# The purpose of transistor and zener in this circuit?

simulate this circuit – Schematic created using CircuitLab enter image description here (simulation will not be correct because the diodes are added just by its name.)

Hello, I am a student who is not really familiar with electronics, and these days I start studying several circuits which is being used in our experiments.

I have a circuit we insert between room ground and the circuit with power supplies. I made a diagram from the photo of the circuit that I took.

As far as I know, to avoid too high floating voltage between the circuit with power supplies and the ground, the circuit is to make negative voltage ~ MAX -50V relative to ground. (Zener diode 1N5347B (datasheet: http://www.kynix.com/uploadfiles/pdf/1N5347B.pdf) are actually 5 in series, not one. I omitted for the simplicity in the diagram)

I sort of understood by Googling and learning myself that the capacitor and the diode is a safe circuit for sudden voltage peaks (am I right?) But, the center part in the circuit diagram, still no idea after several days study. Only figured out that 1N5347B Zener diode 10V 5W is to endure max -50 V.

Here are my questions..

What would this circuit be called, if it has a name?
How may the capacitor size 1 uF and resistor size 10 ohms be chosen?
Why and in what role are those resistors (cement resistors) and transistor connected additionally to the zeners? I thought to have max -50V, only the 5 zener diodes may be sufficient.
Is it sort of isolation circuit or floating regulation circuit?
added at 20:52 pm, 6th March, 2016

Thank you for all who had commented.
The following figure is the edited schematics. U1 - U3 are some user equipment boxes powered by DC power supplies 1, 2, 3 sharing Vcg with their '-' port. (PSs have '+, -, GND' ports, and'GND' ports in PSs are not connected to its own '-' ports.) So, the right side of Vcg are grounded, not to earth, but to the node Vcg. The circuit in the photo is the left side of Vcg.

simulate this circuit

So far what I learnt about the circuit (left side of the node, Vcg) are,

to keep 'Vcg' negative potential relative to earth. (which doesn't really have its own power supply), but up to max ~ -50V.
thanks to @transistor, the roles of the capacitor, diode, and shunt regulator.
Here, additional questions came up to me.

What role does the circuit itself have when it doesn't have a power supply to itself? would it be just being connected between Vcg and earth, and would start work when Vcg tends to go further negative than -50V?
In what case (I guess, when what happens in the right side circuit for operation of equipment), Vcg would tend to go more negative?
and let's say when it goes -51V, without any power supplies on the circuit, the current through R2 to turn on Q1 is going to come through the earth?
What kind of improvement would I be able to make for this circuit?
(from the previous question) 5. How may C1 1 uF and R2&R3 10 ohms have been chosen? (got to know about R1 to limits the current through D2, thanks to @transistor)

p.s. Am I asking too many questions in one? is putting additional questions not quite the thing?

10:34 am 7th March 2016, updated
How it makes close circuit..

I think @transistor answer makes sense, and I understood its role. But my schematics may have error still, and the figure in p. 338 doesn't show clearly how the circuit will be closed during the thruster operation, I am trying to give as much information as possible. @transistor mentioned about 'closing circuit', and I added a drawing for that. Generated plasma near anode (inside in U1 in the drawing) accelerate to the vacuum chamber wall which is earth ground, so I expect this is the mechanism closing the circuit. In the drawing, the big square grounded is the vacuum chamber. I hope this help to comprehend the circuit for the additional questions.
asked Nov 10, 2017