I have a bottle of argon, perhaps I can somehow inject it into the
spark gap to trigger the spark?


Might be worth a try, on the ARL spectrometers I play with, the spark will
not ignite at certain voltages when there is air present in the system. The
argon flush to the spark gap is typically about 5 seconds before starting
the spark.

Most of these have a small Teflon, Perspex or going back to the dark ages, a
bakelite insulating bowl that surrounds the gap and a tygon or similar
plastic type insulating gas supply line. These would typically be about 50mm
diameter and 30mm length internal dimension that would surround the
electrodes and keep the gas inside. An argon flow rate of about 4 litres
per minute will give a clean cavity within a couple of seconds, and spark
will normally occur as the gas mixes and gets rid of most of the oxygen in
the camber.

Most of the spark gaps are in the order of 3 to 5 mm and have a secondary
gap in air around the same distance. The secondary gap is in series with the
primary gap. This gives a nice spark that triggers in the 8KV range, but
this can vary quite a bit. The spark is initiated by a circuit similar to a
car ignition system, and as the voltage rises, it triggers at a voltage that
will vary with different gases or spark gaps.

However, be aware that relying on the difference between the argon and air
might not be reliable and you should treat it as if it was going to go off
with "Murphy's Law".

Hope this helps,
Peter