next up previous
Next: CSRDRIFT Up: Element Dictionary Previous: CSBEND

CSRCSBEND

Like CSBEND, but incorporates a simulation of Coherent Synchrotron radiation.
Parallel capable? : yes
Parameter Name Units Type Default Description
L $M$ double 0.0 arc length
ANGLE $RAD$ double 0.0 bend angle
K1 $1/M^{2}$ double 0.0 geometric quadrupole strength
K2 $1/M^{3}$ double 0.0 geometric sextupole strength
K3 $1/M^{4}$ double 0.0 geometric octupole strength
K4 $1/M^{5}$ double 0.0 geometric decapole strength
K5 $1/M^{6}$ double 0.0 geometric 12-pole strength
K6 $1/M^{7}$ double 0.0 geometric 14-pole strength
K7 $1/M^{8}$ double 0.0 geometric 16-pole strength
K8 $1/M^{9}$ double 0.0 geometric 18-pole strength
E1 $RAD$ double 0.0 entrance edge angle
E2 $RAD$ double 0.0 exit edge angle
TILT $RAD$ double 0.0 rotation about incoming longitudinal axis
H1 $1/M$ double 0.0 entrance pole-face curvature
H2 $1/M$ double 0.0 exit pole-face curvature
HGAP $M$ double 0.0 half-gap between poles
FINT   double 0.5 edge-field integral
DX $M$ double 0.0 misalignment
DY $M$ double 0.0 misalignment
DZ $M$ double 0.0 misalignment
FSE   double 0.0 fractional strength error
ETILT $RAD$ double 0.0 error rotation about incoming longitudinal axis
N_KICKS   long 4 number of kicks
NONLINEAR   long 1 include nonlinear field components?
LINEARIZE   long 0 use linear matrix instead of symplectic integrator?
SYNCH_RAD   long 0 include classical synchrotron radiation?
EDGE1_EFFECTS   long 1 include entrance edge effects?

Like CSBEND, but incorporates a simulation of Coherent Synchrotron radiation.
Parameter Name Units Type Default Description
EDGE2_EFFECTS   long 1 include exit edge effects?
EDGE_ORDER   long 1 order to which to include edge effects
INTEGRATION_ORDER   long 4 integration order (2 or 4)
BINS   long 0 number of bins for CSR wake
BIN_ONCE   long 0 bin only at the start of the dipole?
BIN_RANGE_FACTOR   double 1.2 Factor by which to increase the range of histogram compared to total bunch length. Large value eliminates binning problems in CSRDRIFTs.
SG_HALFWIDTH   long 0 Savitzky-Golay filter half-width for smoothing current histogram. If less than 1, no SG smoothing is performed.
SG_ORDER   long 1 Savitzky-Golay filter order for smoothing current histogram
SGDERIV_HALFWIDTH   long 0 Savitzky-Golay filter half-width for taking derivative of current histogram. Defaults to SG_HALFWIDTH (if positive) or else 1.
SGDERIV_ORDER   long 1 Savitzky-Golay filter order for taking derivative of current histogram
TRAPAZOID_INTEGRATION   long 1 Select whether to use trapazoid-rule integration (default) or a simple sum.
OUTPUT_FILE   STRING NULL output file for CSR wakes
OUTPUT_INTERVAL   long 1 interval (in kicks) of output to OUTPUT_FILE
OUTPUT_LAST_WAKE_ONLY   long 0 output final wake only?
STEADY_STATE   long 0 use steady-state wake equations?

Like CSBEND, but incorporates a simulation of Coherent Synchrotron radiation.
Parameter Name Units Type Default Description
IGF   long 0 use integrated Greens function (requires STEADY_STATE=1)?
USE_BN   long 0 use b$<$n$>$ instead of K$<$n$>$?
EXPANSION_ORDER   long 0 Order of field expansion. (0=auto)
B1 $1/M$ double 0.0 K1 = b1/rho, where rho is bend radius
B2 $1/M^{2}$ double 0.0 K2 = B2/rho
B3 $1/M^{3}$ double 0.0 K3 = B3/rho
B4 $1/M^{4}$ double 0.0 K4 = B4/rho
B5 $1/M^{5}$ double 0.0 K5 = B5/rho
B6 $1/M^{6}$ double 0.0 K6 = B6/rho
B7 $1/M^{7}$ double 0.0 K7 = B7/rho
B8 $1/M^{8}$ double 0.0 K8 = B8/rho
ISR   long 0 include incoherent synchrotron radiation (scattering)?
ISR1PART   long 1 Include ISR for single-particle beam only if ISR=1 and ISR1PART=1
CSR   long 1 enable CSR computations?
BLOCK_CSR   long 0 block CSR from entering CSRDRIFT?
DERBENEV_CRITERION_MODE   STRING disable disable, evaluate, or enforce
PARTICLE_OUTPUT_FILE   STRING NULL name of file for phase-space output
PARTICLE_OUTPUT_INTERVAL   long 0 interval (in kicks) of output to PARTICLE_OUTPUT_FILE
SLICE_ANALYSIS_INTERVAL   long 0 interval (in kicks) of output to slice analysis file (from slice_analysis command)
LOW_FREQUENCY_CUTOFF0   double -1 Highest spatial frequency at which low-frequency cutoff filter is zero. If not positive, no low-frequency cutoff filter is applied. Frequency is in units of Nyquist (0.5/binsize).

Like CSBEND, but incorporates a simulation of Coherent Synchrotron radiation.
Parameter Name Units Type Default Description
LOW_FREQUENCY_CUTOFF1   double -1 Lowest spatial frequency at which low-frequency cutoff filter is 1. If not given, defaults to LOW_FREQUENCY_CUTOFF1.
HIGH_FREQUENCY_CUTOFF0   double -1 Spatial frequency at which smoothing (high-frequency cutoff) filter begins. If not positive, no frequency filter smoothing is done. Frequency is in units of Nyquist (0.5/binsize).
HIGH_FREQUENCY_CUTOFF1   double -1 Spatial frequency at which smoothing (high-frequency cutoff) filter is 0. If not given, defaults to HIGH_FREQUENCY_CUTOFF0.
CLIP_NEGATIVE_BINS   long 1 If non-zero, then any bins with negative counts after the filters are applied have the counts set to zero.
WAKE_FILTER_FILE   STRING NULL Name of file supplying wakefield filtering data.
WFF_FREQ_COLUMN   STRING NULL Name of column supplying frequency values for wakefield filtering data.
WFF_REAL_COLUMN   STRING NULL Name of column supplying real values for wakefield filtering data.
WFF_IMAG_COLUMN   STRING NULL Name of column supplying imaginary values for wakefield filtering data.
GROUP   string NULL Optionally used to assign an element to a group, with a user-defined name. Group names will appear in the parameter output file in the column ElementGroup





For a discussion of the method behind this element, see M. Borland, ``Simple method for particle tracking with coherent synchrotron radiation,'' Phys. Rev. ST Accel. Beams 4, 070701 (2001) and G. Stupakov and P. Emma, SLAC LCLS-TN-01-12 (2001).

Recommendations for using this element. The default values for this element are not the best ones to use. They are retained only for consistency through upgrades. In using this element, it is recommended to have 50 to 100 k particle in the simulation. Setting BINS=600 and SG_HALFWIDTH=1 is also recommended to allow resolution of fine structure in the beam and to avoid excessive smoothing. It is strongly suggested that the user vary these parameters and view the histogram output to verify that the longitudinal distribution is well represented by the histograms (use OUTPUT_FILE to obtain the histograms). For LCLS simulations, we find that the above parameters give essentially the same results as obtained with 500 k particles and up to 3000 bins.

In order to verify that the 1D approximation is valid, the user should also set DERBENEV_CRITERION_MODE = ``evaluate'' and view the data in OUTPUT_FILE. Generally, the criterion should be much less than 1. See equation 11 of [20].

In order respects, this element is just like the CSBEND element, which provides a symplectic bending magnet that is accurate to all orders in momentum offset. The field expansion is available to fourth order.

One pitfall of symplectic integration is the possibility of orbit and path-length errors for the reference orbit if too few kicks are used. This may be an issue for rings. Hence, one must verify that a sufficient number of kicks are being used by looking at the trajectory closure and length of an on-axis particle by tracking. Using INTEGRATION_ORDER=4 is recommended to reduce the number of required kicks.

Normally, one specifies the higher-order components of the field with the K1, K2, K3, and K4 parameters. The field expansion in the midplane is $B_y(x) = B_o * (1 +
\sum_{n=1}^4\frac{K_n\rho_o}{n!}x^n)$. By setting the USE_bN flag to a nonzero value, one may instead specify the b1 through b4 parameters, which are defined by the expansion $B_y(x) = B_o
* (1 + \sum_{n=1}^4\frac{b_n}{n!}x^n)$. This is convenient if one is varying the dipole radius but wants to work in terms of constant field quality.

Setting NONLINEAR=0 turns off all the terms above K_1 (or b_1) and also turns off effects due to curvature that would normally result in a gradient producing terms of higher order.

Edge effects are included using a first- or second-order matrix. The order is controlled using the EDGE_ORDER parameter, which has a default value of 1. N.B.: if you choose the second-order matrix, it is not symplectic.

Some confusion may exist about the edge angles, particularly the signs. For a sector magnet, we have of course E1=E2=0. For a symmetric rectangular magnet, E1=E2=ANGLE/2. If ANGLE is negative, then so are E1 and E2. To understand this, imagine a rectangular magnet with positive ANGLE. If the magnet is flipped over, then ANGLE becomes negative, as does the bending radius $\rho$. Hence, to keep the focal length of the edge $1/f = -\tan E_i /\rho$ constant, we must also change the sign of $E_i$.

When adding errors, care should be taken to choose the right parameters. The FSE and ETILT parameters are used for assigning errors to the strength and alignment relative to the ideal values given by ANGLE and TILT. One can also assign errors to ANGLE and TILT, but this has a different meaning: in this case, one is assigning errors to the survey itself. The reference beam path changes, so there is no orbit/trajectory error. The most common thing is to assign errors to FSE and ETILT. Note that when adding errors to FSE, the error is assumed to come from the power supply, which means that multipole strengths also change.

Special note about splitting dipoles: when dipoles are long, it is common to want to split them into several pieces, to get a better look at the interior optics. When doing this, care must be exercised not to change the optics. elegant has some special features that are designed to reduce or manage potential problems. At issue is the need to turn off edge effects between the portions of the same dipole.

First, one can simply use the divide_elements command to set up the splitting. Using this command, elegant takes care of everything.

Second, one can use a series of dipoles with the same name. In this case, elegant automatically turns off interior edge effects. This is true when the dipole elements directly follow one another or are separated by a MARK element.

Third, one can use a series of dipoles with different names. In this case, you must also use the EDGE1_EFFECTS and EDGE2_EFFECTS parameters to turn off interior edge effects.

N.B.: For versions 19.X and ealier splitting dipoles is not generally recommended for CSRCSBEND because the coherent synchrotron radiation computations start over at the beginning of each piece. This is only acceptable when using STEADY_STATE=1. This was changed in version 20.X.


next up previous
Next: CSRDRIFT Up: Element Dictionary Previous: CSBEND
Robert Soliday 2014-03-21