The reaction e+e~e+e m. m has been analyzed using 97 pb ' of data taken with the Crystal Ball detector at the DESY e e+ storage ring DORIS II at beam energies around 5.3 GeV. For the first time we have measured the cross section for yy~m. m. for n m invariant masses ranging from threshold to about 2 GeV. We measure an approximately flat cross section of about 10 nb for 8'=m 0 0 (0.8 GeV, which is below 0.6 GeV, in good agreement with a theoretical prediction 'tr n' based on an unitarized Born-term model. At higher invariant masses we observe formation of the ft(1270) resonance and a hint of the fo(975). We deduce the following two-photon widths: I rr(f, (1270)) =3.19+0. 1620 z, keV and I "(fo( 975)) (0.53 keV at 90% CL. The decayangular distributions show the m~system to be dominantly spin 0 for W &0.7 GeV and spin 2, helicity 2 in the f, (1270) region, with helicity 0 contributing at most 22% (90% C.L.).

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(to be submitted to Zeit. Phys. C} for muons, and ~ 40% for electrons.level, on any new source of lepton pairs is ~ 20% of the hadronic decay contribution of the major hadronic sources is set by the data. The upper limit, at 90% confidence decays, and there is no need to invoke any "unconventional" source. The normalisation the low-mass spectrum can be explained satisfactorily by lepton pairs from hadronic p-Be collisions at 450 GeV/ c at the CERN SPS. For both electron and muon pairsWe report on the production of low-mass electron pairs and muon pa.irs in Area. An overview of the apparatus is shown in Figure la. The main components are OCR OutputThe HELIOS spectrometer is situated in the H8 beam line of the CERN SPS North 2.2 Apparatus reliably and stably throughout the experiment.The intensity was ~ 106 per burst. The targeting of the beam on to the wire worked 0.1%), a transverse diameter less than 50 pm and divergence ~ 0.2 mrad at the target.eriment to match the wire target. This beam has excellent momentum resolution (6p/ p A special 450 GeV/ c proton f'micro"-bea.m was developed for the HELIOS exp from the decay of hadrons produced in the interaction.of only 125 pm diameter, in order to minimize the radiation length traversed by photons the design. Accordingly, we have used a 4 cm long (10% interaction length) Be wire target experiment, and so the suppression of e'*'c" pairs from conversions was a key feature ofThe study of low-mass lepton pairs was one of the prime motivations of the HELIOS analysis is presented in section 4, and in section 5 results are summarised and conclusions ing, and data-taking, followed by the event reconstruction and selection in section 3. TheThe plan of this paper is as follows. In section 2 we describe the apparatus, trigger any "unconventional" source. Upper limits on any new source are presented.can be accounted for by lepton pairs from the decay of hadrons, and there is no need forThe main result is that low-mass lepton pairs, produced centrally at `/E 2 29 GeV, a measurement of the total charged multiplicity of the event.electron identincation by both transition radiation and calorimetry; a double measurement of the momentum (or energy) of both muons and electrons;Other noteworthy points are:of certain Dalitz decay modes;the measurement of photons as well as charged leptons, affording direct measurement the detector, producing two essentially independent measurements of lepton pairs; the analysis of both electron pairs and muon pairs, emphasising different aspects of from conventional sources. The most important features of the experimental approach are:Be collisions (\/Z cx 29 GeV) in the central rapidity region, is compared to the expectationIn this paper the production of electron and muon pairs, produced in 450 GeV/ c p their production level in ordinary hadronic collisions.plasma formation in relativistic heavy-ion collisions [3]: it is essential then to understand process Furthermore, lepton pairs have been suggested as a signature for quark-gluon implied by conven...

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Features incorporated in the design of the International Thermonuclear Experimental Reactor (ITER) tokamak and its ancillary and plasma diagnostic systems that facilitate operation and control of ignited and/or high Q DT plasmas are presented. Control methods based upon straightforward extrapolation of techniques employed in the present generation of tokamaks are found to be adequate and effective for ITER plasma control with fusion powers of up to 1.5 GW and burn durations of ⩾ 1000 s. Examples of simulations of key plasma control functions, including plasma magnetic configuration control and fusion burn (power) control, are given. The prospects for the creation and control of steady state plasmas sustained by non-inductive current drive and bootstrap current are also discussed.