Culham Laboratory Research Articles

The design of a high current, low emittance multiply charged ion source

A novel design of multicharged ion source is currently under construction at Culham Laboratory. The physical processes upon which it is based — stepwise ionisation and magnetic cusp confinement together with energy and particle balance — are described. These principles have been combined in a model from which it is possible to characterise the new source. A zero dimensional computer code has been developed to solve the model equations numerically. Results from the code are presented for the source under construction. From the results the optimum operating conditions in terms of pressure, input power and cusp field strength may be determined.

Small is beautiful

Small satellites and their role in reducing costs were the subject under discussion at a one-day conference organized by the Royal Aeronautical Society and held at Culham Laboratory, UK, on 24 January 1991.

The CHYMES coarse mixing model

This paper contains a complete description of the CHYMES model, developed at Culham Laboratory to study some aspects of the coarse mixing stage of a steam explosion. Coarse mixing is the first stage in a sequence of events which could lead to the explosive release of the heat energy from a high temperature melt which has come into contact with a cold volatile liquid. The assumptions underlying the model are presented, together with the conservation equations and constitutive physics used. A brief description of the numerical method is also included. Results from previous CHYMES simulations are then presented, together with work performed to validate the model. A typical simulation of mixing in the lower head of a PWR is then described. Finally, a summary of the progress made, using CHYMES, in the understanding of the mixing stage of a steam explosion is presented.

Exploitation and development of particle beams at Culham Laboratory (invited)

Particle beams have been developed at the Culham Laboratory for many years, mainly for application to the production of high-temperature plasmas. These developments included demonstration of fundamental plasma physics such as particle trapping, heating, and current drive coupled to continuous advances in the physics and technology of ion beam production and transport. This work reached its zenith in production of multimegawatt high-proton fraction sources for the neutral beam injectors for JET. A new initiative is now coming for the development of negative ion based injectors. This wide ranging base of physics and technology provided a springboard for exploitation of beams in areas other than fusion. The last few years have seen increasing R and D at Culham in areas such as space, defense, surface treatment, and implantation. The applications have ranged from beams of H+ or H− to Xe+, from currents of less than 1 mA to currents of amperes and from energies of 50 eV to energies of hundreds of kilovolts. This paper describes some examples of these activities and relates them to the physics and technology base. Future areas of interest are also discussed.

Experimental results from a diagnostic beamline for the PBX-M experiment

The experimental performance of a high-brightness neutral-beam system is described which has been designed and manufactured by Culham Laboratory under contract to the Princeton Plasma Physics Laboratory. The neutral beam serves as a diagnostic on the PBX-M experiment and is used to perform studies of high-beta, bean-shaped, neutral-beam-heated plasmas, and plasma-current profile shaping using ion Bernstein wave and lower hybrid rf systems. A maximum extracted current of ∼2.7 A of positive hydrogen ions at 80 keV has been obtained from a multiaperture extraction system. The full-energy H0-equivalent neutral-current density measured 370 cm from the extraction plane was ∼6 mA cm−2.

The suppression of electrons extracted from a negative-ion source

When a suitable accelerating voltage is applied to extract negative ions from a volume production source, electrons are also extracted. If no means of suppression is utilized, the electron current in a hydrogen plasma would typically be of the order of 80–100 times the negative-ion current. In the accelerator designs in use at Culham Laboratory, electrons entering the accelerator are dumped in a trap within the second electrode at a small fraction of the beam energy. Even using this technique, a considerable amount of power would be expended unnecessarily in accelerating electrons to the trap. The suppression of electrons at the source/accelerator interface can be achieved by means of a magnetic field applied across the extraction aperture. This traps electrons, which are then collected on an electrically biased subsection of the first electrode. Experiments performed with an electromagnet configuration have shown that the electron flux entering the accelerator can be reduced as approximately the reciprocal of the square of the magnetic field strength.

The Culham Laboratory hypervelocity test facility

Culham Laboratory has designed and constructed a hypervelocity test facility (HTF) using electromagnetic launcher technology. Aimed at launching projectiles of approximately 1 gm to a velocity approaching 15 km/s, the facility is configured around an existing 1.1-MJ, 8-kV capacitor bank. The authors describe the electromagnetic launcher design and its power supply system and also report on its early operation. Theoretical performance is predicted and compared with experimental performance. A preliminary assessment of structural integrity, including rail and insulator damage, is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Negative Ion Beam Heating (Report on the IAEA Technical Committee Meeting, Culham Laboratory, 15-17 July 1987)

The IAEA Technical Committee Meeting on Negative Ion Beam Heating was hosted by the Culham Laboratory (United Kingdom Atomic Energy Authority) at the instigation of the International Atomic Energy Agency. The meeting was attended by 38 participants from nine countries; 24 papers were presented and two panel sessions held.

Fusion farewell

Plasma physicists gathered at Culham in December to acknowledge the debt the fusion community owes to Dr Rendell (Bas) Pease, FRS, who retired in November after 30 years with UKAEA. In that time he established, and was Director of, the Culham Laboratory and was a President of The Institute of Physics.

A Hybrid Blanket for a Reversed-Field Pinch Reactor

A hybrid blanket for a reversed-field pinch (RFP) reactor is presented for breeding 233U from thorium. This study focuses on the shell/blanket design and assumes a plasma like that used by Culham Laboratory (CL) in its designs. The 233U bred helps the important neutron economy and allows the tritium breeding ratio to be 0.96 at beginning of life for a mean of 1.06 for a 9 MW·yr/m2 burnup. A thick conducting shell is assumed for discharge stability and field reversal. This need for a good conductor requires that only pure copper or aluminum or alloys thereof be used.Two designs were investigated, one with a pure copper first wall/shell, the other with an aluminum alloy. In these designs 3.4% of the thorium is converted to 233U. This corresponds to 18.3 tonnes U in both cases for average thermal powers of 4590 and 4450 MW, respectively, at a wall loading of 2.2 MW/m2 during the burn phase. The breeding rates of 233U are, respectively, 0.66 and 0.69 kg/MW·yr, representing ratios of fission events per bred atom of 0.26 and 0.22, which is naturally better than in the fast breeder. In both designs the low metallurgie temperature limit means the large amount of power deposited on and in the shell is not attractive thermodynamically. The resulting large temperature differences in the shell cause high mechanical stresses.The design as it stands is not feasible from the point of view of radiation damage to materials. The pure metal copper shell swells too much (life limit 1.3 to 2 MW·yr/m2), the transmutations limit the electrical life to 3 MW·yr/m2, and low alloys of copper have not yet been irradiated tofluences sufficient for consideration. The aluminum alloy becomes brittle. This and low cycle fatigue each limit the life to 4.5 MW·yr/m2. Further thoughts at CL show that the RFP should work with a thinner shell as well; this would considerably reduce the thermal stresses in the shell and increase its lifetime sufficiently.

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Development of a 200 kV high current accelerator with post acceleration mass analysis for O + implantation into silicon

The formation of a buried insulating layer by oxygen ion implantation into silicon is a main contender for the bulk production of silicon on insulator material and has led to the development of dedicated implanters for the industry. To be economic, high currents of O + are required with good uniformity and impurity control. The beam system described here, which comprises the plasma source, ion accelerator and mass analyser, has been developed at Culham Laboratory for the OXIS 100 ion implanter in a collaborative programme with VG Semicon and Harwell Laboratory and with financial support from the Department of Trade & Industry. The key areas of development have been the large volume radio frequency plasma source with its inherent low impurity yield and long lifetime, the triode single stage accelerator providing multiple beamlets at energies up to 200 kV, and finally the double sector analysing magnet with short focal length to disperse the beamlets of the separated atomic oxygen ions over the wafer target.

Safety Guidelines for the Design of Next-Generation Tokamak Fusion Machines

The purpose of this paper is to provide an outline of a possible route for the development of safety guidelines for the design of a next-generation tokamak fusion machine, such as that being considered within the European Community for operation in the early 21st century. This next European torus (NET) will not only be fueled by deuterium and tritium (D-T) but will also be designed to provide demonstrations of tritium breeding and significant positive net thermal energy generation. The design of such a European machine will gain much from the knowledge currently being acquired from the Joint European Torus situated at the UK Atomic Energy Authority's Culham Laboratory in England.

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The Joint European Torus: installation, first results and prospects

The paper provides a brief introduction to the main aims, overall design philosophy and the planned parameter range of the large tokamak device (major radius R = 2.96 m; horizontal and vertical minor radii, respectively: a= 1.25 m, b = 2.10 m; plasma current, Ip= 4.8 MA), the Joint European Torus (JET), situated on the Culham Laboratory site, UK, whose main objective is to obtain and study plasmas in conditions and with dimensions approaching those needed in a fusion reactor. The main emphasis is on initial operation in the Ohmic-heating phase, in which results are presented covering a wide range of parameters: plasma currents Ip < 3.7 MA; toroidal magnetic fields BT= 1.3–3.4 T; elongation ratios b/a= 1.2–1.7; and safety factor values, q = 2.3–10. Average electron densities ne = (1–4) X 1019m−3, with high central electron temperatures (Te up to 5 keV) and ion temperatures (Ti up to 3 keV) have been achieved, although Zeff was in the range of 2.4–10. Energy confinement times (τE) of up to 0.8 s have been obtained. Some problems with metallic and low-Z impurities are noted, causing high radiation levels. Initial experiments, with ion cyclotron resonance frequency (ICRF) heating of H and 3He minorities in deuterium plasmas at MW levels, are reported. The paper concludes with a description of planned future experiments on impurity control, additional heating (ICRF ≈ 15 MW, and neutral injection ≈ 10 MW), and preparations for tritium operation.

Experiences of wall interaction in JET

The first year of operation of JET has recently terminated, and the first thorough interior examination has taken place. The work has been carried out jointly by JET staff and a Culham Laboratory team working under a Task Agreement. Erosion features attributable to run-away fast electrons, to unipolar arcs, and to rare power arcs have been identified. Sputtering and evaporation processes are seen and redeposition and cross-contamination of elements within the torus are clearly observed. Severe local effects of thermal excursions are seen on protection plates. Detritus recovered from the vacuum vessel includes metallic droplets and films, and fibrous material, probably from clothing. Surface analysis on long term samples, on limiter components and on probes exposed for short periods at selected phases through the pulse have shown that during tokamak operation more material overall is transferred from the main vacuum chamber wall than from the limiters themselves.

The power transfer and frequency detuning in a gyrotron

Abstract A rigorous small-signal theory of the power transfer and the associated complex frequency shift of a gyrotron oscillator is developed. Experimental results taken from a 28 GHz gyrotron operating in the TE02 mode, (measurements made at UKAEA, Culham Laboratory) and theoretical results on the frequency detuning are compared. It is seen that the frequency detuning increases as the frequency of the electric field approaches that of the cyclotron frequency. The imaginary component of the theoretical complex frequency detuning is shown to indicate the region of operation of the device and this is also compared with that obtained experimentally.

Olympus a standard control and utility package forinitial-value fortran programs

Abstract Title of program: OLYMPUS Catalogue number: ABUF Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland (see application form in this issue) Computer: ICL 4–70; Installation: UKAEA Culham Laboratory Operating system: ICL Multijob Programming languages used: STANDARD FORTRAN High speed store required ∗ : 14464 words; No. of bits in a word: 32 Overlay structure: None No. of magnetic tapes required: None Other peripherals used: Line printer No. of cards in combined program and test deck: 2425 Card punching code: EBCDIC

Non-Inductive Current Drive in Tokamaks (Report on the IAEA Technical Committee Meeting,Culham, 1983)

Non-inductive methods of generating plasma currents offer the possibility of operating tokamak reactors in steady state, which would alleviate the cyclic-stress-related problems of pulsed operation. The last few years have seen a rapid intensification of both the theoretical and experimental research in this area and much progress in evaluating the efficiencies of the potential schemes has been achieved. In order to review the progress and to identify areas for further research, the first IAEA Technical Committee Meeting on Non-inductive Current Drive in Tokamaks was held at Culham Laboratory from 18 to 21 April 1983. The meeting was attended by about 60 scientists from eleven countries who represented almost all the laboratories with substantial programmes on current drive.

JET starts up at Culham Laboratory

The Joint European Torus, second and largest of the new generation of tokamaks designed to achieve breakeven plasma conditions, was operated for the first time in June at the British Atomic Energy Agency's Culham Laboratory. This marks the culmination of a five‐year, £175 million construction effort, coordinated and largely financed by EURATOM, with additional contributions from associated European governments.