Suggestion: Visit
http://www.bertan.com/xrfamily.html
http://www.advance-hivolt.co.uk/html/index.htm
etc. for more info.

neiradio,
     Just a suggestion- look at a voltage double or triple capacitor set as you might find in the hv end of a TV set. if you can maybe charge the doubler caps up on the primary side after the transistor. don't forget the voltage rating. Try a fet or mosfet "transistor" to reduce power /voltage drop over the junction

let us know how you go
Don

You may try the circuit with dual transistor with high frequency transformer , & pulses to be triggerred thru high speed thyristor , with surge suppressors ( to prevent spurious firings). Voltage drop may have been noticed if the voltage regulation of the transformer may be low - then you may have to increase the rating of the transformer.

Suggestion: Try Reference
1. Giacoletto L. J., "Electronics Designer' Handbook," Second Edition, McGraw-Hill Book Co., 1977, Figure 23.125 "Picture-Tube Anode Voltage Supply Obtained by Tripling the Voltage Produced across the Line Output Transformer TR
Essentially, triangular connection of high speed diode with capacitors can increase the voltage. The high voltage production depends on the energy of the magnetic field that  produces high voltage. The harmonic resonance is supposed to be eliminated by a tight coupling with high-voltage coil.
2. Also, Marcus Handbook of Electronic Circuits is useful.

neiradio,
  Had a new thought or two after logging off the other night. I have in the past built hobbiest kits for car CDI ignitions. Now these had the disadvantage that they pushed the secondary voltage too high and the car was prone to cross firing.
   A 25000 hz would relate too a 6 cyclinder car runnig at a bit over 4000 rpm. So, hunt thru the hobby electronics mags / shops for a simple capacitor discharge ignition unit (or a commercial one at the auto accessory shop) use a oscilator cct on say a jfet or fast opto isolator (if there is such a thing) instead of pionts and you should start to approximate 50000V.
  Beware :-
     The insulator rating of the coil
     The coil heating up

Regards Don

Suggestion: DC-DC boost converter may meet your specifications. Visit
http://powerweb.lerc.nasa.gov/elecsys/DOC/scbc.html
http://www-s.ti.com/sc/psheets/slaa105/slaa105.pdf
http://www-s.ti.com/sc/psheets/slvu004/slvu004.pdf
http://www.ednmag.com/ednmag/reg/1997/121897/26di_04.htm#Figure 1
etc. for more info