VACUUM TECHNOLOGY FOR NUCLEAR FUSION

In the long term, on a scale of the order of 100 years, nuclear fusion arguably offers the only sustainable means for the generation of electric power in a quantity sufficient to satisfy the world’s needs. Solar, wind, and wave power can make valuable but limited contributions to the energy budget, and fossil fuels, reserves of which are finite, become increasingly scarce and costly, with oil needed for purposes other than energy generation. In addition, their burning creates soundly based environmental concerns. Electricity generation by nuclear fission, once thought to be the solution to the perceived long-term problem, is no longer so regarded for reasons partly political and partly technical, particularly those to do with long-term storage of its radioactive waste products.

Although the technological challenges of controlled thermonuclear fusion and capital cost of research facilities are enormous, and many challenges remain, the fact that the ingredients that fuel the reaction are abundant in nature and easily acquired, that there are no long-lived radioactive waste products, and that considerable progress has already been made, are powerful incentives for the furtherance of the work to its long-term goal. The underlying principles of the subject and its current state are described in a recent book by Harms et al. (2000), and an essay by Pert (2002) reviews past work and future prospects. Research in this subject has been under way since the 1950s, and from nationally financed activities initially, often with military connections, it has evolved to one of substantial open international collaboration. Designs and planning for ITER, the International Thermonuclear Experimental Reactor, are well advanced.

Of a number of thermonuclear reactions between light nuclei that yield products of less total mass and the consequent release of energy equivalent to the mass difference, attention is concentrated on that between the hydrogen isotopes deuterium D and tritium T to produce an alpha particle and a neutron, in which 17.6 MeV of energy is released as kinetic energy of the products:

1D2 + 1T3 → 2He4 (3.5 MeV) + 0n1(14.1 MeV)

For this reaction to occur, the positively charged D and T nuclei must approach each other with sufficient speed to overcome the strong repulsive Coulomb force. For a gaseous deuterium–tritium mixture, this requires that the temperature be of the order of 108 K, so hot that the mixture is completely ionized to form electrically neutral plasma of intimately mixed positive ions and electrons. Heating and confining the plasma are problematic. In order to have a sustained reaction in an ignited plasma, such that the energy released is greater than that lost from it by radiation, a criterion due to Lawson has to be satisfied, namely, that the “fusion

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triple product” of ion density N, energy containment time tE, and temperature T exceed a specified value. The value of tE is determined by the rate of energy loss from the plasma by radiation. At temperature 108 K, the product N tE has to be greater than about 2 × 1020 m−3 s for the net release of energy. For a tE value of 1 second, the N value is that associated with gas at 10−2 mbar. Since the early 1980s, experimental progress towards this goal has been quite rapid, with increases in the realizable values of N, tE, and T. Values of the triple product have increased by many orders of magnitude and are now only a factor of about 10 below that for ignition. The ITER machine previously mentioned is intended to take investigations into the realm of ignited plasmas. Ultimately, the generation of electricity will be achieved in plant that surrounds the burning plasma with a lithium blanket. While the energetic alpha nuclei generated will interact strongly with the plasma to oppose cooling losses by radiation and maintain its temperature, the neutrons will escape to be absorbed in the blanket. Here they have two functions: by further nuclear reactions to produce more tritium that is sent back to feed the reaction, and, from the heat generated as they are slowed down, to raise steam for the electrical power plant. Helium nuclei that would otherwise accumulate in the plasma from the spent D/T fuel constitute “ash” and have to be removed.

Containing and controlling the hot plasma against instabilities remains one of the central problems of the activity, and since the earliest days, strong magnetic fields in various special configurations have been investigated for this purpose. The considerable progress of recent decades has been due to the adoption of the tokamak configuration in which the plasma is contained in a toroidal (essentially doughnut-shaped) vacuum vessel, subject to a toroidal magnetic field that is augmented by other stabilizing components, such that the charged particles tend to spiral around field lines. The diagrams to be found in texts need to be consulted to appreciate how the field affects particle trajectories. The plasma is created by pulsed transformer action — electric fields induced by rapidly changing magnetic fields initiated by a large current pulse through an external, primary winding cause ionization of a low-pressure D/T gas mixture and its ohmic heating. The plasma pulse lasts for seconds or more, in which interval the diagnostic measurements are made under sophisticated computer control. In this interval, higher temperatures may be achieved by additional RF heating and, in some cases, the injection of beams of energetic neutrals tangentially into the plasma. Such beams are created by partial neutralization of energetic ion beams directed towards the plasma, with the deflection of the non-neutralized fraction electromagnetically into a dump.

To illustrate the scale and operation of these large fusion experiments, we will consider the vacuum system of the JET tokamak. The acronym signifies Joint European Torus, reflecting its funding by the European Communities, now transferred to the European Fusion Development Agreement, EFDA. First

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operated in the early 1980s and, like related experiments in the U.S., Japan, and elsewhere, continuously developed in advancing programs of work to discover the complex physics of high temperature plasmas and their confinement, a full description of the project will be found at www.jet.efda.org. The vacuum systems are described by Duesing (1987).

The JET toroidal vessel is a very large ultrahigh vacuum system with major radius of about 3 m and D-shaped torus cross section of height 4.2 m and minor radius 2.6 m as indicated in Figure 20.1. The need for ultrahigh vacuum conditions, commensurate leak tightness, and special attention to wall conditioning in order to minimize outgassing products and control their identity is discussed below. The torus, constructed from eight octants welded together, has an all-metal double-walled structure with numerous access ports of various sizes to allow for diagnostic probes, robotic manipulation, additional heating, and other services. The independently pumped interspace between the relatively thin inner and outer walls, appropriately braced for strength, facilitates leak detection and allows the circulation of hot helium gas for bakeout. The surface area of the inner wall is about 1000 m2 and the torus volume 189 m3. Two chambers C, one of which is indicated, have horizontal entry to the torus via 1.2-m diameter ports and at their base carry 400-mm inlet turbomolecular pumps T, with matching all-metal gate valves V, backed by primary pumps. The total effective pumping speed at the torus is 6000 l s−1. Prior to this secondary pumping, the rough pumping fromatmosphere to 0.1 mbar, using several pumping stations each with capacity 2000 m3 h−1, takes less than 2 h. The torus wall is degassed by heating it at up to 500°C by blowing hot helium through the interspace, and in operation the wall is held at 300°C. In addition, surface conditioning by extensive glow discharge cleaning is carried out, after which partial pressures of H2O, CO, CH4, and C2H4 are less than or of order 10−9 mbar, with a hydrogen partial pressure about 10−7 mbar. Higher hydrogen partial pressures are tolerable because the torus is backfilled with a very pure mixture of its D and T isotopes to a pressure of about 10−1 mbar prior to the creation of the plasma. The pulsed operation of a tokamak is a relatively violent event. The large toroidal currents induced, 5 MA or more, impose large mechanical forces of magnetic origin on the structure, and occasionally if plasma reaches the wall inadvertently, where it is quenched, a small perforation may result. An ingenious scheme of sectioning the interspace to detect such leaks in theinner wall is described by Orchard and Scales (1999).

One of the most critical aspects of operation is to minimize the presence of impurities in the background gas and their entry into the plasma by wall outgassing and by the surface interactions that occur when the hot plasma touches the plates L that serve as limiters to define its shape and keep it away from the

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inner vessel wall. This is because energy loss from the plasma, as well as due to a small amount of actual particle loss, is principally by radiation, predominantly Bremsstrahlung (braking radiation) due to the inevitable accelerations and decelerations of electrons in the plasma as they move in the electric fields of positive ions. This radiation scales with the atomic number as Z2. Low Z materials are therefore necessary for the limiter surfaces that define the “first-wall” with which the plasma comes into direct contact. Graphite (Z = 6) and Be (Z = 3) are used, the latter where the

plasma flux is especially high. As noted by Dylla (1998), in an article on the evolution of large machines for particle physics and fusion experiments, it wasadvances and collaborations in solving first-wall problems that have enabled machines such as JET, TFTR in the U.S., and JT60U in Japan to achieve plasmaparameters sufficiently near to the Lawson criterion minimum to justify the next phase of development of the significantly larger and more expensive ITER machine.

Synchrotron Radiation Sources:A particle of mass m and charge q moving with velocity v in a direction perpendicular to a uniform magnetic field B experiences a force qvB perpendicular

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FIGURE 20.1 Schematic diagram of torus cross section.

to both v and B, causing it to move in the arc of a circle of radius R. The value of R, determined by equating qvB to the product of m and the centripetal acceleration v2/R, is R = mv/qB. This relationship, which we will refer to as the “orbit equation” is of prime importance for machines in which charged particles, guided by magnetic fields, move in circular orbits at high energies. Because the acceleration of electrical charge causes the emission of electromagnetic radiation, there will be radiation from these orbits associated with the continuous inward acceleration.

Synchrotrons are large modern machines for accelerating electrons, protons, or their antiparticles to very high energies. The synchrotron principle may be described with reference to Figure 9.10(a), which is a schematic plan view of a ring-shaped vacuum system in which particles of charge q may circulate at radius R. Over the area indicated by dotted lines, external electromagnets create a vertically directed magnetic field B that can be increased with time in a controlled way. A radio frequency voltage source, whose frequency may also be varied, provides an electric field of the correct phase to accelerate particles circumferentially as they pass through a resonant cavity. Particles are injected tangentially into the system at a velocity and magnetic field B such that the orbit equation is satisfied, and then accelerated to higher and higher energies by applying an RF field of increasing frequency while increasing the value of the magnetic field commensurately so as to maintain them in the same circular orbit. Such a scheme allows for the relativistic mass increase of the particles with energy, which limited the attainment of earlier types of accelerating machines.

Radiation from these orbits has come to be known as synchrotron radiation. In electron synchrotrons, it is particularly strong and the principal mechanism by which the circulating beam loses energy. The electrons travel with speeds very close to that of light, and the radiated photons are emitted tangentially in a forward direction in a very narrow cone — for 1 GeV electrons, the semi-angle of this cone is only about 0.2°, so the radiation is highly collimated. From the whole circulating beam, therefore, there is a pattern of emission that resembles the spray of water that is thrown off the rim of a wet wheel rotating at high speed. It occupies a thin disc of space that extends radially outwards in the plane of the circulating charges. The emitted radiation is intense and spans a broad spectral range, typically from millimeter infrared wavelengths through the visible and UV to soft and hard X-rays, corresponding to photon energies less than 1 eV up to more than 10 keV. Furthermore, it is highly polarized in the plane of the orbit. Typical synchrotron sources are brighter than traditional X-ray tubes by a factor of order 105 or more.

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In recent decades, a number of electron synchrotrons have been built specifically to provide radiation for scientific investigations on the structure of materials. They have beam energies in the range 1 to 10 GeV and require vacua of about 10−10

mbar so that the high-energy beam, once established, can circulate continuously to provide a steady supply of well-characterized radiations for a number of hours. They are described as electron storage rings. The lifetime of the beam is determined by the loss of electrons due to their scattering by residual gas molecules. The RF voltage maintains the energy of those remaining against the radiation losses. It is in the nature of the processes involved that the current is bunched, consisting of very short pulses of less than a nanosecond duration and this is therefore also true of the output radiation.

While Figure 20.2(a) illustrates the principle, Figure 20.2(b), though highly simplistic, is more representative of matters in practice. Storage rings are not simply circular but are made up of alternate curved and straight sections. In the curved sections, dipole magnets cause beam-bending and therefore synchrotron emission as described in the preceding text. The limited arc length means that the radiation is in the form of a thin fan shape. The straight sections accommodate the primary beam injection arrangement and accelerating cavities and allow the insertion of special devices that enhance performance. Also located externally at these sections are magnetic quadrupole lenses that, allied with the focusing role that the dipole magnets have for electron trajectories close to the central orbit, are part of the beam focusing and control system. The focusing role of the dipole magnet arises by making their pole faces slightly divergent rather than parallel, so that small changes in the vertical field are created in a direction transverse to the beam. Because the vacuum tube has to pass between the poles of magnets, its cross section is relatively small and typically elliptical with vertical minor axis of a few centimeters, and the horizontal major axis rather more.

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Figure: 20.2 (a) Principle of synchrotron, (b) practical arrangement with curved and straight sections.

A fraction of the fan-shaped radiation from a particular dipole magnet may be selected by apertures and extracted into a tangentially directed beam line, while the rest impinges on the vacuum walls. There are, therefore, a number of junctions made with the main vacuum ring to allow this. Inside a beam line are housed apertures, mirrors, monochromators, and other optical elements appropriate to the wavelength components of the selected radiation. Beam lines are dedicated to specific purposes, infrared, ultraviolet, and various x-ray investigative techniques, diffraction, microscopy, and grazing incidence diffraction, for example. Woodruff and Delchar (1986) describe particular techniques that are valuable in surface science, and Field (2001) discusses further applications over a wider range.

The Synchrotron Radiation Source (SRS) at Daresbury, U.K. (www.srs.ac.uk) is an example of a machine offering world-class facilities to a large community of researchers in diverse fields. The main storage ring is 96 m in circumference and contains 16 bending magnets, each 2 m long. Inside it a current in the range of 150 to 300 mA circulates at an energy of 2 GeV. The injection system consists of a 12 MeV linear accelerator that feeds pulses of electrons into an intermediate booster synchrotron, accelerates them up to 600 MeV, and injects them into the main ring. When sufficient current has been built up at this energy, it is accelerated up to 2 GeV. Useful currents are maintained for 20 h or more, and 36 experimental stations are supplied with electromagnetic radiations of various wavelengths. Some of these are obtained by the insertion of “wigglers” and “undulators” in thestraight sections to produce enhanced effects. In these devices the electron beam is made to pass through magnetic field configurations that, in the former case, locally force it to be highly curved with the enhancement of radiation brightness at very short wavelengths. In the latter case, a periodic field variation leads to interference and the emergence of a number of very bright, almost monochromatic, beams.

The vacuum system of the SRS is rather complex, and aspects of its design and early performance were described by Trickett (1987). Reid (1997) has given a brief account of some aspects of its operation more recently. It is and all-metal UHV system in which scrupulous attention is paid to the avoidance of hydrocarbon contamination, which would have serious effects in beam scattering and the degradation of surfaces. The main ring is divided into sections that can be isolated by gate valves, and the beam lines can be similarly isolated. The machine stays at UHV for long periods during which many users simultaneously carry out experiments on a nonstop shift basis at its various stations. The pumps for ultrahigh vacuum are distributed ion pumps at the bending sections that utilize the field of the bending magnets in an extended pumping cell parallel and adjacent to the beam path. In the straight sections there are large 400 l s−1 ion pumps on top of which are mounted titanium sublimation pumps. Sections are brought to atmosphere rarely, but when this is necessary, re-establishment of UHV starts with

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oilfree rough pumping by diaphragm pumps and turbomolecular pumps with magnetic bearings. Items to be inserted into the system are rigorously cleaned, subjected to prior vacuum degassing in special vacuum furnaces, and also glow-discharge cleaned.

Pressure is sensed at places distributed throughout the systems by BAG gauges that have to be carefully located so that their function is not impaired by the various fields and the electron beam. Quadrupole residual gas analyzers are also strategically located to monitor the system and give warning of undesirable changes such as the opening of small leaks. The pressure around the ring is not uniform because there is some distance between pumps, and the pipe conductance is limited by its cross section and the presence of apertures. A pressure profile as a function of distance therefore shows minima nearest to pumps and maxima remote from them. The profiles with and without the beam are considerably different because when the beam is on it, it produces a large gas load due to photon stimulated desorption of molecules off the wall in the vicinity of the bending regions where the high photon flux of the radiation impinges. This dynamic effect is the main gas load in electron storage rings. Fortunately, it progressively diminishes with accumulating photon dose to acceptable levels, an effect referredto as “beam cleaning.” This can frequently be exploited in restoring UHV conditions when such occasions arise, rather than the lengthier procedure of sector bakeout. The modeling of the photon-induced gas load is therefore a crucially important aspect in the design of the vacuum systems of an electron storage ring. The vacuum in the beam lines has of course to be compatible with that in the ring, except in the case of beam lines that can be isolated from it by physical windows that are transparent to the radiation, as is beryllium for high energy x-rays. The vacuum conditions are then slightly less stringent.

The pressure measured in the straight sections of the ring in operation can be related to the mean pressure around it, which determines the beam lifetime. The state of this very complex machine is monitored by numerous pressure, temperature, and other sensors together with status indicators for valves, pumps, etc., and associated interlocks. The strategy for the control of the machine by computer and its implementation was first discussed by Reid (1982). Recent developments in this area are described on the Web site.

The SRS was the first large high-current synchrotron built as a dedicated x-ray light source, and its value to the scientific community has paved the way for the design and building of Diamond, a third-generation synchrotron source, and currently U.K.’s largest scientific project, whose first beam lines are scheduled to come into operation in 2006. This machine will be 560 m in circumference, with beam energy 3 GeV, current 300 mA, and sophisticated facilities at many user stations. Details may be found at the web site www.diamond.ac.uk

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