What about a Fireye brand system? I used them years ago on large fuel oil / natural gas heaters. There are also UV sensors that will detect this I have only seen them about a foot or so away. They may have units that work from farther away than that.
What are you sensing and why? Is this a fire system, heating, or light generator. You could use a current sensor to detect the arc. With no arc you will have no current flow. What voltage is this ignitor working at 6,000 or 10,000 V What type of electrode single element or dual? What power supply?
Those are just a few off the top of my head.