no vtk file exported

I am simulating a two-dimensional biaxial compression experiment and I would like to use paraview for post-image optimisation (e.g. to form shear bands), but I am not sure why I am always unable to generate the appropriate VTK files. Here is my code.
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# -*- encoding=utf-8 -*-
# the script demonstrates a simple case of triaxial simulation using TriaxialCompressionEngine. More elaborated examples can be found in the triax-tutorial folder
from yade import pack

sp=pack.SpherePack()
## corners of the initial packing
mn,mx=Vector3(0,0,0),Vector3(2,2,.02)

## box between mn and mx, avg radius ± ½(20%), 2k spheres
sp.makeCloud(minCorner=mn,maxCorner=mx,rRelFuzz=0.5,num=4000)

## create material #0, which will be used as default
O.materials.append(FrictMat(young=15e6,poisson=.4,frictionAngle=radians(30),density=2600,label='spheres'))
O.materials.append(FrictMat(young=15e6,poisson=.4,frictionAngle=0,density=0,label='frictionless'))

## copy spheres from the packing into the scene
## use default material, don't care about that for now
O.bodies.append([sphere(center,rad,material='spheres') for center,rad in sp])
## create walls around the packing
walls=aabbWalls(thickness=1e-10,material='frictionless')
wallIds=O.bodies.append(walls)

triax=TriaxialCompressionEngine(
 wall_bottom_id=wallIds[2],
 wall_top_id=wallIds[3],
 wall_left_id=wallIds[0],
 wall_right_id=wallIds[1],
 wall_back_id=wallIds[4],
 wall_front_id=wallIds[5],
 internalCompaction=True,
 ## define the rest of triax params here
 ## see in pkg/dem/PreProcessor/TriaxialTest.cpp:524 etc
 ## which are assigned in the c++ preprocessor actually
 sigmaIsoCompaction=-2e4,
 sigmaLateralConfinement=-2e4,
 max_vel=10,

 label="triax"
)

O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Box_Aabb()]),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom(),Ig2_Box_Sphere_ScGeom()],
  [Ip2_FrictMat_FrictMat_FrictPhys()],
  [Law2_ScGeom_FrictPhys_CundallStrack()]
 ),
 GlobalStiffnessTimeStepper(active=1,timeStepUpdateInterval=100,timestepSafetyCoefficient=0.8),
 triax,
 TriaxialStateRecorder(iterPeriod=100, file='nocyclic2.txt'),
 NewtonIntegrator(damping=.4),
 VTKRecorder(iterPeriod=50,recorders=['all'],fileName='/tmp/vtk/p1-')
]

triax.strainRate=0.02

O.engines=O.engines[0:5]+[PyRunner(iterPeriod=20,command='history()',label='recorder')]+[PyRunner(iterPeriod=20,command='stop()',label='stop')]+O.engines[5:7]

def stop():
 if O.iter>50000 :
  O.pause()

from yade import plot

def history():
 plot.addData(e11=-(triax.strain[0]),
         e22=-(triax.strain[1]),
  s11=-(triax.stress(0)[0]),
  s22=-(triax.stress(2)[1]),

  i=O.iter)

plot.plots={'e11': ('s11',),'e22': ('s22',)}

O.saveTmp()

plot.plot()
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Any reply would be helpful.
Thank you.