How to make a circuit that generates 50mA current at +1.8v within 10ns rise time?
First some analysis:

from \$ i = C \frac{\partial V}{\partial t} \$ we can determine that you constrains are equivalent to 277 pF load capacittor. If we use a MOSFET that has a \$ I_{ds,sat} = 30 mA\$ then this curve can be obtained since MOSFETs are transconductors (i.e. current sources).

Perhpas the easiest way of accomplishing this is to have a storage cap that is 20X larger - so say 0.006 uF and a PMOS switch.







simulate this circuit &ndash; Schematic created using CircuitLab

So that things you need to look out for is if the C1 has sufficiently low enough ESL and ESR to ensure that it can flow the charge fast enough. Larger capacitors may have resonant frequencies that are lower and couldn't respond fast enough. barring that then you should use a very high quality (Teflon film) cap.

I would suggest doing some sims in SPICE to ensure that you can get  the right parametric response.