Microwave example with FreeFEM++

FreeFEM++ is a very good Finite Element Method (FEM) engine program which can solve Poisson or Helmholtz equation

One example with microwave is they ex 3.11 of their doc. it is solving Helmholtz equation, or should I say a particular case of the Helmhotz equations.
They also use the program to solve the thermal dissipation inside the box.(quite clever)

As concern the microwave case I try to figure out how they interpreted the Helmholtz equation and the simplifications... Above image: Tested example FreeFEM++ 3.11 of the doc(page45-46) run on windows.