The modified ring test is used to determine the fracture toughness of synthetic, granular, fresh-water ice average density 0.891 Mg m−3 and firn (average density 0.605 Mg m 3) from depths between 26 and 27.2 m in the E core of the Greenland Ice Sheet Project II. Average fracture toughness is 145.7kPa m2 for the manufactured ice and 108.6kPam½ for the firn. Comparison between the ice and firn suggests that ice-fracture toughness decreases with decreasing density (i.e. increasing porosity), suggesting lateral and vertical variations in the near-surface fracture resistance of glaciers and ice sheets may be related to firn densification. The modified ring test has many advantages over conventional, notch-based specimens in that complications which arise in notched specimens due to crack-length, loading-rate, notch-acuity and specimen-size effects are irrelevant for a modified ring-specimen geometry.

We studied the recovery of memory and executive function in 10 patients following anterior communicating artery aneurysm (ACoA) rupture and repair. Patients were tested at 2 consecutive points in time following surgery (approximately at 2 and 3 months). At the first testing, the patients divided into 2 groups based on the severity of impairment on executive measures. Both groups had severe anterograde amnesia, but only patients with severe executive impairments had retrograde amnesia with a temporal gradient. At second testing, both groups had persistent severe anterograde amnesia. The dysexecutive group showed significant improvement in executive deficits and in retrograde amnesia, with attenuation of the temporal gradient. Patients with more severe executive impairments had more extensive bilateral frontal lesions than other patients. These results suggest that the cognitive profile following ACoA rupture changes with time. Time postonset following aneurysm rupture and lesion site are both critical for defining the neuropsychological profile, and determining the underlying cognitive mechanisms in this neurological disorder. (JINS, 1996, 2, 565–570.)

This talk will summarize the present state of the art of the quasioptical gyrotron (QOG), an alternative gyrotron configuration which has been under development primarily in the U.S. and Switzerland for heating fusion plasmas in tokamaks and other high-power millimeter-wave applications. The QOG features an open-mirror Fabry-Perot resonator instead of the conventional waveguide cavity used in conventional gyrotrons. This gives the QOG the potential for wide tunability, advantages for high-power operation, and facilitates the use of a depressed collector for spent electron beam energy recovery. An experimental QOG has been tuned from 85 to 130 GHz by varying the applied magnetic field. QOGs have produced peak powers up to 600 kW for 13 μs pulses and 100 kW for 10 ms pulses. Electronic efficiencies up to 22% have been achieved at 85 GHz, and operation with a depressed collector yielded an overall efficiency of 30%. The design of a multi-kW CW QOG tunable from 80 to 120 GHz is discussed.