  AXEL is a high-pressure xenon gas time projection chamber for neutrinoless
double-beta decay (0n2b) search. The AXEL has a unique readout system called
ELCC which has a cellular structure and photosensors to detect
electroluminescence light produced by ionization electrons. We demonstrated the
performance of the ELCC with a small prototype detector (AXEL-HP10L). The
obtained energy resolution corresponds to 0.82 -- 1.74% (FWHM) at the 0n2b
Q-value. We are constructing a new prototype (AXEL-HP180L) in order to study
the energy resolution at the Q-value of 0n2b with a new design of ELCC with
unit structure, newly developed electronics board, field-shaping electrodes,
and Cockcroft-Walton-type high voltage power supply. For a future 1-ton scale
large AXEL detector, we are developing new background-reduction techniques;
topology identification with machine learning, positive-ion detection, and
active-shield options.
