An attempt to avoid the grid-collision problems was made by Robert Bussard in his Polywell design. This uses cusp magnetic field arrangements to produce "virtual electrodes" consisting of trapped electrons. The result is to produce an accelerating field similar to one produced by the grid wires in the fusor, but with no wires. Collisions with the electrons in the virtual electrodes are possible, but unlike the fusor, these cause no losses via spalled-off metal ions.
The polywell's biggest flaw is its ability to hold a plasma negative for any significant amount of time. In practice, any significant amount of negative charge vanishes quickly. Further, analysis by Todd Rider in 1995 suggests that any system that has non-equilibrium plasmas will suffer rapid losses of energy via bremsstrahlung. Bremsstrahlung occurs when a charged particle is rapidly accelerated, causing it to radiate x-rays, and thereby lose energy. In the case of IEC devices, including both the fusor and polywell, the collisions between recently accelerated ions entering the reaction area and low-energy ions and electrons forms a lower limit on bremsstrahlung that appears to be far higher than any possible rate of fusion.