RFQ development for high power beams

skillfulbuyerUrban and Civil

Nov 16, 2013 (3 years and 10 months ago)

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Imperial College London, FETS

1

RFQ development for high power beams


1. Introduction

2. Particle dynamics in the RFQ

3. Electrodynamic design of
the RF resonator

4. Mechanical design &
construction of the RFQ

5. Conclusions

Imperial College London, FETS

2

Introduction

The first accelerator structure is most critical because of the high space charge
forces at low beam velocities


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Injector
RFQ
DTL
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kin
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average
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average
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z [m]
Imperial College London, FETS

3

Introduction

RFQ :

Using a set of four electrodes to build an electrostatic focussing channel and to
create longitudinal electric field components for the acceleration of the
particles by modulation of the electrodes.

Requirements : High transmission, low emittance growth, low power consumption
by use of high Impedance resonator

Imperial College London, FETS

4

An RFQ has to fulfil several functions like beam matching, bunching and
acceleration at once. These functions can only be provided by changing the
modulation of the RFQ electrodes along the beam path.

Particle dynamics in the RFQ

In the traditional design philosophy in
different parts of the RFQ different
parameters are kept constant
according to the function of this part.

Imperial College London, FETS

5

Particle dynamics in the RFQ

Influence of the electrode design on the beam current limit of the RFQ :

To increase the current limit of the RFQ in modern designs all parameters of the

RFQ electrodes are varied along the beam axis.

Improved design

CRYRING

Classic design

Classic design

Improved design

HERA

Imperial College London, FETS

6

Particle dynamics in the RFQ

Influence of electrode voltage, injection energy and RFQ length on beam current
limit, RFQ length and power consumption and transmission

Imperial College London, FETS

7

Particle dynamics in the RFQ

Due to sparks and dark discharge,
the maximum potential on the
Electrodes is limited. This limit is
not only a function of the
aperture of the RFQ, but also of
the RF frequency and the quality
of the electrode surfaces.

The Kilpatrick factor (usually
between 1.5 and 2 for RFQ’s) is
the factor between the applied
potential on the electrodes and
the spark limit given by the
theory of Kilpatrick.


Electrode potential as a function of gap
distance and RF frequency

Imperial College London, FETS

8

Electrodynamic design of the RF resonator

To provide the electrodes with the necessary potential
different types of resonant RF structures can be used.

4 rod structure(e)

Split coaxial (d)

Double
-
H (c)

4 Vane structure (a & b)

Imperial College London, FETS

9

Electrodynamic design of the RF resonator

Challenges are : High shunt impedance, low resistive losses, concentration of
fields onto axis

R’ [k
W
m]

4
-
Vane

4
-
Rod

Split coaxial

D
-
H
-
resonator

f [MHz]

)
4
(
2
exp
exp
2
0
2
Vane
Q
Q
R
R
dV
H
W
N
W
Q
L
N
P
P
U
R
SF
th











Imperial College London, FETS

10

Electrodynamic design of the RF resonator

Challenges are : Field flatness is strongly influenced by endplates and
mechanical design of the resonator (tuners, couplers…)

Field flatness of 4 rod RFQ

4 Vane RFQ with large coupling windows
(left) and according longitudinal potential
distibution (upper)

Imperial College London, FETS

11

Electrodynamic design of the RF resonator

Challenges are : Unwanted modes (
dipole
, multipole) near the working
frequency of the RFQ

Use of VCR rings

Improvement of
mode structure
at HERA RFQ
by the use of
RLC couplers
in end flanges

Imperial College London, FETS

12

Electrodynamic design of the RF resonator

Challenges are additional support : Coupling, Endplates, Tuners, Feedback systems etc.

have influence on RF properties of the cavity

RLC coupler

end tuner

adjustment ring

positioner

Vane

beam axis

Imperial College London, FETS

13

Mechanical design & construction of the RFQ

Challenges are : Production tolerances have influence on the
particle transport (mismatch) and resonator characteristics
(esp. 4 vane and dipole modes)

RIA : high tech

assembly

SNS : massive parts

Imperial College London, FETS

14

Mechanical design & construction of the RFQ

Challenges are : Power dissipation, resistive losses and cooling

Leada: very strong cooling !

4 Rod : uncooled stems at 170
o
C

How much cooling is
necessary ?

China Institute of
Atomic Energy : No
difference between 16
and 20 channels !

Imperial College London, FETS

15

Mechanical design & construction of the RFQ

Challenges are : support for power
feed troughs, pumping, mode
stabilizing, active control of
tolerances, etc… without
influencing the RF properties

(shunt impedance, additional modes)

SNS

Leda

Imperial College London, FETS

16

Conclusions

Frequency, RFQ length etc. are not independent from each other

But strongly coupled (
“one knob machine”)


No optimum design strategy for particle dynamics known.


Choice of resonator structure strongly influences the mechanical
design


Dynamic control of resonator by electro
-
mechanic systems (piezo)
should be considered for 4
-
Vane structure


=> Design of an RFQ is not straight forward but a process of
several iterations.


Imperial College London, FETS

17

Work to be performed

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Year 1




















1) Decision of frequency


2) Particle dynamic design (field level, Kilpatrick) => Length of RFQ


3) Electro dynamic design of resonator and Endplates => Choice of RFQ type


4) Electro dynamic design of tuners, couplers


5) Design of models, production and tests of models


6) Mechanical design of RFQ, Endplates, Positioners


7) Mechanical design of tuner, couplers, etc


8) Production of RFQ


9) Production of tuner, couplers, and support


10) Assembly of RFQ, test and commissioning