Coupling Coefficient - k

The Coupling Coefficient (k) is a ratio representing how much of the primary coils electromagnetic field is coupled to the secondary coil. Typically, this ratio is between 0.13 and 0.22. The coupling coefficient is purely a function of the geometry's and relative placement of the primary and secondary coils. The reason k is low for Tesla coils is because of the voltage stress limitation on the secondary coil. In otherwords, high k values result in fast energy transfer and voltage stress high enough to cause electrical breakdown (unwanted racing sparks).

The degree of inductive coupling determines the rate at which the energy transfer occurs, the intensity of current, the tuning response, and among other things, the voltage gradients developed along the secondary coil and at the topload. If coupling is too tight, some points along the secondary will have a voltage gradient high enough to cause corona inception and ultimately spark breakout of the secondary winding. Typically, we observe this phenomenon as racing sparks traveling down the length of the secondary. When this occurs, the coupling is too tight and needs to be loosened up (position the coils further away from each other).

Coils which are poorly tuned may also cause racing sparks. Although the mechanism is different, the result is the same (secondary voltage stress too high). For maximum performance, it is desirable to transfer energy quickly. The faster energy is transferred, the fewer cycles that are required for total energy transfer. It is also desirable to quench as soon as possible (stop the gap conducting). This translates to less conduction time at the sparkgap resulting in lower gap losses.

Before adjusting coupling, first tune the coil at low power. Once the coil is tuned, the coupling can be adjusted. Typically, this is a vertical adjustment (raising or lowering one of the coils). Once coupling is adjusted at low power, run the coil at normal power levels. At higher powers, the secondary will see higher voltages. With higher voltages comes higher voltage gradients. Some coils will need to loosen up the coupling at higher powers. The reason to adjust the coupling at low power first is to reduce the risk of component damage during adjustment.