After a short while a significant amount of current is put into a set of special divertor coils situated just below the plasma itself. The field generated by these coils is so strong that it is stronger than the field generated by the plasma itself in the vicinity of the coils. The so-called x-point occurs at a position where these two fields cancel out. Magnetic field lines just above the x-point will trace out doughnut-shaped surfaces which never get near the vacuum vessel.

Just a little further out from this surface magnetic field lines no longer form closed donuts, they will in fact all finish by hitting the divertor in the bottom of the machine, which is designed to absorb most of the power. When the x-point is formed the bottom of the machine, the divertor becomes bright. Once the plasma is in this x-point configuration, additional power is applied.

This power, mainly from the neutral beam injection system, puts up to 35 million Watts of power into the plasma, heating the plasma up to ~150 million degrees Celsius. At this point the plasma surface, and in particular the divertor becomes very bright. A strong ‘shaking’ of the image is also observed on the dedicated screen. This shaking is associated with phenomena called ELMs (Edge Localised Modes). These ELMs can be seen as small solar flares as they expel significant burst of energy at regular intervals, typically 10-50 times per second.

The phase with high power lasts from 5-20 seconds after which the power is switched off. Then the plasma current and the magnet field are slowly ramped down and the plasma extinguishes when this plasma current approaches zero.

What happens next

Once the pulse is over, up to 60GB of data is collected. Some is reviewed immediately but the majority is stored for long-term analysis. The pink glow on the screen in this experiment lasted 40 seconds, the longest pulse ever run was one minute long.

The next pulse will be run in around 30 minutes. Before the next pulse ‘story’ can begin the Session Leader will analyse the behaviour of the pulse to check that it did what he requested, and use this information to finalise the ‘pulse schedules’ for future experiments.

The length of the pulse on JET is limited by engineering design and cost consideration. Two factors mainly limit the duration. The magnetic coils, though cooled, heat up during a pulse and when the temperature reaches a limit, the current has to be ramped down to avoid cumulative heating over many pulses which could cause damage to the coils. This duration was part of the original design and much stronger cooling, or superconducting coils would be required to extend the pulse length significantly. The second thing that limits the pulse length, is the fact that the plasma current is maintained by a voltage induced in the plasma by varying the current of the main transformer coil.

The voltage can only be maintained as long as the current in this ‘primary’ coil varies. To get the longest possible pulse the current in the primary coil is ramped up before the pulse to it’s maximum value. During the pulse, it is then ramped down at the rate required to have the desired plasma current. When the current in this primary coil reaches its maximum negative value the plasma current can no longer be sustained.

In the design of JET the pulse length was fixed, in order to have 10-20 seconds at full power and field. In plasmas, most things vary on timescales of less than 1 second and hence after 10-20 seconds very little changes in the behaviour of the plasma. This means scientists can learn almost everything they need to from plasma pulses of 10-20 seconds duration.

Future fusion machines will be designed for significantly longer pulse durations and, though continuous operation is not a necessity for reactor operations, methods to drive current continuously are being developed.