Thanks for your replies. My mistake was that I though I could solve V^{1/2} d^2 V = dx^2 , which stemmed from an incomplete understanding of the method of separation of variables (I took the fact that you could solve something like d^2x/dt^2= F/m using double integration, and incorrectly...
I have found the 1D differential equation relating voltage and position for a vacuum diode in the space charge region, which is
\frac{d^2V}{dx^2} = constant * V^{-1/2}
and I know the solution to be
V(x) = V_0 \left(\frac{x}{d}\right)^{4/3}
which is found by multiplying both...