Behavior of electron beam in a high current betatron

The UCI High Current Betatron has a uniform toroidal magnetic field in addition to the betatron field. Electrons are injected continuously from a thermionic emitter while both the toroidal and the betatron fields are rising. Electron capture takes place when more than 3 A is emitted from the injector either tangentially or non-directionally. Acceleration continues under a condition that the ratio between the average betatron field inside the orbit and the betatron field at the orbit is in the range of 1.2 – 3. At the initial phase of acceleration, the beam current increases rapidly with time. Then the beam suffers a gradual loss. Finally the whole beam disrupts and generates a burst of X-rays. If the injection is stopped in the middle of the course of the acceleration, the disruption of the beam happens at that moment. Influence of the space charge on the beam behavior is evident. The life of the beam becomes shorter as the injection current is reduced down to the threshold. The final electron momentum p inferred from X-ray measurement is related to the betatron field B and the major radius R by p ≈ 0.5 eBR, instead of p eBR. The beam current and electron energy increase with the increase of the injection voltage. Beam current of 150 A and electron energy of 500 keV are obtained at an injection voltage of 30 kV.