Re: Re: Re: High voltage in vaccuum

Subject: Re: Re: Re: Re: High voltage in vaccuum

Comments:
: : : : Hello, : : : : I wonder what is the maximum field that vaccuum : : : : can withstand without breakdown ? : : : : I also wonder if this is the right question : : : : : - It depends on the quality of the vaccuum. : : : : Let's assume a vaccuum of high quality : < 1e-8 bar : : : : But isn't there also a question of polarity and geometry ? : : : : A high field at the cathode should be more dangerous than : : : : a high field at the anode ? (cold cathode emission) : : : : ... : : : : -Chris : : : : : : I have found some papers where it is explained : : : that a high field on the cathode produces : : : electron emission by field effect according : : : to the "corrected" Fowler-Nordheim equation : : : : Theorically fields as high as 10MV/cm and more : : : could be reached on a metal surface but a correction : : : must be taken into account due to gas adsorbed : : : at electrodes and this values goes down... : : : This value can then be improved by a special : : : treatment of the surfaces. : : : Does anyone know down to what value in practice ? : : : Thanks, : : : -Chris : : Hi Chris, : : I also am interested in learning about this subject. I always wondered what would happen if one were to apply a voltage (say, 1000vdc) between two electrodes, widely separated, in the "perfect" vacuum of outer space. Then slowely bring the electrodes together until they make contact. Would the voltage "jump" an arc at some critical distance between the electrodes? Would the same thing happen at say, One Volt, but at a shorter distance? What if you applied 100 billion volts? I have a suspicion that there are many variables to consider in all this (temperature of electrodes, out-gassing of electrode material, electrode surface shape and whether they are smooth or sharply-pointed, etc.). I wonder if anyone has ever done any serious research in this field (no pun intended). : : Mike Gray : Mike, Chris : The topic is fairly widely discussed in various physics texts and in some IEEE documents. I found most of my information over the years in conflicting versions from Gearge Farrall (GE) and R.V. Latham in England. : For my money, Mr. Latham provided much better insight into the topic than did George, a personal friend in years past. Both men are certainly brilliant, and, though they worked in somewhat different arenas, were occasionally at odds over topical analyses. : I treat Mr Lathams book on High Voltage, High Vacuum insulation as a bible for my personal researches. : Much of the data available in the industry is useful for lining a pigeon's cage. Much of these gentlemen's work is sacred. : The breakdown effect is highly surface dependent, and material dependent as well. Both of you gentlemen are involved in the utilization and/or design of devices used to produce x-Rays. In order to benefit from Mr. Latham's studies, you will have to think more heavily about the source of X-Rays themselves, than you may have originally intended. : As a labor of the latter half of my life, I have found it rewarding and even lucrative. Should you care to expend some effort in understanding their work, it will certainly be mentally rewarding. Bear in mind that the results they achieved stand on the backs of thousands of other researchers who laid the groundwork, from physics and engineering, who are reliably referred to in their writings. : Bear in mind that there is no pure vacuum anywhere and that there are some very interesting contact resistance results available from a husband and wife pair of researchers from St Mary's, PA. (Comment about one volt) : DAve Dilick