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A pulse of methane gas is injected into the DIVA tokamak. The major portion of the injected impurity is shielded by the outer edge plasma, i.e. only 20% of the injected carbon are introduced into the discharge. After radial penetration of the injected carbon has been accomplished, accumulation doubles and a new steady state is sustained over the period of proton confinement. The transient radial diffusion process of the carbon is compared with calculations using an impurity transport code. The behaviour of the carbon impurity in the whole minor radius is well described by assuming perfect re-cycling at the gold-plated shell surface and using neoclassical diffusion superposed by anomalous diffusion of protons. It is shown that spreading caused by anomalous diffusion is important. The calculated result indicates that the carbon ions are lost dominantly in a He-like state.
The transport of a relativistic electron beam was studied experimentally and numerically in a magnetic field configuration which consists of a focusing wiggler as well as an additional axial guide field. A new beam divergence was found for the lower side of the magnetoresonance. It was identified as due to the effect of the second harmonic mode of the cyclotron frequency in the resonance, determined by /jft i i -A^rn for n =2. The harmonic resonance was detected for the planar wiggler first, and it could appear also for the helical one. PACS numbers: 41.75.Ht, 41.85.Ja, 52.75.MsSuccessful operation of a Raman free electron laser (FEL) using a relativistic electron beam (REB) has been reported for both planar and helical wiggler configurations [1][2][3][4]. To obtain a high-power performance of the FEL, a stable transport of the beam in the wiggler is one of the key issues. The introduction of an axial uniform guide field superimposed on the wiggler is usually required to keep the orbital stability in the transport, while it will involve some other complex features like the magnetoresonance between the cyclotron motions and the wiggling of the electrons [5]. It is well recognized that the beam diverges and spills out when the electron cyclotron frequency flu in the guide field B g approaches the wiggling frequency K w v\\, where k w is a wiggler wave number, Clw^eBg/yorrie, yo = {l "~ (v 2 /c 2 )} _1 *, v and c are electron and light speed, respectively, and the suffix II indicates axial components [6].In this Letter we describe an experimental and numerical study on the transport of a REB (£6~~0.8 MeV and /6~300 A) through a combination of a focusing type planar wiggler [7] and an axial field: We retain a relatively low additional B g to control the beam dynamics at the entrance of the wiggler and the FEL performance. We found a new type of beam loss at an axial field far lower than the value of B g corresponding to the fundamental magnetoresonance flii^&wm; e.g., the loss is in the parameter region of the so-called group I. Therefore it clearly differs from the "harmonic" gyroresonance which was predicted to occur in group II at flu ^nk w v\\ (n an integer) by Chu and Lin [8], where ft H is a timeaveraged cyclotron frequency in B\\. We need another explanation for the new resonance. We made an analytical study and a numerical simulation on the resonance and its effect on the beam transport through the wiggler. We carried out a similar numerical study for the case of a helical wiggler and the result suggests we will also have the same kind of resonance and beam degradation. The magnetic field in a planar focusing wiggler with a uniform field B g G z is given by B(r) = s
We have developed a high-current laser ion source that can provide low-charge-state ions for induction accelerators. A frequency-doubled Nd:YAG laser (λ=532 nm) irradiated a copper target with relatively low intensities of 108–109 W/cm2. The laser-produced plasma supplied a large number of Cu+ and Cu2+ ions over 1014 ions/cm2 at a distance of 10 cm from the target. We have successfully extracted high-current copper beams over 0.1 A/cm2 using an inductive acceleration module. The beam emittance measured by a pepper-pot method was about 40 πmm mrad. © 2000 American Institute of Physics.
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