Apologies, apparently the first time I sent this reply it only went to Doug. Here it is for public view.
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Your primary circuit and your secondary circuit make up two resonant circuits, both ideally "tuned" to the same resonant frequency. Your secondary coil and topload generally set the resonant frequency. The resonant frequency is determined by the formula Fres = 1/[2*pi*sqrt(L*C)], where L is the inductance of your secondary coil and C is the capacitance of the topload (we're ignoring parasitic inductances and capacitances here, for simplicity sake). The same formula is used to find the resonant frequency of your primary tank circuit, where L is the inductance of your primary coil and C is the capacitance of your tank capacitor. Based on this formula, in order to maintain the same Fres (to match your secondary circuit) when increasing capacitance C you need to decrease inductance L. Similarly if you increase inductance L, you need to decrease capacitance C. So I was wondering if you could decrease your capacitance in order to allow you to use more turns on your primary coil to improve the strength of the generated electromagnetic field.
Best,
Matt Lewis, Fairlee VT