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RE: [Ext-GDE-87] comment from LLRF team (CCR#20)



Dear Chris,

This is my personal comment and not discussed inside LLRF team yet.
So if I misunderstood your comment, please tell me.

I think we do not have enough specifications about
#1. Voltage regulation of modulator
#2. Beam current stability (pulse to pulse, bunch by bunch)
#3. Stability of the Lorentz force detuning (LFD) control by piezo
#4. preciseness of the rf distribution

In case of the 10% in power (5% in amplitude) overhead, we can not compensate >5% request from cavities. (Since not all the power can be used for the compensation, roughly 3% of perturbation can be accepted.) If the rf distribution is not precise, some of the cavities having higher power will be detuned and rf power is wasted at the dummy load without using a beam acceleration (Thus the overhead decreases). As for #3, I have never heard about the performance of piezo compensation at >30 MV/m with long time operation.

In addition
#5 The performance of the FB is limited by the overhead.

The operation gain is restricted by the overhead. Typically, in the case of the 10% in power overhead, FB gain (G) is maximum around 50. (Because in the case of 0.1% perturbation pick-up, 5% extra amplitude (10% extra power) will be necessary.) FB can compensate the error with 1/G so that the perturbation (within the bandwidth of the FB) can be suppressed to 2% (G=50) or 1% (G=100, in the case of the 10% amplitude overhead).
And FB bandwidth(-3 dB)  are 35kHz and 57 kHz for G=50 and 100, respectively.
In the case of 20% power overhead, we can use G=100 and compensate the field perturbation having the frequency component of less than 57 kHz.

As for 6.3% extra margin:
We should distinguish rf power and amplitude. 33.5 MV/m max and 31.5 MV/m operation have 6.3% in amplitude (13% in power). It looks good but maybe we can not use this margin. 1. Even all of the cavities are processed at 33.5 MV/m, field patterns are not uniform under vector-sum control (due to the different Qe, coupling ratio of the rf distribution, LFD, and so on ). The operation is limited by the cavity having worst performance of highest gradient. One of the example is shown in llrf report by Stefan and Brian (ILC-MAC at KEK p.26)
http://ilcagenda.cern.ch/materialDisplay.py?contribId=11&materialId=slides&confId=985
Thus we can only operate with 33.5 MV/m with identical cavities (which is impossible). 2. If we avoid one of the cavity with worst performance, we will have to detune the cavity little bit and we lose some of rf power to dummy load (the power is not used for beam acceleration.)

As for Qe and Pin-cavity control:
If fixed rf distribution system will be adopted, we can not control the input power. (I am not sure that the cavity coupling can be changed after installation, or during rf operation.) Even we can change the Pin-cavity, the vector sum control results in the spread of the cavity fields (because of the difference of Qe and coupling ratio). From the view point of cavity performance, I am not sure this works well or not. (Further quantative discussion will be necessary.)

5% amplitude overhead is better than 3% but if we consider the klystron life time (degerneration) , I hope we will have 10 % amplitude overhead (20% in power) If the LFD is controlled with piezo completely, I agree with the reduction of the overhead. (This is only my opinion and further discussion will be necessary.)

Best wishes,
Shin

At 16:36 06/11/03, Adolphsen, Chris wrote:

Note that for the proposal we submitted, a 10% power overhead is included (see page 35 if the Main Linac BCD - an earlier version of the proposal with 27 cavities had only 6% power overhead) - the rf power distribution math is as follows

33.5 MV/m * 9.5 mA * 1.038 m = 330.3 kW  (Cavity Input Power)
* 26 Cavities
* 1 / 0.95 (Distribution Losses)
* 1 / 0.90 (Tuning Overhead)
= 10.0 MW

Note also that the 33.5 MV/m maximum gradient is 6.3% above the design gradient of 31.5 MV/m, so there is some additional overhead to allow for the variation in sustainable gradients among the cavities. Finally, when running at IP energies < 500 GeV, there will be more overhead.

The current BCD has 11% power overhead, which is close to that originally proposed in the TESLA TDR:

23.4 MV/m * 9.5 mA * 1.038 m = 230.7 kW  (Cavity Input Power)
* 36 Cavities
* 1 / 0.94 (Distribution Losses)
* 1 / 0.88 (Tuning Overhead)
=  10.0 MW

The cavities are assumed to have piezo-electric tuners, which should make the overhead required for Lorentz force detuning compensation small. Partial-quenches should be rare (if not, the gradient will be lowered) and the FB system should probably not try to compensate for them (remember that even with the full loss of one cavity, the IP energy changes by only ~ 1/8000). Beam loading may be more of a problem with the large variation in cavity gradients - we are working on a way to adjust power to individual cavities without dumping power (a prototype is being built) - with control of the cavity input power and Qext, one can compensate beam loading at gradient G by choosing

External Q = Qe = Qeo * ln(2) / ln (1 + G/Go * Qeo/Qe)

Input Power     = PI = PIo * (1/4) * (1 + G/Go * Qeo/Qe)^2 * (Qe/Qeo).

where Go and Qeo are the nominal values (from which the cavity fill time is determined). In this case, the energy gain along the bunch train is uniform but there is non-zero reflected power,

Reflected Power = PR = PIo * (1/4) * (1 - G/Go * Qeo/Qe)^2 * (Qe/Qeo).

which is relatively small.

SNS has a large overhead (times 2-3 in power) because it is a proton machine that includes low beta cavities, and because they want to eventually increase the beam current. TTF generally has a large overhead because they run relatively few cavities per klystron and do not have piezo-tuners in most of the cavities. No doubt they find the extra overhead useful, but it is also a short linac with bunch compression, so fine local control is needed.

XFEL will have a fairly large overhead (40-60% in power), but they also have a higher Qext (times 2), and with a 'smaller' machine, they can afford to be safe. As the linac BCD says, ' ... an overhead based on operation experience with ILC-like cryomodules should eventually be used'.





-----Original Message-----
From: Shinichiro Michizono [mailto:shinichiro.michizono@xxxxxx]
Sent: Thursday, November 02, 2006 10:23 AM
To: Nobu Toge; Warren Funk; GDE CCB; ml-ext-gde@xxxxxxxxxxxx
Cc: Brian Chase; Stefan Simrock; Shinichiro.michizono@xxxxxx
Subject: [Ext-GDE-87] comment from LLRF team (CCR#20)

Dear Toge-san,

This is the comment about CCR#20 from ilc-llrf team.

1.  6% overhead (3% amplitude overhead) is too small for feedback
compensation of Lorentz force detuning, beam loading, exception
handling (such as quench).
2. Operationa experiences at FLASH, SNS indicate >10% (in amplitude)
will be necessary for stable operation.

Best wishes.
Shin



-----------------------------------------------------
Shinichiro MICHIZONO
Accelerator Laboratory
High Energy Accelerator Research Organization(KEK)
1-1 Oho, Tsukuba, Ibaraki 305-0801 Japan

  e-mail: shinichiro.michizono@xxxxxx
tel:+81.298.64.5697
      +81.298.64.5200 + 4641 (PHS)
fax:+81.298.64.3182
-----------------------------------------------------

-----------------------------------------------------
Shinichiro MICHIZONO
Accelerator Laboratory
High Energy Accelerator Research Organization(KEK)
1-1 Oho, Tsukuba, Ibaraki 305-0801 Japan

 e-mail: shinichiro.michizono@xxxxxx
tel:+81.298.64.5697
     +81.298.64.5200 + 4641 (PHS)
fax:+81.298.64.3182
-----------------------------------------------------