csrf token is valid just in first submit using ajax

Hi, i want to submit my form using jquery ajax, it is ok but just for first time i click submit, here is my code ,

my view that contains js :

Code:

<?php

echo form_open(site_url().'/user/ajaxreg' , array('class' => 'email', 'id' => 'form1'));
echo form_label('نusername : ');
echo form_input('username', set_value('username'));
echo form_label('password  : ');
echo form_password('password' , set_value('password'));
echo form_label('  repassword  : ');
echo form_password('repassword');
echo '<br/>';
echo form_submit('submit', 'Sign Up','id="submit"');
echo form_close();

?>

<script>
    $(function(){
        $("#submit").click(function(event){
            event.preventDefault();

            $.ajax({
                    url: "<?=site_url().'/user/ajaxreg';?>",
                    type:'post',
                    data : {
                        <?=$this->security->get_csrf_token_name();?> : "<?=$this->security->get_csrf_hash();?>" ,
                        'username' : 'username01'
                    } ,
                    success:function(result){
                        alert(result);

                    }

            });
        });
</script>

and this is my controller :

Code:

public function ajaxreg()
    {
        $data_post = $this->input->post(array('username','password','repassword'),TRUE);
        var_dump($data_post);
    }

i have enabled csrf protection in config.php

when csrf is disabled in config.php, every thing works fine, but after enabling that, my code works first time i clicking, and after that it doesn't works until i refresh page ,

how i can use ajax+csrf to submit forms in CI#?
thanks

ressan.ir
CI is nice