How to generate highest voltage possible without sparking?
Well I would really stop bieng nosy in the physics involved(though its unclear why one wanna charge object to medium breakdown condition and stop due to arching and start all over again)


Here where you can get started,you can apply many type of ionization sensor,some of them are very simple like pressurized Helium(much better option here) tube and PM tube design try googling them.

To control the this tube with a circuit you will need a control logic,well if you do a deep study,you will see the physics behind these tube operation is the key for designing the controller.The design is based on proportional design in ionization region where the charge in tube is the measure of voltage outside,ahead of it lies the G.M region but I dont think there is need of that.

As generating high voltage you must have something like Van De Graft,or some other classic device,it wont be hard for you to get a Scintillator.


Well want a cheap Solution use Photo detector to detect spark but a less ideal solution,
thing that I wont use is thermal detectors,the error would increase a lot and may be you cannot reach peak value.

Where I cant Tell you the circuit until unless the motive and the fucntionality of design are known.
Thanks for making the queries clear ,this is what I think may help,
A device that could  generate charge, I would suggest you to make Van de Graft Generator becasue it has a very simple design,


For more Help you can see these how to make Van de graft generator
This guy's approach is much better  The Org van de Graft

As you would Want to make it upto a certain voltage you will have to calculate the capacitance of the upper doom,which you may change as per desire.
The working of device is Simple, you induce charge on the belt which is then spread on the doom using corona discharge.

There are other alternatives also,but I think this is the best, and has the most simple control mechanism of on off keying,

You could use on the detectors I told previously Pm tube produces a pulse train which can be used to trigger switching process
You can count the pulse set equivalent voltage to each frequency,
Compare this to the opamp,using a SETPOINT of Voltage and add a little voltage to SETPOINT in the next cycle until is in range of -10uC of the limit of Object ,depending on the medium in which you use the setup,as you can this is a little bit complex.

Some Things about the circuitry I suggest that you use stepper motors of small step-size,If the current required by the motor is very high,then try switching circuit use a really to switch power lines ON/OFF ,If You have a background in power electronics you could also use Thyristors as switch,its better to use low current motors(700~850 mA) as for those with high current you will need to design a extra controller.

If you are not able to afford a Scintillator, or PM tube,I can suggest you to buy a Cold cathode tube,like(tube-lights etc),you should be able to light them up if you keep you cathode ray in direction of field,it will light up if you have sufficeint voltage,Now whenever a arch will occur voltage at Van de Graft will fall so  does the current in your tube,and you could use this to turn off the Vande Graft,and start again after some time(As electric field per distance ,result in voltage drop you can get enough voltage to light up the tube)
Using the Van De Graft you can generate upto 500kV using right material and shape.