COMSOL: cloaking for Helmholtz equation

Hello,

I have successfully done cloaking simulations with COMSOL's RF module. Trying to do the same for the Helmholtz equation, I ran into some problems. I think this has to do with incorrect boundary conditions, but I can't see the solution right now. Maybe someone can help me!?

The simulation results are shown in the following pic:

It looks like scattering from the boundaries.


I've used the following setup in COMSOL:

2D > Mathematics > Classical PDEs > Helmholtz equation

Global Definitions > Parameters: k (wavenumber)

Equations: grad(-c grad(u)) + a u = f
Outside the ring: c=1, a = -k^2, f=0
Inside the ring: c = usual cloaking parameters, a = -k^2, f=0

Zero flux: top and bottom boundary
Flux/Source: left and right boundary g = -k*sin(k*x)


For the RF wave equation I used PMLs and scattering boundary conditions. That's not possible for the Helmholtz equation and I thought that it wouldn't be necessary. Can someone tell me what's going wrong there!?

Regards,
RB

plz, i need an example for cloaking in comsol