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T3P

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is

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a

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3D

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parallel

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finite-element

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time-domain

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solver

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to

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calculate

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transient

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field

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response

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of

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a

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electromagnetic

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structure

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to

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imposed

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fields,

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and

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dipole

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or

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beam

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excitations.

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In

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our

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approach,

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Ampere's

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and

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Faraday's

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laws

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are

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combined

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and

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integrated

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over

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time

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to

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yield

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the inhomogeneous

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vector

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wave

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equation

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for

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the

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time

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integral

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of

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the

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electric

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field

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E

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:

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Latex

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\begin{eqnarray*}\left( \epsilon \frac{\partial^2}{\partial t^2} + \sigma_{eff} \frac{\partial}{\partial t} + \nabla \times \mu^{-1}\nabla\times \right) \int^t \emph{E} d \tau = -\emph{J}
\end{eqnarray*}

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with permittivity

Latex
 $\epsilon = \epsilon_0 \epsilon_r $

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and

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permeability

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Latex

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$\mu = \mu_0 \mu_r$
. In the current implementation, a constant value of the effective conductivity
Latex

$\sigma_{eff} = \tan \delta \cdot 2 \pi f \cdot \epsilon $

is assumed by fixing a frequency
Latex

 $f$ 
, and the losses are specified by the loss tangent
Latex
 $\tan \delta $ 
. As is common for Wakefield computations of rigid beams, the electric current source density J is given by a one-dimensional Gaussian particle distribution, moving at the speed of light along the beam line.

The computational domain is discretized into curved tetrahedral elements and

Latex
 $\int^t \emph{E}d \tau $
in Eq. (1) is represented as an expansion in hierarchical Whitney vector basis functions
Latex
$ \emph{N}_i (x) $

Latex

\begin{eqnarray*} \int^t \emph{E} (\emph{x}, \tau) d \tau = \sum^{N_p}_{i=1} e_i (t) \cdot \emph{N}_i (x) \end{eqnarray*}

up to order

Latex
 $p$ 
within each element. For illustration, the numbers of basis functions for first, second and sixth order are
Latex
 $N_1 = 6 $
,
Latex
 $N_2 = 20 $
and
Latex
 $N_6 = 216 $
, respectively.  Higher order elements (both curved and with higher-order basis functions) not only significantly improve field accuracy and dispersive properties, but also generally lead to higher-order accurate particle-field coupling equivalent to, but much less laborious than, complicated higher-order interpolation schemes commonly found in finite-difference methods.