32. TOOLHEAD Namelist

The TOOLHEAD namelist is used to model an external energy source that is attached to a moving toolhead like those employed in additive manufacturing processes. Two choices of laser model are currently implemented. The motion of the toolhead relative to the computational domain is specified using a TOOLPATH namelist.

Note

Required/Optional:: Optional
Single/Multiple Instances:: Multiple

32.1. Namelist Variables

name 

A unique name used to identify a particular instance of this namelist. Clients will reference the toolhead using this name.

Type:: case sensitive string (31 characters max)
Default:: none

toolpath 

The name of a TOOLPATH namelist that defines the motion of the toolhead relative to the computational domain. The toolpath position defines the reference point for the energy source associated with the toolhead. The toolpath also specifies when the source is on and off via the setting of flag 0: on (flag set), and off (flag clear).

Type:: case sensitive string (31 characters max)
Default:: none

laser_direction 

The direction of laser energy flow. The magnitude of this 3-vector is not significant.

Type:: real 3-vector
Valid values:: any non-zero vector
Default:: none

laser_power 

The total power of the laser.

Type:: real
Units:: E/T
Valid values:: \(\ge0\)
Default:: none

laser_power_func 

Description : The name of a FUNCTION namelist that defines the laser power as a function of time. That function is expected to be a function of (t,x,y,z).
Type        : string
Default     : none

laser_time_const 

The exponential-decay time constant for the laser power employed when the laser is turned on/off. Rather than the laser power changing discontinuously at such a time \(t_0\), it fades in continuously to full power or out to zero power with an exponential decay \(\exp((t_0-t)/\tau)\) where \(\tau\) is the time constant of the decay. To have the power change discontinuously, specify 0.

Type:: real
Units:: T
Valid values:: \(\ge0\)
Default:: none

Note

When the laser changes state between on/off this creates an abrupt change in forcing which results in a short-time transient which must be resolved by the time stepping procedure. Because of this, Truchas automatically reduces the step size by a significant factor whenever this occurs. In spite of this it can still pose significant numerical difficulties for time stepping. Judicious use of this exponential fade feature to soften the change will ease the time stepping without noticeably changing the long-time solution.

laser_type 

Specifies the type of laser model used. See the following section for a description of the available models and their associated namelist variables.

Type:: string
Valid values:: “gaussian” and “gaussian beam”
Default:: none

32.2. Laser Models

Gaussian

In the simple Gaussian model, the laser energy propagates along rays parallel to the beam axis in the direction specified by laser_direction. The axis passes through the toolpath position, and the radiant flux \(E_e\) on any plane orthogonal to the axis is a Gaussian in the radius \(r\) from the axis:

\[E_e(r) \propto \frac{1}{2\pi\sigma^2}\exp\left(\frac{-r^2}{2\sigma^2}\right).\]

The model parameter \(\sigma\) is specified using this namelist variable:

laser_sigma

The value of \(\sigma\), which characterizes the size of the beam. The relationship between \(\sigma\) and the full-width-at-half-maximum (FWHM) value of the beam is given by \(\text{FWHM} = 2\sqrt{2\ln2}\;\sigma \approx 2.35\,\sigma\).

Type:: real
Units:: L
Valid values:: \(>0\)
Default:: none

Gaussian Beam

The Gaussian beam model models a focused Gaussian laser beam whose energy propagates along converging/diverging rays that are approximately parallel to the beam axis with direction specified by laser_direction. The focal point of the beam lies on the axis and is located at the toolpath position. The radiant flux \(E_e\) on planes orthogonal to the axis is a Gaussian in the radius \(r\) from the axis, with a profile (amplitude and width) that varies with distance \(z\) of the plane from the focal point:

\[E_e(r,z) \propto \frac{2}{\pi w^2}\exp\left(\frac{-2r^2}{w^2}\right), \quad w(z) = w_0 \sqrt{1 + \left(\frac{z\lambda M^2}{\pi w_0^2}\right)^2}.\]

The model parameters \(w_0\), \(\lambda\), and \(M^2\) are specified using these namelist variables:

laser_waist_radius

The value of \(w_0\), which characterizes the size of the beam at its focal point or waist. The relationship between \(w_0\) and the full-width-at-half-maximum (FWHM) value of the beam at its waist is given by \(\text{FWHM} = \sqrt{2\ln2}\;w_0 \approx 1.18\,w_0\).

Type:: real
Units:: L
Valid values:: \(>0\)
Default:: none

laser_wavelength

The wavelength \(\lambda\) of the laser radiation.

Type:: real
Units:: L
Valid values:: \(>0\)
Default:: none

laser_beam_quality_factor

The unitless beam quality factor \(M^2\) which characterizes the deviation of a laser beam from an ideal Gaussian beam. This is a widely-used measure in the laser industry.

Type:: real
Valid values:: \(\ge1\)
Default:: none