Help me: Optimization the pressure!!

Greetings,

Thanks for reading my problem.
I have some problem about setting the optimization module.
My model is simple, In Acoustic Physic, I use an acoustic lens to converge the pressure at the focal point (You can see the attached file). I want to find the optimal value of the lens radius (R1, R2) to maximize the pressure at the focal point. So I use the optimization module (including Optimization and Deformed Geometry).
I set up the valuables R1 and R2 in these equations (which represents the shape of the acoustic lens):
y=sqrt(R1^2-(x+a1)^2)+b1
y=sqrt(R2^2-(x+a2)^2)+b2
in order to minimize the following equation:

(p-150)^2
Where p is the pressure at the focal point and 150 is a reference pressure, both applied to domain.
The algorithm to optimize in comsol as follows:

1) Assign a value to R1, R2
2) Estimate the value of pressure at the focal point
3) Verify if optimality condition is satisfied, otherwise start again from 1
But I always got the error as follows:
Warning: Inverted mesh element near coordinates (0, 0.05996).
Warning: Inverted mesh element near coordinates (0, 0.023996).

Does anyone has any ideas about this weird warning?

Thanks in advance,

Tran Quang Dat