Focus On : Pumping Systems

Introduction

A newcomer to JET can get easily confused when pumping systems are discussed. Indeed, there are two large and completely different pumping systems, both quite impressive and deserving respect for their technology and performance: the cooling system of the JET machine (fluid pumping), and its vacuum system (gas pumping).

Cooling the JET machine

As mentioned previously in Focus On Power Supplies, most of the electrical power consumed by JET is transferred into heat. The main reason for this is that all of JET's massive coils which produce the strong, plasma confining magnetic fields are made from copper - and although copper is a very good conductor, it still has a small resistance to the electric current. At the very high electric currents needed to achieve the strong magnetic fields, this resistance causes significant heating in all of JET's coils. They must be continuously cooled down to prevent overheating of the facility.

Apart from the toroidal and divertor coils, all of JET's coils are cooled by forced circulation of water (See Figs. 1 to 6). The cooling water needs ongoing effective demineralisation and deionisation in order to keep its conductivity very low, and therefore a special water treatment facility has been installed at JET. However, in the event of leaks, the water becomes re-ionised and conductive after a short period, so that it can cause electrical short-circuits. That is why the most vulnerable toroidal (Fig. 5) and divertor coils use a special cooling liquid - Galden HT55, a non-flammable heat transfer fluid that maintains its high resistance under all conditions.

Tens of powerful pumps that force the circulation of the water and Galden can be found just below the JET Torus Hall, in its basement area. It is an extremely noisy environment during JET operations! Deionised water and Galden circulate in closed loops and exchange their heat with the main water circuit in heat exchangers that are situated next to the pumps and look like large engine radiators. The main water circuit then carries the excess heat  to JET's four cooling towers (Fig. 7), each with a two speed fan. Although these towers are very small in comparison to the cooling towers of nearby Didcot Power Plant, they still have a significant capacity of 4 x 35 MW (million watts) corresponding to 4 x 1000 m³ (one million litres) of water per hour cooled down from 50 °C to approximately 20 °C. Next to the towers are five large pumps which drive the water circulation in the main circuit - one per tower, the fifth is spare – each with 200 kW power and operating on 3300 Volts.  An additional booster pump supports the water flow at the far end of the main circuit.

During the plasma discharge, when large electric currents flow in the coils to generate the magnetic fields, the temperature of the coils increases sharply. After the discharge their temperature slowly decreases to the level at which  the next discharge is feasible. Overheating of the JET coils is the main limiting factor for the duration of the JET discharges. A typical JET plasma discharge lasts for 20 seconds - but it can be longer (even 60 seconds) when lower magnetic fields are required. The cooling system has been designed so that after each discharge the facility can be cooled down in 15 minutes, to match the similar time intervals required to spin up the flywheel generators (see Focus On Power Supplies) and to download and save all data acquired from the JET diagnostic systems.

To further boost the performance of JET - whenever high magnetic fields or long discharges are required - two massive 3 MW chillers (large refrigerators - Fig. 8) have been installed on site. When operational, these chillers are connected to the heat exchanger of the Galden-cooled units (toroidal field coils and divertor coils), replacing the main water circuit. The chillers can push the temperature of the Galden fluid down to 12 °C.

The JET magnetic field coils are not the only reason why a substantial cooling system is required on site. Other major "customers" of the system are Neutral Beam Injectors (Fig. 9), principally for their ion dumps and deflection magnets (see Focus On: JET Plasma Heating and Current Drive), and the JET flywheel generators. In addition, many minor systems need to be connected to the cooling pipework, including the air conditioning plant, cryogenic plant (see the next section) and individual plasma diagnostics.

JET's main cooled items

In order to achieve much longer plasma discharges, recent and future fusion facilities (including ITER) have to be equipped with superconducting magnetic field coils. A beneficial side-effect of this important upgrade is that power consumption of the superconducting coils is negligible compared to copper coils. However, there is a price to pay. Not only is the production of the superconducting coils very expensive, but - more importantly - a sophisticated liquid Helium cooling system to maintain very low temperatures (about -268 °C) in the coils will be required, otherwise the phenomenon of superconductivity (complete disappearance of electric resistance) would not occur. Therefore, building and operating a superconducting facility means opting for a considerably more complicated undertaking. It can be said that at JET, the copper coils were chosen for the sake of simplicity in the 1970s, when JET was an unprecedented technological step as it was.

Nevertheless, notice that even in ITER an extensive water cooling system will be required to support, for example, operation of its cryoplant and its neutral beam injectors.

Vacuum Pumping of the JET Torus

The other major pumping system is the JET Vacuum System. It is responsible for pumping out gas from the large volume of the JET torus - the doughnut-shape vacuum vessel in which plasma discharges take place (Fig. 10). The total vessel volume to be pumped is more than 200 m³ - similar to the volume of an average apartment!

Why is this vacuum pumping required? The densities of the hydrogen plasma that can be confined by magnetic fields are very low, about one million times lower than the density of air. Even a much smaller amount of non-hydrogen elements remaining in the vessel (e.g. nitrogen or oxygen from the air) would considerably damage discharge performance. During JET's shutdown periods, however, the vessel is vented to air allowing maintenance and new installations. Therefore, after each shutdown, all air must be thoroughly pumped out. The working gas for the plasma experiment - usually deuterium (heavy hydrogen isotope), occasionally protium (common light hydrogen isotope), helium and, in special campaigns, tritium - are puffed in just before and during the plasma discharge in accordance with the real-time plasma control requirements. In addition, JET plasmas can be "fuelled" by Neutral Beams and by pellets, i.e. by small capsules of frozen deuterium fired right into the hot plasma core. These working gasses are sometimes complemented by precisely defined minuscule amounts of impurities to diagnose the plasma parameters.

In order to keep plasmas as clean as possible, the vacuum system pumps the JET vessel continuously, even during the plasma discharge. The continuous pumping has negligible influence on plasma fuelling (i.e. on supplying the working gas), because at very low densities the fuel gas expands immediately to the whole vessel. The gas influx is electrically neutral, therefore not guided by the magnetic field.  Plasma exhaust, to the contrary, is guided by magnetic fields towards the bottom of the JET vessel, to the divertor, where it is continuously collected by dedicated cryopumps (see below).

JET is unique in the world as a fusion research experiment able to work with tritium, and, as a consequence, it has to be operated with all precautions required for active isotope handling. All the gases that are pumped from the vessel must go through a dedicated pipeline to the Active Gas Handling System (see Focus On: Fusion Technology and Fig. 13). In this system, chromatography and cryodistillation processes allow for safe separation and storage of the different isotopes from the pumped gases - namely of tritium (active), deuterium and helium (stable). This procedure is required at all times, even when JET is not operating with tritium, as traces of tritium continuously desorb from the vessel structure into the main pumped volume.

JET can achieve a very good level of vacuum, up to a millionth of a millionth of the density of air (in technical terms, the final pressure of impurities can achieve up to 10^-9 mbar, that is 10^-7 Pa). The procedure required to achieve and maintain that good vacuum is actually quite complicated, and several techniques must be employed.

Turbomolecular Pumps

First, a medium-level vacuum is achieved by pumping directly from the Active Gas Handling System. When the pressure in the vessel goes down below 1 x 10^-2 mbar , four large turbomolecular pumps are switched on (Figs. 12 & 13). These turbine pumps, which rotate at ~33,000 rpm (550 revolutions per second!) and have a pumping capacity for nitrogen of 2000 litres of gas per second each, operate continuously and effectively to produce a very low gas pressure in the vessel. The vessel is further pumped by the cryopumps in the divertor region (see below) and JET would not be routinely operated with the cryopanels warm. With the pumped divertor panels at helium temperature a well conditioned torus will typically be pumped to ~1 x 10^-8 mbar. Several smaller turbomolecular pumps are installed to maintain vacuum in some of the JET diagnostic systems.

Cryopumps

At several specific regions of the JET facility, a very high pumping speed is required: in the Neutral Beam Injector box, where it is necessary to prevent the gas flow from the beam neutraliser into the plasma, in the divertor region at the bottom of the vessel, where the plasma exhaust is directed by magnetic field lines, but also in the Lower Hybrid Current Drive system and in the deuterium pellet source. Very fast pumping in these regions is achieved by cryopumps - large surfaces that are at extremely low temperatures (Figs. 14 to 17). On these surfaces nearly all gases immediately freeze and collect as frost. The only troublesome gas at JET which does not freeze is helium. In order to cope with helium at JET, argon frosting can be applied in the Neutral Beam Injector box. The six cryopumps at JET have the following pumping speeds (in litres per second) :

6,000,000 l/s in each of the two Neutral Beam Injectors,

130,000 l/s (total) in the Torus Divertor Region - two separate pumps,

50,000 l/s in the Lower Hybrid system,

10,000 l/s in the Pellet Centrifuge.

During operation, the JET cryopumps are cooled down to -269 °C (5K) by liquid helium that is supplied from the JET cryoplant. In order to maintain the required amount of liquid helium for the facility, the JET cryoplant has a helium liquefier, an extreme member of the broad family of high capacity refrigerators. During JET operations, the JET’s helium liquefier unit - with two main compressors and several ancillaries - needs around 1 MW power continuously in order to produce about 8,000 litres (i.e. one tonne) of liquid helium per day.

Unlike the turbo molecular pumps, the cryopumps have limited operation times – they collect pumped gas on their surfaces that needs to be removed periodically. The procedure, known as regeneration, consists of the controlled heating of the cryopumps: the gas evaporates from the cryopumps and is pumped out from the vessel by the turbo pumps. Obviously, regeneration can only be undertaken when there are no experiments - at JET it is typically done weekly on Saturdays, or overnight in case more frequent regeneration is required.

JET Vessel Baking

The structure of the JET vacuum vessel is quite complex, with a large number of components and materials. Only vacuum-safe materials may be accepted for new installations, which do not evaporate and do not easily absorb and release gases. Even then, pumping the vacuum vessel to a very good vacuum (very low pressure) is not straightforward, namely because the gas molecules tend to 'hide' - adsorb on the surfaces of the solid state materials of the vessel. A very basic and efficient method to release the gas molecules from their hiding places is material baking. At JET, the whole structure of the vacuum vessel can be baked at up to 320 °C, and the baking system (Fig. 19) keeps the JET vessel hot continuously (even during plasma experiments), usually at about 200 °C. As a matter of fact, JET cannot be operated without baking - this is because its thermal expansion moves it free from the packing blocks.

The JET vessel baking is driven by two systems: hot gas and electrical. To allow for the hot gas baking, the JET vacuum vessel was built in two layers so that the baking gas can circulate in their interspace. Helium, which is used as the baking gas, runs in a closed loop - from the JET vacuum vessel to a massive blower (280 kW electric motor) that forces 22 m³ of the gas every second through heat exchangers (total 780 kW of heating power) and back into the interspace of the double-layer vacuum vessel. To also sustain the baking process on vessel components which project from the doughnut-shape vessel, (eg the diagnostic windows), the electrical baking system was installed. This complementary system consists of hundreds of electrical heaters mounted directly onto the outside surface of the vessel components.

Discharge Cleaning

Vessel baking is a key tool in the "first wall conditioning", which is necessary in order to achieve high plasma purity, however its effect can be boosted if, in parallel, the inner surfaces are bombarded by charged particles. While by keeping materials hot we can 'shake out' the gas particles adsorbed to surfaces, by bombarding the surface the particles get "kicked out". In most tokamaks, including JET, the walls are conditioned by baking combined with the effect of particle bombardment using "cold" gas glow discharge (Figs. 20 & 21) as well as "hot" plasma discharges – hence the term "discharge cleaning".

At JET, a glow discharge can be struck in the whole volume of the vacuum vessel either in deuterium, or in helium. Sometimes, though rarely, a hydrogen glow can be performed. Deuterium glow cleaning is more chemically reactive than helium, for example it reacts with oxides attached to the wall, releasing heavy water. Helium glow cleaning acts mainly by the electric charge of the glow particles. The glow discharge can be switched on continuously in the JET vessel - more than 24 hour continuous glow discharge cleaning in deuterium followed by a similarly long glow discharge cleaning in helium is not exceptional after long shutdowns. During experimental campaigns, an overnight glow discharge cleaning may be requested to improve the first wall condition. The glow discharge cleaning is typically run once a week after regeneration of the pumped divertor helium panels.

High temperature plasma discharges themselves act as a rather efficient tool to further clean the first wall from adsorbed atoms and molecules, as the plasma particle energies (i.e. velocities) are much higher in hot plasmas than in the glow discharge. For fusion experiments this is an adverse effect, as it increases the amount of impurities in the plasma. However, after a shutdown it is a common practice to run a few standard, scientifically uninteresting plasma discharges prior to the actual research with "tuned up" discharges. In any case, there are many other reasons for doing so: other systems, including power supplies, real-time control and plasma diagnostics, need a few simplified plasma discharges after each shutdown, to be recommissioned.

Beryllium Evaporation

Last, but not least, the first wall conditioning process is usually complemented with a technique that can deposit a microscopic layer of a suitable light element on the first wall. The layer helps to keep good vacuum conditions, in particular by gettering oxygen. Many tokamaks use boron in a glow discharge process known as boronisation. JET, the only fusion facility worldwide to do so, has opted for beryllium in-vessel evaporation (Fig. 22). This conditioning technique is typically applied once a week, often just after the glow discharge cleaning.

JET's unique beryllium handling capability is of an extreme importance today, as the design of the next step facility, ITER, relies on a beryllium first wall. Consequently, JET is being prepared to accept a large and challenging project, the replacement of the present Carbon Fibre Composite first wall by a beryllium first wall, which is planned for 2008 - see JET's programme in support of ITER.

"What comes to your mind if you hear someone say, "The Shut-Down is over: they restarted the pumping today!"? Perhaps you think of a water circulation pump like the one at home in the central heating system, an oil impeller like the one in your car, or maybe a vacuum pump similar to the roughing pumps you used in project work at university. Actually the pumping referred to is the set of large turbopumps mounted on the torus that produce the ultra-high vacuum needed for the tokamak to operate, but of course JET includes a wide variety of pumps in its plant, many of them of colossal capacity compared to those more familiar examples."
Tom Todd, Chief Engineer

Author: Jan Mlynar, Public Relations Officer (photo) with input from Liam Worth (Tritium & Vacuum Unit) and Phil Butcher (Power Supplies Unit)