Update run_linac_segment.py

Reduce resolution for reduced runtime

examples/linac_segment/run_linac_segment.py

@@ -14,7 +14,8 @@
 
 # set numerical parameters for space charge
 sim.max_level = 0
-sim.n_cell = [64, 64, 64]
+#sim.n_cell = [64, 64, 64]  #use this for high-resolution runs
+sim.n_cell = [32, 32, 32]
 sim.particle_shape = 2  # B-spline order
 sim.space_charge = True
 sim.poisson_solver = "fft"
@@ -28,7 +29,8 @@
 sim.init_grids()
 
 # npart = 2  # number of macro particles
-npart = 1000000
+#npart = 1000000  #use this for high-resolution runs
+npart = 10000
 ns = 25  # default number of slices per element
 
 # beam reference parameters