The behavior of a long annular Josephson junction is studied in presence of an externally applied magnetic field. We report on measurements of the critical current dependencies on magnetic field with various numbers of trapped fluxons. The magnetic field produces opposite-located potential wells for fluxon and antifluxon. The images obtained by low temperature scanning electron microscopy prove that fluxon and antifluxon are pinned by these field-induced wells. Good agreement is found between experiment and numerical simulations.
The financial support from the DAAD is greatly acknowledged.