What percentage of admitted applicants receive Assistantships?
100% of admitted PhD applicants are offered an Assistantship. Normally, we do not offer admission to US or to international applicants unless we are able to provide both an Assistantship and a full tuition scholarship. This applies whether the student joins in fall (August) or in spring (January). Incoming Masters students are generally not considered for Graduate Assistantships.
Will my chance of admission be improved if I indicate in my application that I don't need an Assistantship?
As explained in the previous answer, we automatically assume that every applicant needs an Assistantship. Sometimes an applicant already has a special fellowship from some organization, or has an agreement with an employer who will support the applicant's graduate education either part-time or full-time. Of course, we look very favorably on applications of this kind, and we will "go the extra mile" to help such applicants to meet our academic criteria for admission into our graduate-level physics degree programs. The Masters of Science degree is a full time program that normally requires external support.