RESUMO
Laser-driven accelerators, in which particles are accelerated by the electric field of a plasma wave (the wakefield) driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV m(-1) (refs 1-3). These fields are thousands of times greater than those achievable in conventional radio-frequency accelerators, spurring interest in laser accelerators as compact next-generation sources of energetic electrons and radiation. To date, however, acceleration distances have been severely limited by the lack of a controllable method for extending the propagation distance of the focused laser pulse. The ensuing short acceleration distance results in low-energy beams with 100 per cent electron energy spread, which limits potential applications. Here we demonstrate a laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 10(9) electrons above 80 MeV). Our technique involves the use of a preformed plasma density channel to guide a relativistically intense laser, resulting in a longer propagation distance. The results open the way for compact and tunable high-brightness sources of electrons and radiation.
RESUMO
PURPOSE: We report 52 percutaneous urterolithotomies in 51 patients having large, impacted middle ureteral stones. Direct percutaneous stone removal can be performed as successfully as in cases of renal stones treated with percutaneous nephrolithotomy. METHODS: The operation is performed under local anesthesia; therefore, the procedure is quicker and simpler than the laparoscopic or retroperitoneoscopic intervention. All patients became stone free. In two patients (4%), ultrasound disintegration was necessary; in the remaining cases, there was no need for any fragmentation: the stone was removed intact. A retroperitoneal drain was always left at the end of the procedure. With the exception of two cases, the ureter was always stented without closure of the ureteral incision. RESULTS: Fever (> or = 38 degrees C) was observed in 15 patients (29%) for 2 days. Retroperitoneal hematoma 5 cm in diameter was seen in one patient. One patient had urine leakage through the retroperitoneal drain in the postoperative period for 18 days. Also, one patient came back 3 days after discharge with urine leakage through the percutaneous retroperitoneal tract. CONCLUSION: Direct percutaneous ureterolithotomy is an effective way to remove impacted middle ureteral stones but is advisable only for endourologists with considerable experience.
