disable parent window when child window is being open ? Plz give me Example .

<script>
var popupWindow = null;
function child_open() 
    { 
    popupWindow =window.open('Indent_popup.aspx',"_blank","directories=no, status=no, menubar=no, scrollbars=yes, resizable=no,width=800, height=280,top=200,left=200");
}
function parent_disable() {
    if (popupWindow && !popupWindow.closed)
        popupWindow.focus();
                 }
</script>

<asp:Button ID="Add_btn" runat="server" BorderStyle="Solid" Height="22px" 
                        Text="Add" Width="75px" onclick="Add_btn_Click" BorderColor="Gray" 
            BorderWidth="1px" Font-Bold="True" style="margin-top: 0px" 
  onclientclick="javascript:return child_open()"  CausesValidation="False" />

protected void Add_btn_Click(object sender, EventArgs e)
{
    this.Add_btn.Attributes.Add("onclick", "javascript:return child_open()");
    process();
    //this.Add_btn.Attributes.Add("onclick", "javascript:return b_enable()");
    //Indent_no_dd.Enabled = false;
}