Synchroscan streak camera imaging at a 15-MeV photoinjector with emittance exchange

At the Fermilab A0 photoinjector facility, bunch-length measurements of the laser micropulse and the e-beam micropulse have been done in the past with a fast single-sweep module of the Hamamatsu C5680 streak camera with an intrinsic shot-to-shot trigger jitter of 10-20ps. We have upgraded the camera system with the synchroscan module tuned to 81.25MHz to provide synchronous summing capability with less than 1.5ps FWHM trigger jitter and a phase-locked delay box to provide phase stability of ~1ps over 10s of minutes. These steps allowed us to measure both the UV laser pulse train at 263nm and the e-beam via optical transition radiation (OTR). Due to the low electron beam energies and OTR signals, we typically summed over 50 micropulses with 0.25-1nC per micropulse. The phase-locked delay box allowed us to assess chromatic temporal effects and instigated another upgrade to an all-mirror input optics barrel. In addition, we added a slow sweep horizontal deflection plug-in unit to provide dual-sweep capability for the streak camera. We report on a series of measurements made during the commissioning of these upgrades including bunch-length and phase effects using the emittance exchange beamline and simultaneous imaging of a UV drive laser component, OTR, and the 800nm diagnostics laser.Comment: 26 p

Effects of soil mesofauna and microclimate on nitrogen dynamics in leaf litter decomposition along an elevation gradient

The nutrient dynamics in decomposing litter has proved to play a crucial role in regulating the nutrient status of ecosystems and vegetation productivity. Little is known, however, about the effect mechanism of decomposer and its environment on the nitrogen (N) mineralisation of litter along an elevation gradient in such sites. We investigated the effects of soil mesofauna using litter bags on N dynamics in decomposing Castanopsis carlesii litter along an elevation gradient across four types of ecosystems in southeastern China: evergreen broadleaf forest (EVB), coniferous forest (COF), dwarf forest (DWF) and alpine meadow (ALM). The mean contribution of soil mesofauna to N concentration was 12.6% at EVB, 10.6% at COF, 5.4% at DWF, and 3.1% at ALM. The N concentration of litter with mesofauna was significantly related to H′ (Shannon-Wiener) and GN (group number) across the four sites. The effects of soil mesofauna on N concentration of litter were significantly linked to some special faunal groups, including Oribatida, Mesostigmata and Collembola. The N concentration in litter bags were positively related with mean annual air temperature, soil temperature and litter moisture along the elevation gradient. We concluded that the rapid accumulation of N in lower elevation sites during the first few months can result in the retention of mobile N in soils and the effects of soil mesofauna on N dynamics may be intimately associated with microclimate (warm and humid) and faunal diversity along the elevation gradient.Key words: Nitrogen concentration, mesofauna, faunal diversity, elevation gradient, Wuyi National Nature Reserve, China

Emission Optics of the Steigerwald Type Electron Gun

The emission optics of a Steigerwald type electron gun is re-examined. The virtual and real points of divergence, divergence angles and beam-widths of the electron beams at different telefocusing strength are measured in detail for first time . Two different Wehnelt cylinders are used to establish a contrasting viewpoint. The original `focusing' curves measured by Braucks are reconstructed and will be explained only through a `new' interpretation which is different from the conventional views. While the image of the emitting surface in front of the filament is indeed telefocused beyond the anode, the envelope of the beam does not `focus' as expected. A new model for the emission mechanism is established based on our results.Comment: 14 pages, 10 figure