Can you give more details on how you set all boundary conditions? 

Inanycase, reducing the number of mesh cells will give you unreliable results.. 

Thanks Waddah. The boundary conditions I set up, as the particle beam is moving in z-directon, are as follows: 

X_min: E_t = 0     X_max: E_t = 0   , Y_min: E_t = 0     Y_max : E_t = 0  

Z_min: open  Z_max : open

I have increased the number of mesh cells, however still getting those warnings.

As for your item #3 you need to refine the mesh where the staircase meshcells showed up (maybe only locally, right-click selecting the object and choosing "local mesh properties").

Thanks Marcelo for your suggestion.

Detail Boundary conditions are as follows:

YZsymmetry: none

XZsymmetry: none

XYsymmetry: none

Xmin: electric

Xmax: electric

Ymin: electric

Ymax: electric

Zmin: open, 4 Layer, depth 12.163 mm

Zmax: open, 4 Layer, depth 12.163 mm

PML-Type: Convolution PML

Any suggestion please.

I have met the same problems. have you solved it? Can you share the methods with me?

Hi, I have not resolved what brings up that warning but, you will get still

reliable result if you see "maximum simulation time reached , solver stops" I think.

I also met this problom when using CST Studio with Wakefield solver, but the probles is solved perfectly just follow Marcelo B. Perotoni 's suggestions, the key is refine the mesh size along the beam direction near the boundary.