Try the following.

http://www.ltkcable.com/ltkweb/upload/DOWNLOAD_49.pdf

http://www.ltkcable.com/

jack of all trades

I have looked on their web site and did not find 50kV Co-axial cable. However they do have some high voltage insulated wires.

I will also like to try this RG8/U Solid PE cable as this must come much cheaper and may work up to 25kV easily. Foam will not work.

Be aware my data says that RG8/U has a maximum voltage rating of about 5kV only.

Alternatively you could consider screened silicon car ignition cables such as the following (which are helpfully sold by the metre also).

http://www.magnecor.com/magnecor1/main.htm

These are being successfully used in a HV AC test application up to 110kV.

Right now I am looking for co-axial cables. RG8U used with Glassman power supplies is used for up to 60kV. Single wires can be tried for DC power supplies as they are easily available but are not substitute for shielded co-axial cables.

http://www.glassmanhv.com/ByWattage/mk_series.shtml

I wonder if they have some typical cable designs.

I tried to test PTFE cable similar to RG58U and it could hardly reach 10kV corona free and then at 15kV I could hear corona discharges. Average corona current at 18kV was about 10uA. Corona discharge current less than 1uA will be ideal. Current capacity of the power supply is 250uA at 20kV.

Part of your problem is that most cable companies won't give a DC rating to their coax. cable.

Try Rowe: http://www.roweindustries.com/

P.S. This cable is probably not coax. but you can take a look, as this company usually has good prices:

http://www.surplussales.com/Wire-Cable/HVWire-1.html