You cast it once and go to the cap. The cap is +6.
You cast it again, and the cap is now +9. Go to the cap and your total caster level is 22.
You cast it a third time and the cap is now 22/2 = +11. Go to the cap and your total caster level is 13+11 = 24.
Fourth time the cap is +12, for a total of 25.
All subsequent castings have a cap of +12 unless you can somehow boost your caster level to 26 or higher - and the field itself won't do that because its cap only gets you to 25 with your base of 13.