Essential Physics and Engineering of Cryogenics for Accelerators

173 Pages·2012·3.91 MB·English

Essential Physics and Engineering
of
Cryogenics for Accelerators

Simplified Concepts & Practical Viewpoints

By

VenkataRao Ganni
November 5, 2012

Page 1
Operated by the Jefferson Science Associates for the U.S. Dept. of Energy
What is Cryogenics?

It is the production of temperature below 123K (-150 C)

Examples of Cryogenic Fluids
Cryogenic Fluid Tsat, 1 atm
[K]
Helium 4.22
Hydrogen 20.28
Neon 27.09
Nitrogen 77.31
Argon 87.28
Oxygen 90.19
Methane 111.69 Page 2
Operated by the Jefferson Science Associates for the U.S. Dept. of Energy
History

Cryogenics was primarily used for
 Gas separation

Helium was first liquefied by Heike Kamerlingh Onnes on July
10th 2008, in Leiden (NL)

He Observed superconductivity in 1911
This lead to the application of Cryogenics to:
 Physics research
 Medical Applications (MRI Magnets)
 Instruments

The Other Applications are:
 Biological & Medical
 Space research
Page 3
 Vacuum

Operated by the Jefferson Science Associates for the U.S. Dept. of Energy
Superconductivity

No resistance below a critical temperature

a) Low Temperature Super conductors (below 20K)
Used for Magnets and RF cavities

a) High Temperature Super conductors (around 70K
Level)
Used for power leads

All these need Cryogenics
Page 4

Operated by the Jefferson Science Associates for the U.S. Dept. of Energy
Particle Accelerators

Particle Accelerators use Magnets and RF cavities

At room temperature the iron core saturates at about 2T, where as the
magnets built with super conductors can be designed for large magnetic
fields like10T and more and are compact

Similarly the room temperature RF cavities are built for less than 500 HZ.
Higher frequency designs require low temperature environment for
operation

For a given energy, the accelerators designed with superconductors
require:
• Lower capital cost
— Since it requires fewer number of Magnets and/or RF cavities
— Less length of the accelerator
• Lower Operating cost

There fore for large accelerators, Superconducting Structures at
cryogenic temperatures is a proven cost effective reality
Page 5

All large particle accelerators need Cryogenics

Operated by the Jefferson Science Associates for the U.S. Dept. of Energy
Tentative Schedule

Chapter Duration
(min)

• 1 Introduction 10
• 2 Carnot Helium Refrigeration and Liquefaction Systems 10
• 3 Ideal Helium Refrigeration Systems and Carnot Step 10
• 4 The Theory Behind Cycle Design 05
• 5 System Optimization & Ganni Cycle 15
• 6 Liquid Nitrogen Pre-cooling 05
• 7 Sub-Atmospheric {2-K} Helium Refrigeration Systems 10
• 8 Conclusions 05

• Discussions ??

Page 6
Operated by the Jefferson Science Associates for the U.S. Dept. of Energy
Introduction
1.

• Helium refrigeration and liquefaction systems are an
extension of the traditional household refrigeration systems
• Let’s begin with the question,
“What is a refrigeration system?”

• A refrigeration system transfers heat energy from low
temperature to high temperature.

• Normally, the term refrigeration is used for absorbing heat
energy at a constant temperature, but this does not have to
be the case

Page 7
• Let’s look at an ideal vapor compression cycle, operating
between two constant temperature reservoirs…
Operated by the Jefferson Science Associates for the U.S. Dept. of Energy
Introduction
(cont.)

Carnot Vapor Compression Refrigeration Cycle

#1 to #2: Compressor • Fluid is compressed isentropically (requiring W )
C

#2 to #3: Condenser • Heat, Q , is rejected isothermally (at T )
H H
#3 to #4: Expander • Fluid is expanded isentropically (extracting W )
X
#4 to #1: Evaporator • Heat, Q , is absorbed isothermally (at T )
L L

Net input work: W = W – W = Q – Q = (T – T )·DS
CARNOT C X H L H L
Cooling provided: Q = T ·DS
L L Page 8
Coefficient of Performance: COP = Q / W = (T /T – 1)-1
L CARNOT H L

Inverse COP: COP = 1 / COP = T /T – 1
INV H L

Operated by the Jefferson Science Associates for the U.S. Dept. of Energy
Introduction

(cont.)

• The coefficient of performance (COP) is the ratio of the
cooling provided to the required input power

• Carnot vapor compression cycle, the refrigerator
operating between -10 and +50 °C
COP = {(273+50) / (273-10) – 1}-1 = 4.4 W/W

• The Carnot work (W ) for 4.4 kW of cooling is 1 kW
CARNOT

Note: This is not a violation of the first law of thermo
since a refrigerator is transferring energy from one
temperature to another and not converting it

• Thermodynamic efficiency is the ratio of the ideal input
Page 9
power to the actual required input power
Operated by the Jefferson Science Associates for the U.S. Dept. of Energy
Introduction

(cont.)

• The Carnot cycle is an ‘ideal cycle’ in the sense that it
does not have any ‘irreversibilities’ (i.e., ‘lost work’)

for a given path from state 1 to 2, with no
irreversibilities, the heat transfer is

2
Q   T  dS

1

Note: This is a statement of the 2nd law of thermodynamics
Page 10

Operated by the Jefferson Science Associates for the U.S. Dept. of Energy