  We present an optimized design of our recently realized helical disc dynamo.
Like the original set-up, the optimized dynamo consists of a flat multi-arm
spiral coil and a co-axially placed disc which is connected to the former by
sliding liquid metal contacts. In contrast to the original set-up, the disc and
the coil in the optimized design have different sizes. This allows the disc to
capture more of the high-density magnetic flux generated in the inner part of
the coil and to avoid the reverse flux in the outer part of the coil. By
optimizing the coil and dics radii, the critical magnetic Reynolds number can
be reduced from ${\mathit Rm}\approx34.6$ when the disc and coil have equal
inner and outer radii with the ratio $r_{i}/r_{o}\approx0.36$ to ${\mathit
Rm}\approx11.6.$ This lowest possible disc dynamo threshold is attained when
the disc and coil have relatively narrow widths. Using a slightly suboptimal
but more practical set-up with the inner and outer radii of the disc and coil
equal to to $(0.3,0.9)$ and $(0.74,1),$ respectively, self-excitation is
expected at ${\mathit Rm}\approx14.6.$
