How is combustion flame maintained in the combustion chamber after igniters are switched off?
How does a candles flame sustain itself after you have lit it?  It stays lit by continually burning fuel.  The jet engine works the same way.  Once the fire is lit (by the ignitors) it is constantly burning.  Air is constantly being supplied by the compressor and fuel is constantly being supplied by pumps.  

This is fundamentally different to a traditional combustion engine you might be trying to reconcile this idea against.  In a 4-stroke internal combustion engine, for example, you have intake, compression, power and exhaust happening separately and you need spark plugs (or glow plugs) to ignite the fuel each cycle.  In a jet engine, however, instead of 4 independent cycles you have a constant flow of air through the engine.  Air comes in the front, is compressed by the N2 stage and fed continually into the hot section where the fire is constantly burning and then exhausts through the back of the engine.

Look at it like a propane barbecue grill or a gas stovetop -- you only need the ignitor to start the fire. Once the fire is burning it stays that way until you turn off the flow of gas.