var popupWindow;

function popup(url)
{
  
  if (!popupWindow==null && !popupWindow.closed)
    popupWindow.Close();
  popupWindow=window.open(url, "popup", "toolbar=no, location=no, directories=no, status=no, scrollbars=yes, resizable=yes, copyhistory=no, width=600, height=300, left=300, top=50");
  popupWindow.focus();
}