Title

Studies on secondary electron emission induced charging in dielectric materials: Novel methods and applications for space and plasma technologies

Date of Award

2020

Degree Type

Thesis

First Advisor

Isabel Montero Herrero

Second Advisor

María Eugenia Dávila Benítez

Abstract

The interaction of ionizing radiation with matter is of critical importance in many areas of science and technology such as space and plasma technology. Secondary electron emission is a direct consequence of electron irradiation on materials. To characterize materials in terms of secondary electron emission, the secondary emission yield (SEY) and the energy spectra of the secondary electrons are key physical properties. Secondary emission yields of materials are usually too high to avoid Multipactor effect or other related phenomena in applications for space. In addition, the measurement of electron energy spectra of secondary electrons in dielectric materials is a challenge due to charging issues. For the first time, in this doctoral thesis, a synergy between conductor and dielectric domains in composite materials have been experimentally reported and modeled. Composite materials with SEY < 0.2 up to high primary energies (~1 keV) have been prepared and characterized. These composite materials have been modeled using a deterministic simulation to obtain an insight on the interaction process between domains that produces such extremely low secondary emission yield. To obtain the electron energy spectra of dielectric materials, a method that takes advantage of the charging of dielectric materials during electron irradiation has been developed. The method was first tested on floating conductor samples, specifically for Cu, Ag and Au films. The results show a good fit between the new model proposed in this doctoral thesis and the electron spectra obtained with a hemispherical electron energy analyzer. Once the validity of the method was proved, it was used on dielectric materials. Kapton, Teflon and Ultem polymers were selected due to their applications in the space industry. The energy spectra of secondary electrons of these materials was measured and it showed a peak at 1.9 ± 0.1 eV for Kapton, 2.3 ± 0.1 eV for Teflon and 4.3 ± 0.2 eV for Ultem.

The results shown in this thesis on the composite materials pave the way to design new materials of low secondary emission yield. These materials are needed in certain vacuum applications, such as RF communications, as a high secondary emission yield produces an electron avalanche that limits the maximum working power of the RF devices. Also, the research on the charging of dielectric materials under electron irradiation has provided with a new method to characterize the secondary electron energy spectra of dielectrics. The measurements of these spectra have been traditionally difficult due to the charging of the 4 material as high electron doses were needed. The presented new method allows to obtain the energy spectra with doses of only 10 pC/mm2, which ensures a minimal distortion of the pristine state of the dielectric material by avoiding radiation damage, deep charging, defects, aging and other electron induced phenomena on the insulator. Following the regulations of the Universidad Autónoma de Madrid (UAM), this doctoral thesis is presented as a compendium of publications. The manuscript is divided into four parts. A general introduction exposing the main concepts of Secondary Electron Emission can be found in Chapter 1. This is followed by a global summary of the results and their discussion in Chapter 2. The main publications, Publications 1, 2 and 3 that have been produced as part of this doctoral thesis can be found in Chapter 3. Finally, the conclusions are included in Chapter

Comments

Visiting scholar from Instituto de Ciencia de Materiales de Madrid. CSIC Calle Sor Juana Inés de la Cruz 3, Madrid, Spain, PhD University of Madrid, Madrid, August 2018.

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