the usage of RFMODE element

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xhs05
Posts: 30
Joined: 29 Nov 2012, 21:43

the usage of RFMODE element

Post by xhs05 » 30 Aug 2017, 06:23

Hi Michael,

It's Haisheng Xu. I'm trying to use elegant to simulate the longitudinal coupled-bunch instability driven by the HOMs of RF cavity.

I try to use the RFMODE element to represent the HOM of RF cavity in the following way:

RFM1 : RFMODE, FREQ=700.88e6, Q=1.4e5, Rs=3.25e6, bin_size=1e-12, n_bins=2000, N_CAVITIES=1, BUNCHED_BEAM_MODE=1, RECORD="%s,rfmm", SAMPLE_INTERVAL=100, PRELOAD=1, LONG_RANGE_ONLY=1

However, I'm puzzling on the settings of this element in one of the elegant examples: .../examples/multibunchCollectiveEffects/APS-24Bunch-CBI/lattice.lte. In this example lattice file, two RFMODE elements are used without initializing the FREQ parameter of both the elements. Would you like to help me understand this setting? What's the resonant frequency of the HOM in this case? Is RFMODE the correct element to simulate the HOMs of RF cavity?

Thanks!

Regards,
Haisheng

michael_borland
Posts: 1796
Joined: 19 May 2008, 09:33
Location: Argonne National Laboratory
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Re: the usage of RFMODE element

Post by michael_borland » 30 Aug 2017, 08:59

Haisheng,

In the APS example, the runTemplate.ele file has the following command

Code: Select all

&load_parameters
        filename = <cavitySetup>
        change_defined_values = 1
        verbose = 1
&end
The <cavitySetup> field is filled in by the macro argument when Pelegant is invoked in the runCase script. The script generates a file that contains all the needed rf information. It does this using another script, called beamLoadCalcHHC.

RFMODE can be used for simulating HOMs. However, if you have many HOMs, use FRFMODE or FTRFMODE. These elements read HOM data from a file for longitudinal (monopole) and transverse (dipole) modes. They give far better performance than using multiple RFMODE or TRFMODE elements. In the example, the RFMALL and RFDALL elements are used to invoke the HOMs.

--Michael

xhs05
Posts: 30
Joined: 29 Nov 2012, 21:43

Re: the usage of RFMODE element

Post by xhs05 » 30 Aug 2017, 19:55

Hi Michael,

Thanks a lot for your quick reply and the comments!

I'll check out the runTemplate.ele file and the elements you mentioned.

Regards,
Haisheng

xhs05
Posts: 30
Joined: 29 Nov 2012, 21:43

Re: the usage of RFMODE element

Post by xhs05 » 31 Aug 2017, 07:00

Hi Michael,

Continue the previous question, I attach my scripts this time. The lattice file is lattice.lte. The script I run is Run_me.sh.

As I mentioned above, I try to simulate the longitudinal coupled bunch instability driven by one HOM of the RF cavity. Theoretical calculation shows that the most dangerous frequency of the mode is 700.880 MHz, and once I shift the HOM by 40 kHz, the growth rate drops significantly. However, my simulation shows that when I shift the frequency of the HOM in the range of near 200 kHz, the beam is always unstable. It took me quite a while to look for the problem. However, I haven't figured out where the problem is. Wish to get your help!

The RFMODE element is initialized in the lattice.lte file. The frequency is varied via the Collective_track_Template.ele file.

BTW, I use CHARGE element to represent the total charge of the beam (720 bunches). My understanding is that the CHARGE element represents the total charge of the whole beam instead of the single-bunch charge. Is this understanding correct?

Thanks a lot in advance!

Regards,
Haisheng
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