Multipacting Tests with Magnetic Field for the LHC Beam Screen

In connection with electron-cloud induced heating of the LHC beam screen, multipacting tests with a resonant coaxial cavity have been successfully performed in presence of a solenoid and a dipole magn etic field. We have developed a simple and reliable technique, based on amplitude modulation of the input signal, to detect electronically the onset of multipacting and to monitor the field and power level in the resonator. Several multipacting patterns have been systematically investigated under the effect of a variable DC-bias applied to the inner conductor of the coaxial setup. The results at r oom temperature are qualitatively similar to those obtained during cold tests (below 20 K in a cryostat) with a dipole magnetic field up to 7.5 T. A weak solenoid field of about 50 Gauss is usually su fficient to stop the multipacting, but the same longitudinal field is ineffective in presence of a strong vertical dipole field (up to 1.5 T). We have also measured the rise time of the multipacting v ersus the intensity of the solenoid field. Moreover, a substantial decrease of the multipacting threshold is observed when the dipole magnetic field has an intensity such that the electron cyclotron f requency is equal to the resonant frequency of the coaxial cavity.