Normality condition on graded algebra Let $\mathbb G_a$ denote the additive group of complex numbers.
Definition:
Let $V \subset Y$ be a dense open subset of the affine variety $Y$ and
$\pi : P \longrightarrow V$ a $\mathbb G_a$-principal bundle. An affine extension is an affine $\mathbb G_a$-variety $\hat P$ together with a morphism $ \hat \pi: \hat P \longrightarrow Y$ and an equivariant open embedding $ \iota: P \hookrightarrow \hat P$, such that the diagram
$$
\begin{array}{ccc}
P & \hookrightarrow & \hat P \\\
\downarrow & & \downarrow \\\
V & \hookrightarrow & Y
\end{array}
$$ 
is commutative and $\iota (P)= \hat \pi^{-1}(V).$
I am interested in affine extensions of the trivial $\mathbb G_a$-bundle over the affine plane punctured at the origin, i.e. $\mathbb A^2_*:=Sp(\mathbb C[x,y])\setminus\{\mathbf o\}$ with $\mathbf o:=(x,y)$, and I have the following description:
Proposition:
If $P \longrightarrow \mathbb A_*^2$ is the trivial bundle, then any affine extension
$\hat P \longrightarrow \mathbb A^2$ is of the form
$$
\hat P = Sp (A),
$$
where
$$
A= \bigoplus_{\nu=0}^\infty \mathfrak m_\nu t^\nu \subset \mathbb C [x,y,t],
$$
with a decreasing sequence $(\mathfrak m_\nu)$, $\nu \in \mathbb N$, of ideals
$\mathfrak m_\nu \subset \mathbb C  [x,y]$, such that
1.
$
\mathfrak m_\nu \cdot \mathfrak m_\lambda \subset \mathfrak m_{\nu + \lambda}
$
for all $\nu, \lambda\in \mathbb N$,
2.
$\mathfrak m_0= \mathbb C [x,y]$, and 
3.
$V(\mathfrak m_\nu) \subset \mathbf o$ for $\nu > 0$.
On the other hand, every finitely generated $\mathbb C$-algebra of that form defines an affine extension of the trivial bundle.
Question: Does somebody know of a criterion on the sequence $(\mathfrak m_\nu)$$_{\nu \in \mathbb N}$ so that $A$ becomes normal?
Examples:
a) If $\mathfrak m_\nu= \mathbb C [x,y]$ for all $\nu$ we have $\hat P \cong \mathbb A^2 \times \mathbb G_a$.
b) If $\mathfrak m_\nu  =(x^m,y^n)^\nu$, we have
$
A = \mathbb C [x,y,x^mt, y^nt].
$
In the second example, one can see that $A$ is normal for instance if $\mathfrak m_\nu=(x^2,y)^\nu$, but not if $\mathfrak m_\nu=(x^2,y^2)^\nu$ -- in the latter case I think the normalization would be defined by the sequence $\mathfrak m_\nu=(x^2,xy,y^2)^\nu$. Since $A = \mathbb C [x,y,x^mt, y^nt]\cong\mathbb C[x,y,u,v]/(x^mv-y^nu)$ in example b), $Sp(A)$ is a hypersurface in $\mathbb C^4$, so normality is equivalent to singularities being of codimension at least two.
Edit: If the question is difficult in general, I am also interested in the following special case: For which monomial ideals $\mathfrak m\subset\mathbb C[x,y]$, is the ring defined by the sequence $\mathfrak m^\nu$ normal? In this situation I would expect something like: $A$ is normal iff the support of $\mathfrak m^\nu$ consists of all lattice points in the convex hull of the support of $\mathfrak m^\nu$ in $\mathbb R^2$. Here the support of $\mathfrak m^\nu$ is the set of pairs $(k,l)\in\mathbb N^2$ such that $x^ky^l\in\mathfrak m^\nu$.
 A: Hi Isac, One simple criterion can be given which is analogous to Rees' valuations corresponding to ideals. Set $R := \mathbb{C}[x,y]$, define $\nu: R \to \mathbb{N} \cup \infty$ as $\nu(f) := \max\lbrace k: f \in m_k\rbrace$. Then I believe $A$ is normal if $\nu(f^k) = k\nu(f)$ for all $f \in R$. For other criteria, I would look into Kei-ichi Watanabe's articles.
Edit: Here is a general approach.
Claim 1: $A$ is integrally closed iff every  homogeneous  (with respect to the grading of $A$) element in $\mathbb{C}[x,y,t]$ (i.e. an element of the form $f(x,y)t^k$) which is integral over $A$ is in $A$.
By Claim 1 the integral equation of $ft^k$ (for $f \in R$) over $A$ be of the form $z^d + \sum_{i=0}^d g_ez^{d-e} = 0$ for some $g_e \in m_{ek}$. This proves Claim 2 below.
Claim 2: $A$ is integrally closed iff $\bar m_k := m_k$ for all $k \geq 1$, where
$\bar m_k := \lbrace f \in m_k: f^d + \sum_{i=0}^d g_ez^{d-e} = 0$ for some $d \geq 0$ and $g_1, \ldots, g_d \in m_{ek}\rbrace$.
In the special case that $m_k$ is a monomial ideal for each $k$, it suffices to prove the integral condition only for monomials in $m_k$. And in the special special case that $m_k = m^k$ for some monomial ideal $m$, it follows that your claim is true, i.e. $A$ is integrally closed iff the support of $m^k$ contains all the monomials in the cone spanned by its monomials.
A: The $\nu$-formulation is a little off, you need to incorporate $t$ as well.  Let $ord(\alpha) = \sup\lbrace k: \alpha \in m_k t^k\rbrace$, and let $\overline\nu(f) = \limsup\lbrace ord(f^k)/k : k \in N\rbrace$ (related to Section 6.9 in Huneke-Swanson book). At least if $m_k = I^k$ for some ideal $I$, then for all $f$, if $\overline \nu(f) = ord (f)$, then $A$ is normal.
In a two-dimensional regular local ring or two-dimensional polynomial ring such as here, $k[x,y][It]$ is integrally closed iff $I$ is integrally closed (Chapter 14 in Huneke-Swanson).
A: First of all I don't see that $A$ is finitely generated. For this you need equality in 1. for $\lambda,\nu$ big enough.
Secondly, your special case of monomial ideals: Here $A$ is triply graded and therefore the ring of functions on an affine toroidal variety. Hence it is normal iff the monoid $M$ of all $(k,l,\nu)\in\mathbb N^3$ with $x^ky^lt^\nu\in A$ is saturated. This means that if $M_{\mathbb R}$ is the convex cone spanned by $M$ then $M=M_{\mathbb R}\cap{\mathbb Z}^3$.
Conversely you can generate all examples this way. Since $M_{\mathbb R}$ contains $(1,0,0)$ and $(0,0,1)$ it suffices to consider the slice $M_{\mathbb R}\cap\{\nu=1\}$. Let $P$ be its projection into the $(x,y)$-plane. Then $P$ is of the form
$$
P=\text{convex hull}(p_1,\ldots,p_s)+{\mathbb R}_{\ge0}^2
$$
with $p_i\in{\mathbb Q}_{\ge0}^2$ and one of the $p_i$ is a multiple of $(1,0)$ and $(0,1)$, respectively. In other words, $P$ looks like a Newton polygon. Another way to describe $P$ is by means of linear inequalities
$$
a_ix+b_iy\ge c_i
$$
where alle $a_i,b_i,c_i\in{\mathbb Q}_{\ge0}$ and $x\ge0$, $y\ge0$ are among these inequalities.
Now put
$$
\mathfrak m_\nu:=\langle x^ky^l\mid(k,l)\in\nu P\rangle
$$
and you get lots of normal examples.
I don't think that this criterion is equivalent to each slice $\nu=\text{fixed}$ is saturated.
