Typically, a run consists of slowly ramping the magnetic field while monitoring the beam current for a given set of parameters. The second successful resonance run plot, 91699C, is shown in Fig.11. As expected a strong proton current developed at the magnetic field corresponding to the fr. In run 91699C the resonant frequency was tuned to 13.590 MHz. Other parameters for run 91699C are listed below:
fr 13.590 MHz
Forward RF Power 16 Watts
Theoretical B-field 0.889 Tesla
H2 Pressure in tank 5.1E-5 Torr
Filament Current 5.75 Amps
Filament Voltage 5.0 Volts
Filament Bias -320 Volts
Filament Emission 21.0 m Amps
Max. Ion Radius 7.0 cm
Max. Ion Energy 184 keV
The measured ion peak at 0.885 Tesla tightly corresponded with the theoretical value to 0.6%. The development of the 0.885 peak was rewarding confirmation that this nine-inch cyclotron work as designed. However an unexpected peak at 0.449 Tesla developed. Since hydrogen, being the fundamental element, has a Q/m ratio of 1e+/1amu it was hard to imagine what ion was being accelerated, for it must have fractional mass or excessive charge if it were arising from the fixed fr. Several runs were taken with increased ion current sensitivity, in order to determine reproducibility as well as stability, Fig.12. After an investigation into the matter, it was determined that indeed singly charged protons were being accelerated.
Fig.12 Sensitive beam measurements.
Although, not obvious at first, the RF frequencies required for acceleration of the ions at the low magnetic fields, developed from excitation of higher frequency modes of oscillation in the tank circuit. These are known as harmonics of the fundamental fr. Even at that, the harmonic frequencies that developed would require higher magnetic fields, not lower. At this point some detailed thought of the accelerating electric field is required.
As shown earlier, the RF period needs to equal the angular frequency of the circulating ion, such that the electric field always points in the same direction as the ionís velocity when entering the accelerating gap. If the applied frequency were double that of the fundamental, on itís second crossing of the gap the ion would receive a de-acceleration, thus gaining zero net acceleration. However, if the RF frequency were triple that of the fundamental it is seen that the electric field direction is again in sync with ionís travel. This effect holds true for any odd multiple of the fundamental cyclotron frequency. Acceleration measured at the low magnetic fields were a result of the harmonic frequencies being odd multiples of the ionís required fundamental cyclotron frequency. These harmonics were confirmed and measured with the use of a network analyzer. Because of the shorter period, the duration of the electric field in the appropriate direction is also less, causing increased magnetic field precision requirements, hence the sharpening of the beam current peak seen in Fig.12. In order to maximize the RF systems efficiency these harmonics should be suppressed. Future RF work will attempt this feat.