Hello.
Currently in our gas discharge plasma lab, the discharge unit is connected to discharge system via single wires, the wires to run and return current. It has been clear that rising time of the discharge current is depending on length of the wires thus we always prefers short length to reduce inductance, a major factor for rising time of LC circuits.
I've heard from my friend that maybe HV coaxial cable is better to not only prevent from EMI out from lines to air but also get minimum inductance.
Is this really true? I've thought twisted wire is the best in inductance by canceling magnetic field from input and return current but not considered coaxial cable in this purpose so far.
Is really true, could you also recommend good manufacturer for this cable?
Hi there
You need absolutely use HV coaxial cables, and as shorter as posible. There are various specifications and you need to check the one best fi your purpose. In addition, if you use multi capacitor banks, you need make sure all the cables are same length to make sure the wavefront sharp. You may also need to use triggered spark gaps to contorl the discharge of the current enough large.
kind regards
Mingfang Lu
Professor of Plasma Physics and currently
Chief Representative/Editor-in-Chief of IOP Publishing, China
Thanks! I've got detailed information from you. In this case, could you suggest me any good HV coaxial cable manufacturer? And which line you mean that length is the same? the line between each capacitor in array?
Dear Donggyu Jang
To answer your question, it is worth to consider the basic definition of inductance, which is the ratio between looped magnetic flux and the electric current that created it (L = λ/I). With this in mind, it is easy to understand that coaxial arrangements, not only cables, are the best choice to minimize inductance and this type of arrangement is really used in high voltage labs. This is the case, for instance, with coaxial shunts used in high voltage labs as converting device to measure fast impulse current. What happens in practice is that the current flows through the inner conductor of the coaxial cable in one direction and returns through the outer coaxial shield in the contrary direction. What is particular in coaxial arrangement is that the outer returning current is distributed in such a way that forms a kind of outer cylinder of current establishing a type of cage. In this way, the magnetic flux created by the inner conductor current is canceled by the flux created by the cable shield current so that the flux in the region inside the cable, between the inner conductor and the shield, is ideally canceled. This region is the main electrical insulation of the cable. Then, by definition: no condition for looped flux to occur ==> no inductance (please, see the attached slide. Click three or more times on it to see the slide animation. If it works, it will be easier to understand). In practice, some flux is not canceled inside and it is looped causing a small inductance. The cable has also unavoidable self-inductances due the magnetic flux penetration in the conductor and it counts in the computation of the surge impedance of the cable (Z=√L/C) and, depending on how fast is the current, the coaxial cable must be matched at his end terminal with an non-inductive resistor with resistance equal to the surge impedance of the cable, to avoid reflections. As pointed out by the colleague here, the cable must be as short as possible, among other reasons, to minimize attenuation since it is not lossless. The whole arrangement used in your case must be optimized if you want to keep the inductance as low as possible in order to generate or measure fast current. Among good practices there are: avoid electric current concentration using radial circuit arrangements and copper sheet for connections between high voltage components, instead of ordinary wires, connections as short as possible and compact circuit taking into account necessary clearances due to high-voltages. Good luck.
Thanks Orsino Borges Filho. Very detailed explanation. I just wonder the purpose of the circuit in your presentation. Thus "connector coaxial BNC" is just normal BNC connector for Oscilloscope right? while Left thing is path of the impulse current generated somewhere else. I've just wonder what is gray line at the axis of Resistive cylinder? If it is normal conductor why current choose resistive path instead of center conductor of low resistance?
Please, see attached slide with more detailed information. It is a 2D representation of a coaxial resistive shunt for high impulse current measurements (kA) and its measuring circuit. Rm is typically in milliohm range. Vm is the voltage drop in Rm and is typically in Volts. This kind of measuring circuit is used in surge arrester lightning current test. High frequency current transformer can be also used for the same purpose.