Decline of the self-focusing of a pulsed high intensity electron beam owing to gas breakdown

1981

Ionization of He and Ar by a pulsed high-intensity electron beam from a Febetron 706 has been analyzed self-consistently with an electronic computer by using related data in references at pressures between 1 and 300 Torr. The computational results indicate that the net current Inet at pressures below 30 Torr of He and 10 Torr of Ar keeps a nearly constant value after a certain time which is comparable to the gas-breakdown time assumed previously. As the pressure is further increased, Inet increases gradually with time and E/p induced by the pulsed beam decreases with increasing pressure. Comparing the dose observed on the beam axis with the computational result has revealed that the increase of the dose in the higher pressure region is attributed to the increase of Inet due to the decreasing Iback induced by the pulsed beam. When the doses are compared at a certain pressure among monatomic gases, the larger dose is given by a gas with the larger mean ionization time ti or 1/(αw) for secondary ionization by E/p (α is the first Townsend ionization coefficient and w is the electron drift velocity). The data on self-focusing can be interpreted by this law.

Ionization of gases by a pulsed electron beam as studied by self-focusing. I. Monatomic gases

1981

Ionization of gases by a pulsed electron beam as studied by self-focusing. II. Polyatomic gases

1981

Self Cite

In order to analyze data on the self-focusing of a pulsed electron beam in polyatomic gases, the net current Inet in H2, N2, and CH4 was computed self-consistently as functions of time in the pressure range between 5 and 300 Torr of these gases by using swarm parameters. The computational result indicates that the larger dose Dobs, observed by a piled dosimeter on the beam axis, is attributed to the larger Inet, which is mainly determined by a mean ionization time t1 for secondary ionization by the electric field induced by the pulsed beam. When values of Dobs for different gases are compared at the same pressure, the larger Dobs is given by the larger ti. This relationship is demonstrated for several polyatomic gases by estimating ti from various parameters in a function of secondary electron energy or E/p such as the electron drift velocity, the first Townsend ionization coefficient, the ionization cross section, and so on. For the short pulse duration of a Febetron 706, electron–ion recombination processes scarcely affect Inet except at high pressures of some polyatomic gases, while the effect of electron-attachment processes is appreciable in SF6, CCl2F2, and N2O.

Ionization of gases by a pulsed electron beam as studied by self-focusing. III. He, Ar, and O2 mixtures

1981

The self-focusing of a pulsed electron beam from a Febetron 706 in He, Ar, and O2 is enhanced by 10% addition of polyatomic gases at total pressure higher than 50 Torr. The enhancement by addition of N2 to He is stronger than that expected from pure N2. The effect of additives to Ar appears abruptly from a certain pressure dependent on additives. According to the conclusion in part II, this enhancement is caused by the increasing of the mean ionization time ti of secondary ionization due to electric field E induced by the pulsed beam. Such inference is demonstrated in the present paper on the basis of the time ti for a mixture given by Eq. (1); ti for a mixture is determined by the ionization potential of additives relative to that of a host gas, the initial slope of the energy dependence of the total ionization cross section σi of additives, and the retardation of secondary electrons by additives for acceleration by E. When a gas having small σi and large effect of slowing down is added, the self-focusing is strongly enhanced due to the large increase of ti. This increase of ti is also demonstrated by the increasing of 1/(wα) by addition of additives (w is the electron drift velocity and α is the first Townsend ionization coefficient); the values of both parameters for some cases can be known either from papers of experimental data and the Boltzmann equation analysis or by the average electron method.