## 2 Four Channels Generator
### Marx Modulator Trigger Circuit  
From the circuit topology of Figure 2, one can identify 9 voltage reference nodes, from Ref.1 to Ref.9, which can be used to place the switches driver boards. Hence, to supply energy to all the switch driver boards, with galvanic insulation, a series of toroid transformers was implemented, which is feed be a 150 kHz inverter, as shown in Figure 3.  
As shown in Figure 3, only one switch is placed on Ref.1 and Ref.9, but two switches are placed between Ref.2 and Ref.8, a charge switch from one stage, \(i\), and a pulse switch from a previous stage, \(i-1\), as seen in Figure 2.  
Figure 3: Auxiliary power supply, PS, in each Marx modulator, supplying power to the stage switches.  
Figure 2: Simplified schematic of the positive Marx modulator.  
In a more detailed schematic, Figure 4a shows a generic switch driver board \(k\) placed at voltage reference \(k\), being \(k\in\)[1; 2; 3; 4; 5; 6; 7; 8; 9], as seen in Figure 3. Each switch driver board is capable of triggering two switches, \(\text{S}_{\text{pi}}\) and \(\text{S}_{\text{ci}}\), where each one comprises two MOSFETs in parallel, both triggered by an individual driver to increase switching speed, as seen in Figure 4a.  
Considering Figure 4a, all the components in each board are placed on the same voltage reference, being floating in relation to safety ground. During pulse mode, each  
Figure 4: (a) Schematic of a generic switch driver board \(k\) used for driving each switch in Marx generator of Figure 2, placed at voltage reference \(k\). (b) Simplified schematic of the opto-coupler association for the UART communication. (c) Simplified schematic of the opto-coupler association for the trigger signal transmission for each MM.  
voltage reference \(k\) jumps to the resulting sum of the voltage on each capacitor in the series. Then, the trigger signals to each driver board, must be sent via galvanic isolation.  
In each Marx modulator the trigger signal, generated on the main control board, is sent to the driver boards by associations of optocouplers, as seen in Figure 4b,c. The use of optocouplers was chosen in alternative to optic-fibers for compactness and to reduce the propagation delay times, normally, found on optic-fibers receivers and transmitters. Figure 4b shows the opto-coupler association for the UART communication and Figure 4c the transmission of the trigger signal to each MM reference driver position, where additional delay modules, D, comprising an RC network and gate door, are used, to compensate for the propagation delays in the opto-couplers.  
From Figure 4, it can be seen that when the trigger signal is zero, the charging switches are set on, but when the trigger pulse is high, all charging switches are set off (i.e., there is a NOT logic gate). In relation to the pulse switches, when the trigger signal is zero, the pulse switches are set off, but when the trigger signal is high, the pulse switches state depends on the programming loaded on the \(\mu\)C\({}_{k}\), in each driver board \(k\) (i.e., there is an AND logic gate, with the trigger signal and a signal from the board \(\mu\)C\({}_{k}\)). The \(\mu\)C\({}_{k}\) programming is made by the main board, prior to the trigger signal generation, in order to set which Marx stage contributes to the final pulse, enabling the generation of pulses with different amplitudes in the same pulse sequence, or burst. This is called setting the Mask.  
The \(\mu\)C\({}_{k}\) programming in each Marx modulator is made via UART (i.e., universal asynchronous receiver/transmitter) communication protocol, from main control board, where the two lines use optocoupler strings for insulation from the high voltage.