Maintain Bootstrap Modal Popup opened on DropDownList Selection changed in ASP.Net

dear sir in my below design

there is a model 

inside model there are tow drop down box  

after selection first drop down box take effect on the second drop down box 

in my case after selection on first drop down box model will close and successfully second dropdown  effected

 i want to model do not close 

<asp:LinkButton ID="lnkBtnAssign" class="dropdown-toggle" data-toggle="modal" data-target="#myDetails"
     runat="server" OnClick="lnkBtnAssign_Click"> <i class="fa fa-user-plus"></i></span> 
</asp:LinkButton>

  <!-- Modal -->
    <div class="modal fade" id="myDetails" role="dialog">
        <div class="modal-dialog modal-sm">
            <div class="modal-content">
                <div class="modal-header">
                    <button type="button" class="close" data-dismiss="modal">&times;</button>
                    <h4 class="modal-title">Modal Header</h4>
                </div>
                <div class="modal-body">
                    <div class="row">
                        <div class="col-md-offset-1 col-md-10 col-md-offset-1">
                            <p>Ticket No : <%#Eval("TicketNo") %></p>
                            <div class="form-group">
                                <label>Department</label>
                                <asp:DropDownList ID="ddlDepartment" AutoPostBack="true" class="form-control" OnSelectedIndexChanged="ddlDepartment_SelectedIndexChanged" runat="server"></asp:DropDownList>
                            </div>
                            <div class="form-group">
                                <label>User</label>
                                <asp:DropDownList ID="ddlUser" class="form-control" runat="server"></asp:DropDownList>
                            </div>
                            <button type="submit" class="btn btn-default">Submit</button>
                        </div>
                    </div>
                </div>
                <div class="modal-footer">
                    <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
                </div>
            </div>
        </div>
    </div>

protected void ddlDepartment_SelectedIndexChanged(object sender, EventArgs e)
{
       // ClientScript.RegisterStartupScript(this.GetType(), "Pop", "alert('hello world')", true);
        ClientScript.RegisterStartupScript(this.GetType(), "Pop", "openModal();", true);
        string[] Parameters = new string[1] { "@_loginType" };
        string[] DbTypes = new string[1] {"varchar"};
        string[] ParameterTypes = new string[1] { "input"};
        object[] Values = new object[1] {ddlDepartment.SelectedItem.Text.ToString()};
        string[] Lengths = new string[1] {"50"};
        DataSet ds = du.returnDataSet(System.Data.CommandType.StoredProcedure, "spGetUserByLoginType", Parameters, DbTypes, ParameterTypes, Values, Lengths);
        ddlUser.DataSource = ds.Tables[0];
        ddlUser.DataBind();
        ddlUser.DataTextField = "Email";
}