Field Emission Studies Toward Improving the Performance of DC High Voltage Photoelectron Guns

Field emission is the main mechanism that prevents DC high voltage photoemission electron guns from operating at the very high bias voltages required to produce low emittance beams. Gas conditioning is shown to eliminate field emission from cathode electrodes used inside DC high voltage photoelectron guns. Measurements and simulation results indicate that gas conditioning eliminates field emission from cathode electrodes via two mechanisms: sputtering and implantation, with the benefits of implantation reversed by heating the electrode. The field emission characteristics of 5 stainless steel electrodes varied significantly upon the initial application of voltage but improved to nearly the same level after helium and krypton gas conditioning, exhibiting less than 10 pA field emission at - 225kV bias voltage with a 50 mm cathode/anode gap, corresponding to a field strength ∼ 13 MV/m. Field emission could be reduced with either krypton or helium, but there were conditions related to gas choice, voltage and field strength that were more favorable than others. The field emission characteristics of niobium electrodes were compared to those of stainless steel electrodes using a DC high voltage field emission test apparatus. Out of 8 electrodes (6 niobium and 2 stainless steel), the best niobium electrode performed better than the best stainless steel electrodes. Large grain niobium exhibited no measurable field emission (\u3c 10 pA) at 225 kV with 20 mm cathode/anode gap, corresponding to a field strength of 18.7 MV/m. Surface evaluation of all electrodes suggested no correlation between the surface roughness and the field emission current. Removing surface particulate contaminations and protrusions using an effective polishing and cleaning technique helps to prevent field emission. Mechanical polishing using silicon carbide paper and diamond paste is a common method of obtaining a mirror like surface finish on the cathode electrodes. However, it sometimes results rolled-over tips and embedded contamination. A different polishing technique was considered: electropolishing. Three stainless steel cathode electrodes with different initial surface roughness were electropolished by a commercial vendor, and evaluated inside a high voltage test stand. They exhibited less field emission than the diamond paste polished electrodes at the initial application of high voltage; but they were less receptive to ion implantation, which is a beneficial aspect of gas conditioning that serves to increase the work function of the cathode surface. Ultimately, the electropolished electrodes exhibited more field emission than diamond-paste polished electrodes

Evaluation of electropolished stainless steel electrodes for use in DC high voltage photoelectron guns

DC high voltage photoelectron guns are used to produce polarized electron beams for accelerator-based nuclear and high-energy physics research. Low-level field emission (similar to nA) from the cathode electrode degrades the vacuum within the photogun and reduces the photoelectron yield of the delicate GaAs-based photocathode used to produce the electron beams. High-level field emission (\u3e mu A) can cause significant damage the photogun. To minimize field emission, stainless steel electrodes are typically diamond-paste polished, a labor-intensive process often yielding field emission performance with a high degree of variability, sample to sample. As an alternative approach and as comparative study, the performance of electrodes electropolished by conventional commercially available methods is presented. Our observations indicate the electropolished electrodes exhibited less field emission upon the initial application of high voltage, but showed less improvement with gas conditioning compared to the diamond-paste polished electrodes. In contrast, the diamond-paste polished electrodes responded favorably to gas conditioning, and ultimately reached higher voltages and field strengths without field emission, compared to electrodes that were only electropolished. The best performing electrode was one that was both diamond-paste polished and electropolished, reaching a field strength of 18.7 MV/m while generating less than 100 pA of field emission. The authors speculate that the combined processes were the most effective at reducing both large and small scale topography. However, surface science evaluation indicates topography cannot be the only relevant parameter when it comes to predicting field emission performance. (C) 2015 American Vacuum Society

Surface finish control by electrochemical polishing in stainless steel 316 pipes

Electrochemical machining (ECM) is a non-conventional machining process which is based on the localised anodic dissolution of any conductive material. One of the main applications of ECM is the polishing of materials with enhanced characteristics, such as high strength, heat-resistance or corrosion-resistance, i.e. electrochemical polishing. The present work presents an evaluation of the parameters involved in the ECM of Stainless Steel 316 (SS316) with the objective of predicting the resulting surface finish on the sample. The interest of studying ECM on SS316 resides on the fact that a repeatable surface finish is not easily achieved. ECM experimental tests on SS316 pipes of 1.5" (0.0381 m) diameter were conducted by varying machining parameters such as voltage, interelectrode gap, electrolyte inlet temperature, and electrolyte flow rate. The surface finish of the samples was then evaluated in order to find the significance of each of these parameters on the surface quality of the end product. Results showed that overvoltage, which is dependent on the interelectrode gap and the electrolyte temperature, is one of the main parameters affecting the surface finish; additionally there is a strong relationship between the resulting surface finish and the electrolyte flow. The interelectrode gap and inlet electrolyte temperature also affect the resulting surface finish but their influence was not so evident in this work. Finally, the variation of the electrolyte temperature during the process was found to have a great impact on the uniformity of the surface finish along the sample. We believe that this contribution enables the tailoring of the surface finish to specific applications while reducing manufacturing costs and duration of the ECM process

Evaluation of Niobium as Candidate Electrode Material for DC High Voltage Photoelectron Guns

The field emission characteristics of niobium electrodes were compared to those of stainless steel electrodes using a DC high voltage field emission test apparatus. A total of eight electrodes were evaluated: two 304 stainless steel electrodes polished to mirror-like finish with diamond grit and six niobium electrodes (two single-crystal, two large-grain, and two fine-grain) that were chemically polished using a buffered-chemical acid solution. Upon the first application of high voltage, the best large-grain and single-crystal niobium electrodes performed better than the best stainless steel electrodes, exhibiting less field emission at comparable voltage and field strength. In all cases, field emission from electrodes (stainless steel and/or niobium) could be significantly reduced and sometimes completely eliminated, by introducing krypton gas into the vacuum chamber while the electrode was biased at high voltage. Of all the electrodes tested, a large-grain niobium electrode performed the best, exhibiting no measurable field emission (< 10 pA) at 225 kV with 20 mm cathode/anode gap, corresponding to a field strength of 18:7 MV/m

Studies of Breakdown in a Pressurized RF Cavity

Microscopic images of the surfaces of metallic electrodes used in high-pressure gas-filled 805 MHz RF cavity experiments [1] have been used to investigate the mechanism of RF breakdown [2]. The images show evidence for melting and boiling in small regions of ∼10 micron diameter on tungsten, molybdenum, and beryllium electrode surfaces. In these experiments, the dense hydrogen gas in the cavity prevents electrons or ions from being accelerated to high enough energy to participate in the breakdown process so that the only important variables are the fields and the metallic surfaces. The distributions of breakdown remnants on the electrode surfaces are compared to the maximum surface gradient E predicted by an ANSYS model of the cavity. The local surface density of spark remnants, proportional to the probability of breakdown, shows a strong exponential dependence on the maximum gradient, which is reminiscent of Fowler-Nordheim behavior of electron emission from a cold cathode. New simulation results have shown good agreement with the breakdown behaviour of the hydrogen gas in the Paschen region and have suggested improved behaviour with the addition of trace dopants such as SF6 [3]. Present efforts are to extend the computer model to include electrode breakdown phenomena and to use scanning tunnelling microscopy to search for work function differences between the conditioned and unconditioned parts of the electrodes

RF Breakdown of Metallic Surfaces in Hydrogen

In earlier reports, microscopic images of the surfaces of metallic electrodes used in high-pressure gas-filled 805 MHz RF cavity experiments were used to investigate the mechanism of RF breakdown of tungsten, molybdenum, and beryllium electrode surfaces. Plots of remnants were consistent with the breakdown events being due to field emission, due to the quantum mechanical tunnelling of electrons through a barrier as described by Fowler and Nordheim. In the work described here, these studies have been extended to include tin, aluminium, and copper. Contamination of the surfaces, discovered after the experiments concluded, have cast some doubt on the proper qualities to assign to the metallic surfaces. However, two significant results are noted. First, the maximum stable RF gradient of contaminated copper electrodes is higher than for a clean surface. Second, the addition of as little as 0.01% of SF6 to the hydrogen gas increased the maximum stable gradient, which implies that models of RF breakdown in hydrogen gas will be important to the study of metallic breakdown

Reporting a Right Sided 4th Branchial Anomaly; Case Report and Reporting the Technical Issues

Background: Fourth branchial pouch anomaly (4BPA) is a rare condition that often presents an extreme diagnostic and therapeutic challenge. It usually causes recurrent left thyroid or perithyroid tissue inflammation, but it can be situated in the right side too. Case presentation: In this case report we present a nine-year-old girl with right sided 4BPA. She was referred for evaluation of recurrent right muscular space mass and fistulathat was unresponsive to any medical or surgical procedures. This paper demonstrates the proper technical issues about diagnostic work up and surgical approach that we used to manage her dilemma. Conclusion: In patients with both skin fistula and pyriform sinus opening, external cannulation is more convenient and less traumatizing than internal cannulation

Integrated Internal Stabilization for Saddle Nose Surgery

Introduction: Correction of Saddle nose deformity is one of the most challenging issues in facial plastic surgery.   Materials and Methods: In this study, a single structure in the form of L-strut was attempted to be created by using one 0.035" Kirschner wire and an autologous costal graft out of the 10th and 11th ribs. This study involved 13 cases, most of whom were traumatic. The corrective surgical techniques used in this study will be described in detail.   Results: There was no warping, no rejection, and no infection in the created L-strut and patients’ satisfaction was very good during the follow up period.   Conclusion:  Surgical correction of a saddle-shaped nose using the described technique seems to be an acceptable and uncomplicated technique, and the cosmetic result is totally acceptable

The Efficacy of Clarithromycin in Patients with Severe Nasal Polyposis

Although several treatments have been suggested for nasal polyposis, from medical to surgical, there is no standard guideline for the management of this disease. During recent years increasing attention has been directed toward the effects of macrolide antibiotics on chronic sinusitis and nasal polyposis. In this study,  the efficacy of clarithromycin on severe nasal polyposis were examined. In a Prospective, before - after study, forty patients with severe nasal polyposis received clarithromycin 500 mg twice a day for 8 weeks. At the beginning and end of treatment, the severity of patients' symptoms (using subjective analogue scale), computed tomography (CT) scan and endoscopic findings were recorded. After treatment, the severity of nasal obstruction, smelling problems, Post Nasal Discharge and rhinorrhea decreased significantly (P<0.05). Furthermore, the degree of sinus opacification in CT scan and endoscopic findings showed significant improvement. Most patients completed their treatment course without significant side effects. Although a course of clarithromycin improved nasal symptoms, polyp size and CT findings, further studies with more patients are required to recommend this drug as a general treatment in nasal polyposis

A comparison between cold dissection tonsillectomy with harmonic scalpel tonsillectomy

Introduction: This study aimed to compare operation time, intraoperative bleeding and postoperative pain between cold dissection tonsillectomy and harmonic scalpel tonsillectomy.  Materials and Methods: In this single-blinded clinical trial, 32 patients aged 14–48 were enrolled. Each patient randomly underwent tonsillectomy using the harmonic scalpel on one side and cold dissection on the other side. Operation time and bleeding volume were measured during surgery. The pain intensity level on each side was recorded on the first and seventh postoperative days.  Results: The mean volume of intraoperative bleeding was 9.59 ml on the harmonic side and 74.38 ml on the cold dissection side; which represents a significantly lower amount on the harmonic side (