1. FEA of the On-Off Mechanism Piston Movement Impact
Riku Raatikainen

2. Introduction Model Description Boundary Conditions Results Conclusion
INDEX
Introduction
Model Description
Boundary Conditions
Results
Conclusion

3. INTRODUCTION
The On-Off mechanism is a component for reflecting or transmitting the high RF power within PETS
Mechanism is driven by a linear actuator, which creates the movement of the piston between the two operation modes
The actuation time for the mechanism is ≤ 18 ms in which time the piston (150 g) moves inside the choke chamber for 8 mm
The aim of this analysis is to study the impulsive shock phenomenon caused by the piston movement impact. Since the mechanism is brazed on the PETS Compact Coupler, forces created by the mechanism are directly interacted with the assembly
ON OFF
Main components of the mechanism
Illustration of the piston movement

4. MODEL DESCRIPTION
At this stage, the impact of impulsive force to the choke chamber (Copper) was modelled in order to see the magnitude of the shock wave travelling through the structure
As the piston is being moved, the structure exposes to impulse forces, which last very short period of time but causes forces of a high magnitude
Ideal impulse excites all modes of the structure → the response of the structure should contain all mode frequencies
However we cannot create an ideal impulse force numerically → force is applied over a discrete amount of time Δt
acceleration
deceleration
(0.89 m/s)
Illustration of an idealized impulse
Velocity of the moving piston

5. BOUNDARY CONDITIONS Fixed (brazed to the compact coupler body)
Force is applied on the contact area, where the choke chamber is connected to the vacuum flanges by brazing

6. Deceleration of 9 ms (optimal)
RESULTS
Shock wave travelling through the structure once the piston has moved inside the choke chamber in two scenarios with a structural damping corresponding to relative damping of 3 % for the first modes:
Piston is decelerated in Δt = 9 ms
Piston is decelerated in Δt = 4 ms
Deceleration of 9 ms (optimal)
Deceleration of 4ms

7. CONCLUSION
According to the results, the current model indicates forces of a magnitude of ̴15 N due to the impulse (for comparison the PETS weight itself ̴ 60 kg)
The response of the structure (choke chamber) is in the order of nanometers
The effect of the impulse decays in approximately 10 ms
As a next step, the response of the PETS will be studied more closely (the impulse could excite critical modes of the PETS and thus, create a response exceeding the high precision tolerances)
A possible PETS movement in case the impulse excites one or more frequencies of resonance