$('#mydiv').clone(true).attr("id", "mytable"+ i).insertAfter('#mydiv');

$(document).on('click', '.add_line', function(e) {
        e.preventDefault();
        var wrapper = $(this).parents('.wrapper');

        var toClone = wrapper.find('table tr:last');
        toClone.after(toClone.clone(true).attr("id", "champs" + i));
    });