No abstract

Pre-ionization experiments have been performed on a tokamak by injecting about 80 kW of microwave power at 35 GHz for up to 15 ms. Microwave absorption occurs at the electron cyclotron and upper hybrid resonance frequencies as predicted by theory. Pre-ionization causes substantial (40%) reductions in loop voltage during the initial phase of the tokamak shot. Flux (volt-second) savings with pre-ionization are about 30% in the first 2 ms or about 2% of the total flux expenditure in a tokamak shot. The plasma current begins 200 μs earlier and rises 1.4 times more rapidly in the pre-ionized case. Electron densities of 5 × 1012 cm−3 can be sustained throughout the microwave pulse with only a toroidal magnetic field during microwave injection. The bulk electron temperature in the pre-ionized plasma is about 10 eV although there are indications of higher electron temperatures (50 eV) in the upper hybrid resonance layer. Although questions exist concerning the quiescent behaviour of the pre-ionized plasma, the observed parameters are shown to be consistent with a theory which employs classical models of energy and particle balance. During the early stages of Ohmic heating, the pre-ionization is effective in decreasing the peak of the radiated power.

Results are presented for the first experiments in which long-pulse (0.4–1 μsec), relativistic (0.8 MV) electron beams have been transported in the ion focused regime (IFR) in ion channels formed in low pressure diethylaniline gas by means of KrF excimer laser-induced ionization. These experiments demonstrate that the most efficient (50%–80%) and longest pulse (0.6 μsec) e-beam transport is obtained with laser-induced channels over a very narrow gas pressure range (0.3–1.7 mTorr). Higher than optimal pressures cause excess e-beam-induced ionization and instability of the electron beam. At lower pressures, the laser-induced ion channel density is insufficient for initial e-beam guidance. Transverse oscillations of the electron beam have been measured at a frequency close to that predicted for the ion hose instability. The growth length and wavelength of the transverse oscillations are comparable to the betatron wavelength, further suggesting that these oscillations result from the ion hose instability.

We report the results of a free-electron laser experiment which has produced l-2-/xsec duration pulses of ~ 30-GHz radiation at power levels up to 4 MW. This is the first freeelectron laser in the high-current operating regime to employ no external electron-focusing field. Consequently, there is no cyclotron emission, and the output radiation is unambiguously identified as free-electron laser emission.PACS numbers: 42.55.-fCurrently, many laboratories in several countries around the world are investigating the freeelectron-laser (FEL) mechanism. 1 This mechanism has the potential to lead to high-power, tunable, and efficient radiation sources that operate over a broad frequency range of the spectrum.In this paper, we report results from a FEL experiment that has two novel features. First, there is not a guide magnetic field in the interaction region, although the electron current is relatively high, i.e., about 200 A. As a consequence of this feature, the cyclotron radiation that has plagued other experiments 2 is absent, and thus the FEL modes can be unambiguously identified and analyzed. Second, the duration of the beam pulse is 2 /xsec, i.e., about 40 times longer than in previous, high-beamcurrent FEL experiments. The long pulse duration of the present experiment can provide valuable information on the FEL saturation mechanism and other nonlinear effects.Although our experiment is uniquely suited to operate in the oscillator mode, the results reported in this paper are from a superradiant amplifier experiment, and can be briefly summarized as follows: After propagating through the accelerator for ~ 3 m, the 700-keV, 2-/xsec-long electron pulse enters the interaction region. A helical magnetic wiggler drives the FEL and provides all the electron focusing. Since there is no cyclotron emission, the observed microwave output (4 MW at -30 GHz) is unequivocally FEL emission. The radiation is tunable with beam energy as predicted. The duration of the microwave pulse is as long as 2 /JLSQC and its linewidth ~ 10%. Finally, the shape of the radiation pulse suggests that saturation does not occur for at least 50% of the beam-pulse duration.The electron beam is generated by a linear induction accelerator (LIA), which was designed and constructed at the National Bureau of Standards. 3

Faraday rotation has been measured at 10.59 and 9.66 μm in n-type InSb at ∼6 K having a carrier concentration of ∼2×1014 cm−3, using magnetic fields up to 50 kG. In addition to the interband and free-carrier plasma contributions to the Faraday rotation, an appreciable contribution from the conduction electron spins has been observed. The latter contribution is proportional to the spin alignment that saturates at magnetic fields exceeding ∼5 kG. Its saturation value of −17±1°/cm at 10.59 μm is in good agreement with the value calculated from published theoretical results. A value of −1.8±0.1°/kG cm has been deduced for the interband contribution at 10.59 μm, in disagreement with the previously published value of −3.8°/kG cm. Design for a 10 μm Faraday isolator is proposed for operation at ∼5 kG, a field well within the range of permanent magnets.