Christos Iatrou

A 1.5-MW, 140-GHz, TE/sub 28,16/-coaxial cavity gyrotron

The design of a 1.5-MW, 140-GHz, TE-/sub 28,16/-coaxial cavity gyrotron is presented and results of experimental operation are given. A cavity with a cylindrical outer wall and a radially tapered inner rod with longitudinal corrugations was used. A maximum output power of 1.17 MW has been measured in the design mode with an efficiency of 27.2%. Single-mode operation has been found over a wide range of operating parameters. The experimental values agree well with the results of multimode calculations. Frequency-step tuning has been performed between 115.6 and 164.2 GHz. In particular, an output power of 0.9 MW has ben measured in the TE/sub 25,14/ mode at 123.0 GHz and 1.16 MW in the TE/sub 32,18/ mode at 158.9 GHz. At frequencies its with strong window reflections the parameter range for which stable operation is possible is reduced significantly. In order to obtain results relevant for a technical realization of a continuously operated gyrotron, a tube with a radial radio frequency (RF)-beam output through two output windows and a single-stage depressed collector has been designed and is under fabrication. A two-step mode conversion scheme-TE-/sub 28,16/ to Te/sub +76.2/ to TEM/sub 00/-which generates two narrowly directed (60/spl deg/ at the launcher) output wavebeams has been chosen for a quasioptical (q,o) mode converter system. A conversion efficiency of 94% is expected.

Single-stage depressed collectors for gyrotrons

Two 140 GHz gyrotrons with a single-step depressed collector have been operated. The different position of the isolating collector gap in the stray magnetic field causes the electron motion in the retarding region to be in one case adiabatic and in the other case nonadiabatic. The kind of motion within the retarding field influences strongly the behavior of the gyrotron with a depressed collector. In the case of nonadiabatic motion a significant amount of transverse momentum is given to the electrons reflected at the collector potential. This causes the reflected electrons to be trapped between the magnetic mirror and the collector. The electrons escape from the trap by diffusion across the magnetic field to the body of the tube thus contributing to the body current. Despite the high body current there is no observable influence of the collector voltage on the RF output power. In the case of adiabatic motion the reflected electrons do not gain a sufficient amount of transverse momentum to be trapped by the magnetic mirror. They pass the cavity toward the gun and they are trapped between the negative gun potential and the collector. The interaction with the RF field by electrons traveling through the cavity enhances the diffusion in the velocity space thus enabling the trapped electrons to overcome the potential barrier and escape toward the collector. Therefore the body current stays at low values since in this case the reflected electrons do not contribute to it. However, at higher collector voltages a reduction of RF power occurred and some noise in the electron beam was observed.

Design and experimental operation of a 165-GHz, 1.5-MW, coaxial-cavity gyrotron with axial RF output

The development of a coaxial-cavity gyrotron operating in TE/sub 31,17/ mode at 165 GHz is presented. The selection of the operating frequency and mode are based on the limitations imposed by the maximum held of the superconducting (sc) magnet at Forschungzentrum Karlsruhe, Institut fur Technische Physik (FZK), the use of the inverse-magnetron injection gun (IMIG) of the 140-GHz, TE/sub 28,16/ coaxial gyrotron and the possibility of transforming the cavity mode to a whispering gallery mode (WGM) appropriate for the dual-beam quasioptical (q.o.) output coupler and the two output windows, which are foreseen for the next lateral output version of the tube. The tube with axial output has been tested at FZK to deliver maximum output power of 1.17 MW in the designed TE/sub 31,17/ mode with 26.7% efficiency at 164.98 GHz. Maximum efficiency of 28.2% was achieved at 0.9-MW output power. The design operating point with output power 1.36 MW and 36.7% efficiency was net accessible because of beam instabilities at high electron-velocity ratio /spl alpha/, presumably caused due to high electron-velocity spread. Power at higher frequencies was also detected: 1.02 MW at 167.16 GHz in TE/sub 32,17/ mode with 26.88 efficiency, 0.63 MW at 169.46 GHz in TE/sub 33,17/ mode with 18% efficiency, and 0.35 MW at 171.80 GHz in TE/sub 31,17/ mode with 13.3% efficiency.

Factors Influencing the Decision to Start Renal Replacement Therapy: Results of a Survey Among European Nephrologists

BACKGROUND Little is known about the criteria nephrologists use in the decision of when to start renal replacement therapy (RRT) in early referred adult patients. We evaluated opinions of European nephrologists on the decision for when to start RRT. STUDY DESIGN European web-based survey. PREDICTORS Patient presentations described as uncomplicated patients, patients with unfavorable clinical and unfavorable social conditions, or patients with specific clinical, social, and logistical factors. SETTING & PARTICIPANTS Nephrologists from 11 European countries. OUTCOMES & MEASUREMENTS We studied opinions of European nephrologists about the influence of clinical, social, and logistical factors on decision making regarding when to start RRT, reflecting practices in place in 2009. Questions included target levels of kidney function at the start of RRT and factors accelerating or postponing RRT initiation. Using linear regression, we studied determinants of target estimated glomerular filtration rate (eGFR) at the start of RRT. RESULTS We received 433 completed surveys. The median target eGFR selected to start RRT in uncomplicated patients was 10.0 (25th-75th percentile, 8.0-10.0) mL/min/1.73 m(2). Level of excretory kidney function was considered the most important factor in decision making regarding uncomplicated patients (selected by 54% of respondents); in patients with unfavorable clinical versus social conditions, this factor was selected by 24% versus 32%, respectively. Acute clinical factors such as life-threatening hyperkalemia refractory to medical therapy (100%) and uremic pericarditis (98%) elicited a preference for an immediate start, whereas patient preference (69%) and vascular dementia (66%) postponed the start. Higher target eGFRs were reported by respondents from high- versus low-RRT-incidence countries (10.4 [95% CI, 9.9-10.9] vs 9.1 mL/min/1.73 m(2)) and from for-profit versus not-for-profit centers (10.1 [95% CI, 9.5-10.7] vs 9.5 mL/min/1.73 m(2)). LIMITATIONS We were unable to calculate the exact response rate and examined opinions rather than practice for 433 nephrologists. CONCLUSIONS Only for uncomplicated patients did half the nephrologists consider excretory kidney function as the most important factor. Future studies should assess the weight of each factor affecting decision making.

165 GHz, 1.5 MW-coaxial cavity gyrotron with depressed collector

A further step in the development of a coaxial-cavity gyrotron operated in the transverse electric TE/sub -31,17/ mode at 165 GHz is presented. The gyrotron has been equipped with a quasi-optical output system consisting of a Vlasov launcher with a single cut and two mirrors, one with a quasi-elliptic and the other with a nonquadratic phase correcting surface. The radio frequency (RF) power is transmitted through a single output window. A maximum output power of 1.7 MW has been achieved. At the nominal operational parameters an RF power of 1.3 MW with an efficiency of 27.3% has been measured. The efficiency increases to 41% in operation with a single-stage depressed collector.

Long-pulse operation of a 0.5 MW TE/sub 10.4/ gyrotron at 140 GHz

The operation features of a TE/sub 10.4/-mode gyrotron oscillator with a quasi-optical mode converter and a single-stage depressed collector at 140 GHz with an output power of 500 kW in long pulses of 0.2 s are presented. Measurements on long-pulse operation of the tube are described in detail, and the significant differences between short- and long-pulse operation concerning efficiency and output power are pointed out. The variation of frequency during a pulse and an irreversible frequency shift during long-pulse operation were measured and are discussed with respect to gyrotron design.

Coaxial cavity gyrotron with dual RF beam output

A 140-GHz, 1.5-MW, TE/sub 28,16/-coaxial cavity gyrotron with a dual RF beam output has been designed, built, and tested. For the first time, the generated RF power has been split into two parts and coupled out through two RF output windows in order to reduce the power loading in the windows. The quasioptical output system is based on a two-step mode conversion scheme. First, the cavity mode TE/sub -28,16/ is converted into its degenerate whispering gallery mode TE/sub +76,2/ using a rippled-wall mode converter. Then, this mode is transformed into two TEM/sub 00/ output wave beams. A maximum rf output power of about 950 kW with an output efficiency of 20% has been measured. According to numerical calculations, an rf power above 1.5 MW is expected to be generated in the cavity. Even if all losses are taken into account, a discrepancy between experiment and calculations remains. The power deficit seems to be partly caused by the influence of the stray radiation captured inside the tube. However, the two main reasons are probably an incomplete mode conversion from TE/sub -28,16/ to TE/sub +76,2/ and a large energy spread of the electron beam due to trapped electrons. An increased amount of captured stray radiation resulted in a reduced stability of operation. A single-stage depressed collector was used successfully, increasing the RF output efficiency from 20% to 29%.

140-GHz gyrotron with multimegawatt output power

The operational features of a 140-GHz, transverse electric TE/sub 22,6/ mode gyrotron oscillator with an advanced quasi-optical mode converter, a Brewster window, and a single-stage depressed collector at 140 GHz with an output power of 2.1 MW and an efficiency of 34% without depressed collector and 53% with depressed collector are presented. The high output power level is possible due to an almost reflectionless termination of the radio frequency (RF) beam line outside the tube. The operation of the TE/sub 22,6/ mode gyrotron is described in detail, and the significant features for achieving the high-output power are pointed out.

Design considerations for powerful continuous-wave second-Cyclotron-harmonic coaxial-cavity gyrotrons

The mode-selection ability of coaxial gyrotron cavities with a longitudinally corrugated inner conductor is systematically examined, as regards powerful, continuous-wave (CW) operation at the second harmonic of the electron-cyclotron frequency. Suitable cavity geometries for the excitation of a high-order mode of eigenvalue around 100 at the second harmonic have been identified, employing coaxial inserts both of high and of low conductivity. Focusing on submillimeter-wave radiation, we investigate frequency and CW limitations on efficiency, output power, and cavity machining and we present pertinent design considerations. The feasibility of powerful and efficient, CW, submillimeter-wave operation of coaxial cavities at the second harmonic is illustrated by two realistic designs of a second-harmonic gyrotron delivering a power over 100 kW CW at 340 GHz with electronic efficiency around 29%. The performance of the designs is confirmed by independent numerical simulations, which in addition show weak sensitivity to a spread in the electron velocity ratio.

A coaxial Gyro-TWT

The theory of a coaxial waveguide interaction structure for probable application in gyro-TWTs is presented. Relevant dispersion and gain are computed and appreciated. The basic motive behind the coaxial insertion is addressed and results are discussed.