Back in June I posted a blog discussing what made APELC Marx Generators special, which included a description of Marx generators and then discussed how APELC designs our Marxes to have tailored source impedances and waveform characteristics (rise-time, pulse-width etc…). If you have not read that yet, I would recommend giving that entry a look before, or shortly after reading this blog to get a better understanding of our Marx generators (and Marx generators in general). That can be found here: 5 Things to know about APELC Marx Generators
In this blog, I would like to focus more on the practical aspects of down-selecting a Marx generator based upon your application. Customers who already know the exact specifications they are looking for can simply go to our main Marx selection table and peruse the various generator specifications to find the closest Marx that meets their needs. You can find that here.
If the exact specifications are not being met, it may be useful to understand our part numbering scheme and a little bit about Marx generator design to proceed. Our Marx generator part numbers describe the number of stages, number of capacitors per stage, and the individual stage capacitor value, as can be seen in the example below.
Using the MG15-3C-940PF as an example, let us say that the MG15 meets the customers desired temporal specifications (rise-time and pulse-width) as can be seen in the specification table for the Marx (see here), however, the customer needs a higher erected voltage. For the sake of this example, let’s say they need 400kV onto 50 ohms. The first step is for the customer to discuss this requirement with us so that we can help design the best possible generator. However, this blog is meant to give some understanding behind that design process to help our customers better understand the technology, so we will take things a step further.
We would want to first know the load impedance from the customer. This is something that can often be difficult, because some loads (e.g. vacuum diodes) can have variable impedances. That said, understanding this impedance profile is critical in designing the source. To help you understand this reasoning, let’s continue with the MG15 example. Knowing that a Marx generator multiplies the output voltage based upon the number of stages would lead us to intuitively believe that simply adding stages to the MG15-3C-940PF would achieve the desired results. This is not however the case if we are to maintain a specific impedance, pulse width and rise-time. First let us take a look at some estimated specifications for the MG15. In this example we can see that we are only achieving 255kV onto a 50 ohm load.
Table 1 MG15-3C-940PF into 50ohms at 35kV charge
|
|
Now let’s see what happens when we simply try to increase stages:
Table 2 MG25-3C-940PF into 50ohms at 35kV charge
|
|
Whereas we would expect the addition of 10 stages to give us an additional 175kV (assuming a matched load), we are only seeing an additional 62kV. So why is this? You can see in the table that we are assuming a stage inductance of 35nH. Remember, this inductance adds with the number of stages, and as a result our source impedance increases from 52 Ohms to 88 Ohms- thereby reducing our voltage efficiency. Moreover, we have now decreased our erected capacitance from 188pF to 113pF, thereby also changing the pulse characteristics. As a Marx designer, our next step would be to look at available capacitance values and attempt to design a generator that meets the customers’ requirements with existing components. The example below shows what happens when we increase stages and capacitance:
Table 3 MG32-3C-2000PF into 50ohms at 35kV charge
|
|
With the configuration above we are able to exceed the customers voltage requirement by 10%, thereby allowing us more head-room in our charge voltage (and less component stress), and we are achieving the same erected capacitance as the MG15-3C-940PF. As can be seen in Table 3, the capacitor used for each stage was raised from 940pF to 2000pF. Because the impedance is the square root of the erected inductance divided by the erected capacitance, the corresponding change in “C” balances out the increase in “L”.
Why is this important information for our customer to know? Most customers will want to make a cost benefit analysis of their desired specifications vs. the total system cost. Whereas the MG32-3C-2000PF meets the stated requirement, it is also a more expensive generator. At this point, the customer could discuss what specifications are variable and work with us to find the best Marx generator to meet both their budget and their technical requirements.
How can we help you with your Marx Generator needs? Let’s talk!
