Optimal Timing for Spark Recovery in the Triumf Cyclotron

In the TRIUMF cyclotron when a spark occurs it is necessary to shut off the RF drive and to initiate an RF restart procedure. It is also desirable to restore the full operational dee voltage as soon as possible in order to prevent thermal detuning of the resonant cavity. However, when the RF drive is shut off, the disappearance of Lorentz force on the resonator hot-arms causes the hot-arms to vibrate at their mechanical resonant frequency. When the RF field is being restored, the electromagnetic resonance is coupled to the mechanical resonance through the Lorentz force, and the amplitudes of the both the mechanical vibration and the RF field depend on when RF drive is re-applied. Computer simulations and experimental results will be presented to demonstrate that an optimum exists for the RF restart timing. With this optimal timing, the Lorentz force is used to suppress the mechanical vibration of the hot-arms. The reduction in hot-arm vibrations increases the probability of successful restarts as well as reduces the stress on the RF components. INTRODUCTION When sparking occurs inside the TRIUMF cyclotron resonator, the RF power is cut off in order to extinguish the spark[1]. If it takes more than a few seconds before voltage comes back to its nominal value, then the cyclotron thermal balance is lost. This is due to the substantial amount of heat removed from the structure by efficient cooling which is not being replenished by the RF power. Temperature variations affect the cyclotron resonant frequency. In the operational mode, automatic control of the resonant frequency is performed by varying the cooling water pressure which gives a limited tuning range of about 3 kHz. Thermal imbalance due to recovery time causes a resonant frequency shift of about 10 kHz. It requires about 3 minutes for the system to come back to the operational frequency, when it can be switched to generator driven mode. Beam injection restart requires some 10-20 seconds more. Recovering from a spark can cause 4-5 minutes of total machine downtime. In order to prevent this lengthy downtime, the restart procedure is initiated as soon as possible. Spark detection is based on cyclotron voltage drop analysis. When the voltage drop differential exceeds 10 kV/μs, the event is recognized as a spark. At this point the RF drive turns off for 60 ms to let the spark products diffuse away from the arc channel. Then the RF drive instantly comes back to generator driven mode. In the past, we have observed that more than half of these fast recovery attempts were unsuccessful. Also, in every attempt, measurements have shown that there were large fluctuations in field voltage and reflected power during the restart attempts. THEORY Suppose a resonator is operating in a steady state condition with field voltage V0. This produces a Lorentz force and causes the resonant cavity to be detuned by Δω0. At time interval 0 < t < tR, the field voltage is removed and the cavity will start a mechanical oscillation with frequency Ωμ and damping time constant τμ. Neglecting Coulomb friction, the mechanical equation for this detuning is 2 2 2 0 2 1 0 V k dt d ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ Ω − + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ Δ Δ ⎟⎟ ⎟