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We report the experimental operation of a voltage tunable gyrotron backward wave oscillator (gyro-BWO) in the frequency range near 140 GHz. Voltage tunability is an important feature of the gyro-BWO for application as a fast tuning source for driving high power free electron lasers or cyclotron autoresonance maser amplifiers. The gyro-BWO operated in an overmoded cylindrical waveguide structure in the TE1,2 mode. The electron beam source was a Pierce-wiggler gun producing an 80 kV, 6.2 A beam. Frequency tuning with voltage between 134 and 147 GHz was achieved in the TE1,2 mode with constant magnetic field. However, this tuning was found to be discontinuous. Output powers of up to 2 kW and 2% efficiency were found, significantly below theoretical predictions for a cold beam. The theoretical beam velocity spread was modeled by a 3D beam transport code. The code results show that space charge forces, coupled with the wiggler-induced helical motion and the short cyclotron wavelength of the beam, produce large increases in velocity spread in the magnetic compression region. A beam with smaller velocity spread would be needed to make the gyro-BWO operate at the desired efficiency.

IntroductionSexual behaviour changes during a person’s life course. Insights in sexual careers of men who have sex with men (MSM) may optimise timing of HIV prevention methods. Our objectives were to develop a behavioural risk score for HIV seroconversion and study trajectories of sexual behaviour over time.MethodsLongitudinal data from the Amsterdam Cohort Studies on HIV and AIDS (ACS) were used. We included HIV-negative MSM who visited the ACS at least once between May 2007 and April 2016 (n=820). MSM were bi-annually tested for HIV and completed a questionnaire about their sexual behaviour in the preceding six months. The following behavioural variables were selected a priori: number of casual partners with insertive or receptive anal intercourse (AI), condom use during AI, number of condomless AI partners with unknown or positive HIV status, and AI during group sex. Using Poisson regression, we calculated a risk score for HIV seroconversion. This score was used in growth mixture modelling to examine trajectories of sexual behaviour since first male sexual contact.ResultsDuring follow-up 49 MSM seroconverted. Predictors of seroconversion in multivariable analyses were number of casual partners with receptive AI (log transformed IRR: 1.63, 95% CI: 1.20–2.22) and number of condomless insertive AI partners with unknown or positive HIV status (log transformed IRR: 1.73, 95% CI: 1.24–2.42). We identified 3 sexual risk behaviour trajectories: Decreasing high risk (7% of MSM) with high risk behaviour at start of sexual career and decreasing risk over time; Increasing high risk (3%) with low risk behaviour at start of sexual career and increasing risk over time; and Low risk (90%) with relatively low risk throughout the career. MSM in the decreasing high risk trajectory were younger at first sexual contact than MSM in the Low risk trajectory.ConclusionWe identified 3 distinct sexual career trajectories. Increasing risk behaviour during the life course was found in a small group of MSM. The trajectories of these men might provide clues for time-tailored interventions.