JQuerey issue!

Hi im rotating banners in my header with some jquerey code

<!--
--><script language="JavaScript" type="text/javascript">
<!-- Begin
var how_many_ads = 2;
var now = new Date()
var sec = now.getSeconds()
var ad = sec % how_many_ads;
ad +=1;
if (ad==1) {
url="http://envyusmedia.com/track?campaign=CPAFIX&lander=/index.html";
alt="EnvyUs Media";
banner="http://cpafix.com/images/banners/envyus.gif";
width="600";
height="90";
}
if (ad==2) {
url="http://limelightcpa.com/";
alt="LimeLight CPA";
banner="http://www.cpafix.com/images/banners/limelightcpa.gif";
width="600";
height="90";
}
document.write('<center>');
document.write('<a href=\"' + url + '\" target=\"_top\">');
document.write('<img src=\"' + banner + '\" width=')
document.write(width + ' height=' + height + ' ');
document.write('alt=\"' + alt + '\" border=0><br>');
document.write('<small>' + txt + '</small></a>');
document.write('</center>');
// End -->
</script>

Code (markup):

just wondering how i can open the link in a new page?

I've tried adding blank after link but it crashes the code

thanks