Hi,
I am using CSRCSBEN with HIGH_FREQUENCY_CUTOFF and for a given (too high?) number of BINS Elegant gives me the following warning:
"low pass filter resulted in negative values in 42 bins" etc...
What does it mean "negative values"? Is it amplifying modulations instead of damping them?
Thanks,
Simone
CSR filter
Moderators: cyao, michael_borland
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michael_borland
- Posts: 1933
- Joined: 19 May 2008, 09:33
- Location: Argonne National Laboratory
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Re: CSR filter
Simone,
What that message means is that after elegant applies the filter, the histogram has some negative values, which is unphysical. You can appreciate how this might happen by thinking about a square current pulse subjected to a high-pass filter. Due to Gibbs phenomenon, it will have ringing that includes negative-going portions. This should not affect simulation results significantly, except perhaps in unrealistic cases of longitudinal distributions with very precipitous edges.
When negative histogram values are detected, elegant sets the negative values to zero. It also adjusts the normalization of the histogram to restore the total current. You can look at the CSRCSBEND output file to see what the resulting histogram looks like. The magnitude of the artifacts (although not the number of negative-going bins) can be reduced by increasing the difference between HIGH_FREQUENCY_CUTOFF0 and HIGH_FREQUENCY_CUTOFF1.
--Michael
What that message means is that after elegant applies the filter, the histogram has some negative values, which is unphysical. You can appreciate how this might happen by thinking about a square current pulse subjected to a high-pass filter. Due to Gibbs phenomenon, it will have ringing that includes negative-going portions. This should not affect simulation results significantly, except perhaps in unrealistic cases of longitudinal distributions with very precipitous edges.
When negative histogram values are detected, elegant sets the negative values to zero. It also adjusts the normalization of the histogram to restore the total current. You can look at the CSRCSBEND output file to see what the resulting histogram looks like. The magnitude of the artifacts (although not the number of negative-going bins) can be reduced by increasing the difference between HIGH_FREQUENCY_CUTOFF0 and HIGH_FREQUENCY_CUTOFF1.
--Michael
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simone.dimitri
- Posts: 46
- Joined: 09 Jun 2008, 01:19
Re: CSR filter
Ok Micheal, I think it is clear now. Thanks.
By the way, I have found another error message but this time related to LSC in RFCW elements (ACCT_L2, etc.. It says:
Error: distance between LSC kicks for ACCT_L2 at z=6.350740e+001 is too large.
Suggest reducing distance between kicks by factor 1.000579e+000
I am noticing the error depends on "n_kicks" option while it does not seem to depend on "LSC_BINS" option of the element.
Could you please give me some additional explainations? thank you in advance,
S.
By the way, I have found another error message but this time related to LSC in RFCW elements (ACCT_L2, etc.. It says:
Error: distance between LSC kicks for ACCT_L2 at z=6.350740e+001 is too large.
Suggest reducing distance between kicks by factor 1.000579e+000
I am noticing the error depends on "n_kicks" option while it does not seem to depend on "LSC_BINS" option of the element.
Could you please give me some additional explainations? thank you in advance,
S.
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michael_borland
- Posts: 1933
- Joined: 19 May 2008, 09:33
- Location: Argonne National Laboratory
- Contact:
Re: CSR filter
Simone,
The LSC message is indeed related to the number of kicks. There are two criteria
1. If the distance between kicks is too large compared to the space charge oscillation wavelength, the integration will be unstable and so elegant aborts. This will depend on the bunch charge and length.
2. If the fractional change in beam energy between kicks is too large, the integration will be inaccurate, so elegant aborts.
--Michael
The LSC message is indeed related to the number of kicks. There are two criteria
1. If the distance between kicks is too large compared to the space charge oscillation wavelength, the integration will be unstable and so elegant aborts. This will depend on the bunch charge and length.
2. If the fractional change in beam energy between kicks is too large, the integration will be inaccurate, so elegant aborts.
--Michael