Verification of computer-aided designs of traveling-wave tubes utilizing novel dynamic refocusers and graphite electrodes for the multistage depressed collector

A computational procedure for the design of TWT-refocuser-MDC systems was used to design a short dynamic refocusing system and highly efficient four-stage depressed collector for a 200-W, 8- to 18-GHz, TWT. The computations were carried out with advanced, multidimensional computer programs which model the electron beam as a series of disks of charge and follow their trajectories from the RF input of the TWT, through the slow-wave structure and refocusing section, to their points of impact in the depressed collector. Secondary emission losses in the MDC were treated semi-quantitatively by injecting a representative beam of secondary electrons into the MDC analysis at the point of impact of each primary beam. A comparison of computed and measured TWT and MDC performance showed very good agreement. The electrodes of the MDC were fabricated from a particular form of isotropic graphite that was selected for its low secondary electron yield, ease of machinability, and vacuum properties. This MDC was tested (at CW) for more than 1000 hr with negligible degradation in TWT and MDC performances

Part 2: CT characterisation of pancreatic neoplasm: tumour mimics

There are numerous pancreatic and peripancreatic conditions that can mimic pancreatic neoplasms. Many of these can be confidently diagnosed on computed tomography (CT), while others will require further imaging. Knowledge of these tumour mimics is important to avoid misclassification of benign conditions as malignant and to avoid unnecessary surgery. Mimics can be grouped as parenchymal, vascular, biliary and peripancreatic. These are discussed and illustrated in this review